Merge branch 'jh/notes-merge'

* jh/notes-merge: (23 commits)
  Provide 'git merge --abort' as a synonym to 'git reset --merge'
  cmd_merge(): Parse options before checking MERGE_HEAD
  Provide 'git notes get-ref' to easily retrieve current notes ref
  git notes merge: Add testcases for merging notes trees at different fanouts
  git notes merge: Add another auto-resolving strategy: "cat_sort_uniq"
  git notes merge: --commit should fail if underlying notes ref has moved
  git notes merge: List conflicting notes in notes merge commit message
  git notes merge: Manual conflict resolution, part 2/2
  git notes merge: Manual conflict resolution, part 1/2
  Documentation: Preliminary docs on 'git notes merge'
  git notes merge: Add automatic conflict resolvers (ours, theirs, union)
  git notes merge: Handle real, non-conflicting notes merges
  builtin/notes.c: Refactor creation of notes commits.
  git notes merge: Initial implementation handling trivial merges only
  builtin/notes.c: Split notes ref DWIMmery into a separate function
  notes.c: Use two newlines (instead of one) when concatenating notes
  (trivial) t3303: Indent with tabs instead of spaces for consistency
  notes.h/c: Propagate combine_notes_fn return value to add_note() and beyond
  notes.h/c: Allow combine_notes functions to remove notes
  notes.c: Reorder functions in preparation for next commit
  ...

Conflicts:
	builtin.h

20 files changed, 3794 insertions, 162 deletions

diff --git a/Documentation/git-merge.txt b/Documentation/git-merge.txt
index d43416d299..c1efaaa5c5 100644
--- a/Documentation/git-merge.txt
+++ b/Documentation/git-merge.txt
@@ -13,6 +13,7 @@ SYNOPSIS
 	[-s <strategy>] [-X <strategy-option>]
 	[--[no-]rerere-autoupdate] [-m <msg>] <commit>...
 'git merge' <msg> HEAD <commit>...
+'git merge' --abort
 
 DESCRIPTION
 -----------
@@ -47,6 +48,14 @@ The second syntax (<msg> `HEAD` <commit>...) is supported for
 historical reasons.  Do not use it from the command line or in
 new scripts.  It is the same as `git merge -m <msg> <commit>...`.
 
+The third syntax ("`git merge --abort`") can only be run after the
+merge has resulted in conflicts. 'git merge --abort' will abort the
+merge process and try to reconstruct the pre-merge state. However,
+if there were uncommitted changes when the merge started (and
+especially if those changes were further modified after the merge
+was started), 'git merge --abort' will in some cases be unable to
+reconstruct the original (pre-merge) changes. Therefore:
+
 *Warning*: Running 'git merge' with uncommitted changes is
 discouraged: while possible, it leaves you in a state that is hard to
 back out of in the case of a conflict.
@@ -72,6 +81,18 @@ invocations.
 	Allow the rerere mechanism to update the index with the
 	result of auto-conflict resolution if possible.
 
+--abort::
+	Abort the current conflict resolution process, and
+	try to reconstruct the pre-merge state.
++
+If there were uncommitted worktree changes present when the merge
+started, 'git merge --abort' will in some cases be unable to
+reconstruct these changes. It is therefore recommended to always
+commit or stash your changes before running 'git merge'.
++
+'git merge --abort' is equivalent to 'git reset --merge' when
+`MERGE_HEAD` is present.
+
 <commit>...::
 	Commits, usually other branch heads, to merge into our branch.
 	You need at least one <commit>.  Specifying more than one
@@ -142,7 +163,7 @@ happens:
    i.e. matching `HEAD`.
 
 If you tried a merge which resulted in complex conflicts and
-want to start over, you can recover with `git reset --merge`.
+want to start over, you can recover with `git merge --abort`.
 
 HOW CONFLICTS ARE PRESENTED
 ---------------------------
@@ -213,8 +234,8 @@ After seeing a conflict, you can do two things:
 
  * Decide not to merge.  The only clean-ups you need are to reset
    the index file to the `HEAD` commit to reverse 2. and to clean
-   up working tree changes made by 2. and 3.; `git-reset --hard` can
-   be used for this.
+   up working tree changes made by 2. and 3.; `git merge --abort`
+   can be used for this.
 
  * Resolve the conflicts.  Git will mark the conflicts in
    the working tree.  Edit the files into shape and
diff --git a/Documentation/git-notes.txt b/Documentation/git-notes.txt
index 2981d8c5ef..296f314eae 100644
--- a/Documentation/git-notes.txt
+++ b/Documentation/git-notes.txt
@@ -14,8 +14,12 @@ SYNOPSIS
 'git notes' append [-F <file> | -m <msg> | (-c | -C) <object>] [<object>]
 'git notes' edit [<object>]
 'git notes' show [<object>]
+'git notes' merge [-v | -q] [-s <strategy> ] <notes_ref>
+'git notes' merge --commit [-v | -q]
+'git notes' merge --abort [-v | -q]
 'git notes' remove [<object>]
 'git notes' prune [-n | -v]
+'git notes' get-ref
 
 
 DESCRIPTION
@@ -83,6 +87,21 @@ edit::
 show::
 	Show the notes for a given object (defaults to HEAD).
 
+merge::
+	Merge the given notes ref into the current notes ref.
+	This will try to merge the changes made by the given
+	notes ref (called "remote") since the merge-base (if
+	any) into the current notes ref (called "local").
++
+If conflicts arise and a strategy for automatically resolving
+conflicting notes (see the -s/--strategy option) is not given,
+the "manual" resolver is used. This resolver checks out the
+conflicting notes in a special worktree (`.git/NOTES_MERGE_WORKTREE`),
+and instructs the user to manually resolve the conflicts there.
+When done, the user can either finalize the merge with
+'git notes merge --commit', or abort the merge with
+'git notes merge --abort'.
+
 remove::
 	Remove the notes for a given object (defaults to HEAD).
 	This is equivalent to specifying an empty note message to
@@ -91,6 +110,10 @@ remove::
 prune::
 	Remove all notes for non-existing/unreachable objects.
 
+get-ref::
+	Print the current notes ref. This provides an easy way to
+	retrieve the current notes ref (e.g. from scripts).
+
 OPTIONS
 -------
 -f::
@@ -133,9 +156,37 @@ OPTIONS
 	Do not remove anything; just report the object names whose notes
 	would be removed.
 
+-s <strategy>::
+--strategy=<strategy>::
+	When merging notes, resolve notes conflicts using the given
+	strategy. The following strategies are recognized: "manual"
+	(default), "ours", "theirs", "union" and "cat_sort_uniq".
+	See the "NOTES MERGE STRATEGIES" section below for more
+	information on each notes merge strategy.
+
+--commit::
+	Finalize an in-progress 'git notes merge'. Use this option
+	when you have resolved the conflicts that 'git notes merge'
+	stored in .git/NOTES_MERGE_WORKTREE. This amends the partial
+	merge commit created by 'git notes merge' (stored in
+	.git/NOTES_MERGE_PARTIAL) by adding the notes in
+	.git/NOTES_MERGE_WORKTREE. The notes ref stored in the
+	.git/NOTES_MERGE_REF symref is updated to the resulting commit.
+
+--abort::
+	Abort/reset a in-progress 'git notes merge', i.e. a notes merge
+	with conflicts. This simply removes all files related to the
+	notes merge.
+
+-q::
+--quiet::
+	When merging notes, operate quietly.
+
 -v::
 --verbose::
-	Report all object names whose notes are removed.
+	When merging notes, be more verbose.
+	When pruning notes, report all object names whose notes are
+	removed.
 
 
 DISCUSSION
@@ -163,6 +214,38 @@ object, in which case the history of the notes can be read with
 `git log -p -g <refname>`.
 
 
+NOTES MERGE STRATEGIES
+----------------------
+
+The default notes merge strategy is "manual", which checks out
+conflicting notes in a special work tree for resolving notes conflicts
+(`.git/NOTES_MERGE_WORKTREE`), and instructs the user to resolve the
+conflicts in that work tree.
+When done, the user can either finalize the merge with
+'git notes merge --commit', or abort the merge with
+'git notes merge --abort'.
+
+"ours" automatically resolves conflicting notes in favor of the local
+version (i.e. the current notes ref).
+
+"theirs" automatically resolves notes conflicts in favor of the remote
+version (i.e. the given notes ref being merged into the current notes
+ref).
+
+"union" automatically resolves notes conflicts by concatenating the
+local and remote versions.
+
+"cat_sort_uniq" is similar to "union", but in addition to concatenating
+the local and remote versions, this strategy also sorts the resulting
+lines, and removes duplicate lines from the result. This is equivalent
+to applying the "cat | sort | uniq" shell pipeline to the local and
+remote versions. This strategy is useful if the notes follow a line-based
+format where one wants to avoid duplicated lines in the merge result.
+Note that if either the local or remote version contain duplicate lines
+prior to the merge, these will also be removed by this notes merge
+strategy.
+
+
 EXAMPLES
 --------
 
diff --git a/Makefile b/Makefile
index 7eb948d7c8..91567c651e 100644
--- a/Makefile
+++ b/Makefile
@@ -525,6 +525,7 @@ LIB_H += mailmap.h
 LIB_H += merge-recursive.h
 LIB_H += notes.h
 LIB_H += notes-cache.h
+LIB_H += notes-merge.h
 LIB_H += object.h
 LIB_H += pack.h
 LIB_H += pack-refs.h
@@ -615,6 +616,7 @@ LIB_OBJS += merge-recursive.o
 LIB_OBJS += name-hash.o
 LIB_OBJS += notes.o
 LIB_OBJS += notes-cache.o
+LIB_OBJS += notes-merge.o
 LIB_OBJS += object.o
 LIB_OBJS += pack-check.o
 LIB_OBJS += pack-refs.o
diff --git a/builtin.h b/builtin.h
index 8dd4569b3c..c3e5db2685 100644
--- a/builtin.h
+++ b/builtin.h
@@ -16,7 +16,7 @@ extern const char git_more_info_string[];
 extern void prune_packed_objects(int);
 extern int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 			 int merge_title, int shortlog_len);
-extern int commit_notes(struct notes_tree *t, const char *msg);
+extern void commit_notes(struct notes_tree *t, const char *msg);
 
 struct notes_rewrite_cfg {
 	struct notes_tree **trees;
diff --git a/builtin/merge.c b/builtin/merge.c
index c24a7be020..3921cd3040 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -57,6 +57,7 @@ static const char *branch;
 static int option_renormalize;
 static int verbosity;
 static int allow_rerere_auto;
+static int abort_current_merge;
 
 static struct strategy all_strategy[] = {
 	{ "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
@@ -197,6 +198,8 @@ static struct option builtin_merge_options[] = {
 		"message to be used for the merge commit (if any)",
 		option_parse_message),
 	OPT__VERBOSITY(&verbosity),
+	OPT_BOOLEAN(0, "abort", &abort_current_merge,
+		"abort the current in-progress merge"),
 	OPT_END()
 };
 
@@ -919,22 +922,6 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	const char *best_strategy = NULL, *wt_strategy = NULL;
 	struct commit_list **remotes = &remoteheads;
 
-	if (read_cache_unmerged()) {
-		die_resolve_conflict("merge");
-	}
-	if (file_exists(git_path("MERGE_HEAD"))) {
-		/*
-		 * There is no unmerged entry, don't advise 'git
-		 * add/rm <file>', just 'git commit'.
-		 */
-		if (advice_resolve_conflict)
-			die("You have not concluded your merge (MERGE_HEAD exists).\n"
-			    "Please, commit your changes before you can merge.");
-		else
-			die("You have not concluded your merge (MERGE_HEAD exists).");
-	}
-
-	resolve_undo_clear();
 	/*
 	 * Check if we are _not_ on a detached HEAD, i.e. if there is a
 	 * current branch.
@@ -953,6 +940,34 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 
 	argc = parse_options(argc, argv, prefix, builtin_merge_options,
 			builtin_merge_usage, 0);
+
+	if (abort_current_merge) {
+		int nargc = 2;
+		const char *nargv[] = {"reset", "--merge", NULL};
+
+		if (!file_exists(git_path("MERGE_HEAD")))
+			die("There is no merge to abort (MERGE_HEAD missing).");
+
+		/* Invoke 'git reset --merge' */
+		return cmd_reset(nargc, nargv, prefix);
+	}
+
+	if (read_cache_unmerged())
+		die_resolve_conflict("merge");
+
+	if (file_exists(git_path("MERGE_HEAD"))) {
+		/*
+		 * There is no unmerged entry, don't advise 'git
+		 * add/rm <file>', just 'git commit'.
+		 */
+		if (advice_resolve_conflict)
+			die("You have not concluded your merge (MERGE_HEAD exists).\n"
+			    "Please, commit your changes before you can merge.");
+		else
+			die("You have not concluded your merge (MERGE_HEAD exists).");
+	}
+	resolve_undo_clear();
+
 	if (verbosity < 0)
 		show_diffstat = 0;
 
diff --git a/builtin/notes.c b/builtin/notes.c
index c85cbf5a47..4d5556e2cb 100644
--- a/builtin/notes.c
+++ b/builtin/notes.c
@@ -17,6 +17,7 @@
 #include "run-command.h"
 #include "parse-options.h"
 #include "string-list.h"
+#include "notes-merge.h"
 
 static const char * const git_notes_usage[] = {
 	"git notes [--ref <notes_ref>] [list [<object>]]",
@@ -25,8 +26,12 @@ static const char * const git_notes_usage[] = {
 	"git notes [--ref <notes_ref>] append [-m <msg> | -F <file> | (-c | -C) <object>] [<object>]",
 	"git notes [--ref <notes_ref>] edit [<object>]",
 	"git notes [--ref <notes_ref>] show [<object>]",
+	"git notes [--ref <notes_ref>] merge [-v | -q] [-s <strategy> ] <notes_ref>",
+	"git notes merge --commit [-v | -q]",
+	"git notes merge --abort [-v | -q]",
 	"git notes [--ref <notes_ref>] remove [<object>]",
 	"git notes [--ref <notes_ref>] prune [-n | -v]",
+	"git notes [--ref <notes_ref>] get-ref",
 	NULL
 };
 
@@ -61,6 +66,13 @@ static const char * const git_notes_show_usage[] = {
 	NULL
 };
 
+static const char * const git_notes_merge_usage[] = {
+	"git notes merge [<options>] <notes_ref>",
+	"git notes merge --commit [<options>]",
+	"git notes merge --abort [<options>]",
+	NULL
+};
+
 static const char * const git_notes_remove_usage[] = {
 	"git notes remove [<object>]",
 	NULL
@@ -71,6 +83,11 @@ static const char * const git_notes_prune_usage[] = {
 	NULL
 };
 
+static const char * const git_notes_get_ref_usage[] = {
+	"git notes get-ref",
+	NULL
+};
+
 static const char note_template[] =
 	"\n"
 	"#\n"
@@ -83,6 +100,16 @@ struct msg_arg {
 	struct strbuf buf;
 };
 
+static void expand_notes_ref(struct strbuf *sb)
+{
+	if (!prefixcmp(sb->buf, "refs/notes/"))
+		return; /* we're happy */
+	else if (!prefixcmp(sb->buf, "notes/"))
+		strbuf_insert(sb, 0, "refs/", 5);
+	else
+		strbuf_insert(sb, 0, "refs/notes/", 11);
+}
+
 static int list_each_note(const unsigned char *object_sha1,
 		const unsigned char *note_sha1, char *note_path,
 		void *cb_data)
@@ -271,18 +298,17 @@ static int parse_reedit_arg(const struct option *opt, const char *arg, int unset
 	return parse_reuse_arg(opt, arg, unset);
 }
 
-int commit_notes(struct notes_tree *t, const char *msg)
+void commit_notes(struct notes_tree *t, const char *msg)
 {
-	struct commit_list *parent;
-	unsigned char tree_sha1[20], prev_commit[20], new_commit[20];
 	struct strbuf buf = STRBUF_INIT;
+	unsigned char commit_sha1[20];
 
 	if (!t)
 		t = &default_notes_tree;
 	if (!t->initialized || !t->ref || !*t->ref)
 		die("Cannot commit uninitialized/unreferenced notes tree");
 	if (!t->dirty)
-		return 0; /* don't have to commit an unchanged tree */
+		return; /* don't have to commit an unchanged tree */
 
 	/* Prepare commit message and reflog message */
 	strbuf_addstr(&buf, "notes: "); /* commit message starts at index 7 */
@@ -290,27 +316,10 @@ int commit_notes(struct notes_tree *t, const char *msg)
 	if (buf.buf[buf.len - 1] != '\n')
 		strbuf_addch(&buf, '\n'); /* Make sure msg ends with newline */
 
-	/* Convert notes tree to tree object */
-	if (write_notes_tree(t, tree_sha1))
-		die("Failed to write current notes tree to database");
-
-	/* Create new commit for the tree object */
-	if (!read_ref(t->ref, prev_commit)) { /* retrieve parent commit */
-		parent = xmalloc(sizeof(*parent));
-		parent->item = lookup_commit(prev_commit);
-		parent->next = NULL;
-	} else {
-		hashclr(prev_commit);
-		parent = NULL;
-	}
-	if (commit_tree(buf.buf + 7, tree_sha1, parent, new_commit, NULL))
-		die("Failed to commit notes tree to database");
-
-	/* Update notes ref with new commit */
-	update_ref(buf.buf, t->ref, new_commit, prev_commit, 0, DIE_ON_ERR);
+	create_notes_commit(t, NULL, buf.buf + 7, commit_sha1);
+	update_ref(buf.buf, t->ref, commit_sha1, NULL, 0, DIE_ON_ERR);
 
 	strbuf_release(&buf);
-	return 0;
 }
 
 combine_notes_fn parse_combine_notes_fn(const char *v)
@@ -321,6 +330,8 @@ combine_notes_fn parse_combine_notes_fn(const char *v)
 		return combine_notes_ignore;
 	else if (!strcasecmp(v, "concatenate"))
 		return combine_notes_concatenate;
+	else if (!strcasecmp(v, "cat_sort_uniq"))
+		return combine_notes_cat_sort_uniq;
 	else
 		return NULL;
 }
@@ -573,8 +584,8 @@ static int add(int argc, const char **argv, const char *prefix)
 
 	if (is_null_sha1(new_note))
 		remove_note(t, object);
-	else
-		add_note(t, object, new_note, combine_notes_overwrite);
+	else if (add_note(t, object, new_note, combine_notes_overwrite))
+		die("BUG: combine_notes_overwrite failed");
 
 	snprintf(logmsg, sizeof(logmsg), "Notes %s by 'git notes %s'",
 		 is_null_sha1(new_note) ? "removed" : "added", "add");
@@ -653,7 +664,8 @@ static int copy(int argc, const char **argv, const char *prefix)
 		goto out;
 	}
 
-	add_note(t, object, from_note, combine_notes_overwrite);
+	if (add_note(t, object, from_note, combine_notes_overwrite))
+		die("BUG: combine_notes_overwrite failed");
 	commit_notes(t, "Notes added by 'git notes copy'");
 out:
 	free_notes(t);
@@ -712,8 +724,8 @@ static int append_edit(int argc, const char **argv, const char *prefix)
 
 	if (is_null_sha1(new_note))
 		remove_note(t, object);
-	else
-		add_note(t, object, new_note, combine_notes_overwrite);
+	else if (add_note(t, object, new_note, combine_notes_overwrite))
+		die("BUG: combine_notes_overwrite failed");
 
 	snprintf(logmsg, sizeof(logmsg), "Notes %s by 'git notes %s'",
 		 is_null_sha1(new_note) ? "removed" : "added", argv[0]);
@@ -761,6 +773,180 @@ static int show(int argc, const char **argv, const char *prefix)
 	return retval;
 }
 
+static int merge_abort(struct notes_merge_options *o)
+{
+	int ret = 0;
+
+	/*
+	 * Remove .git/NOTES_MERGE_PARTIAL and .git/NOTES_MERGE_REF, and call
+	 * notes_merge_abort() to remove .git/NOTES_MERGE_WORKTREE.
+	 */
+
+	if (delete_ref("NOTES_MERGE_PARTIAL", NULL, 0))
+		ret += error("Failed to delete ref NOTES_MERGE_PARTIAL");
+	if (delete_ref("NOTES_MERGE_REF", NULL, REF_NODEREF))
+		ret += error("Failed to delete ref NOTES_MERGE_REF");
+	if (notes_merge_abort(o))
+		ret += error("Failed to remove 'git notes merge' worktree");
+	return ret;
+}
+
+static int merge_commit(struct notes_merge_options *o)
+{
+	struct strbuf msg = STRBUF_INIT;
+	unsigned char sha1[20], parent_sha1[20];
+	struct notes_tree *t;
+	struct commit *partial;
+	struct pretty_print_context pretty_ctx;
+
+	/*
+	 * Read partial merge result from .git/NOTES_MERGE_PARTIAL,
+	 * and target notes ref from .git/NOTES_MERGE_REF.
+	 */
+
+	if (get_sha1("NOTES_MERGE_PARTIAL", sha1))
+		die("Failed to read ref NOTES_MERGE_PARTIAL");
+	else if (!(partial = lookup_commit_reference(sha1)))
+		die("Could not find commit from NOTES_MERGE_PARTIAL.");
+	else if (parse_commit(partial))
+		die("Could not parse commit from NOTES_MERGE_PARTIAL.");
+
+	if (partial->parents)
+		hashcpy(parent_sha1, partial->parents->item->object.sha1);
+	else
+		hashclr(parent_sha1);
+
+	t = xcalloc(1, sizeof(struct notes_tree));
+	init_notes(t, "NOTES_MERGE_PARTIAL", combine_notes_overwrite, 0);
+
+	o->local_ref = resolve_ref("NOTES_MERGE_REF", sha1, 0, 0);
+	if (!o->local_ref)
+		die("Failed to resolve NOTES_MERGE_REF");
+
+	if (notes_merge_commit(o, t, partial, sha1))
+		die("Failed to finalize notes merge");
+
+	/* Reuse existing commit message in reflog message */
+	memset(&pretty_ctx, 0, sizeof(pretty_ctx));
+	format_commit_message(partial, "%s", &msg, &pretty_ctx);
+	strbuf_trim(&msg);
+	strbuf_insert(&msg, 0, "notes: ", 7);
+	update_ref(msg.buf, o->local_ref, sha1,
+		   is_null_sha1(parent_sha1) ? NULL : parent_sha1,
+		   0, DIE_ON_ERR);
+
+	free_notes(t);
+	strbuf_release(&msg);
+	return merge_abort(o);
+}
+
+static int merge(int argc, const char **argv, const char *prefix)
+{
+	struct strbuf remote_ref = STRBUF_INIT, msg = STRBUF_INIT;
+	unsigned char result_sha1[20];
+	struct notes_tree *t;
+	struct notes_merge_options o;
+	int do_merge = 0, do_commit = 0, do_abort = 0;
+	int verbosity = 0, result;
+	const char *strategy = NULL;
+	struct option options[] = {
+		OPT_GROUP("General options"),
+		OPT__VERBOSITY(&verbosity),
+		OPT_GROUP("Merge options"),
+		OPT_STRING('s', "strategy", &strategy, "strategy",
+			   "resolve notes conflicts using the given strategy "
+			   "(manual/ours/theirs/union/cat_sort_uniq)"),
+		OPT_GROUP("Committing unmerged notes"),
+		{ OPTION_BOOLEAN, 0, "commit", &do_commit, NULL,
+			"finalize notes merge by committing unmerged notes",
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG },
+		OPT_GROUP("Aborting notes merge resolution"),
+		{ OPTION_BOOLEAN, 0, "abort", &do_abort, NULL,
+			"abort notes merge",
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG },
+		OPT_END()
+	};
+
+	argc = parse_options(argc, argv, prefix, options,
+			     git_notes_merge_usage, 0);
+
+	if (strategy || do_commit + do_abort == 0)
+		do_merge = 1;
+	if (do_merge + do_commit + do_abort != 1) {
+		error("cannot mix --commit, --abort or -s/--strategy");
+		usage_with_options(git_notes_merge_usage, options);
+	}
+
+	if (do_merge && argc != 1) {
+		error("Must specify a notes ref to merge");
+		usage_with_options(git_notes_merge_usage, options);
+	} else if (!do_merge && argc) {
+		error("too many parameters");
+		usage_with_options(git_notes_merge_usage, options);
+	}
+
+	init_notes_merge_options(&o);
+	o.verbosity = verbosity + NOTES_MERGE_VERBOSITY_DEFAULT;
+
+	if (do_abort)
+		return merge_abort(&o);
+	if (do_commit)
+		return merge_commit(&o);
+
+	o.local_ref = default_notes_ref();
+	strbuf_addstr(&remote_ref, argv[0]);
+	expand_notes_ref(&remote_ref);
+	o.remote_ref = remote_ref.buf;
+
+	if (strategy) {
+		if (!strcmp(strategy, "manual"))
+			o.strategy = NOTES_MERGE_RESOLVE_MANUAL;
+		else if (!strcmp(strategy, "ours"))
+			o.strategy = NOTES_MERGE_RESOLVE_OURS;
+		else if (!strcmp(strategy, "theirs"))
+			o.strategy = NOTES_MERGE_RESOLVE_THEIRS;
+		else if (!strcmp(strategy, "union"))
+			o.strategy = NOTES_MERGE_RESOLVE_UNION;
+		else if (!strcmp(strategy, "cat_sort_uniq"))
+			o.strategy = NOTES_MERGE_RESOLVE_CAT_SORT_UNIQ;
+		else {
+			error("Unknown -s/--strategy: %s", strategy);
+			usage_with_options(git_notes_merge_usage, options);
+		}
+	}
+
+	t = init_notes_check("merge");
+
+	strbuf_addf(&msg, "notes: Merged notes from %s into %s",
+		    remote_ref.buf, default_notes_ref());
+	strbuf_add(&(o.commit_msg), msg.buf + 7, msg.len - 7); /* skip "notes: " */
+
+	result = notes_merge(&o, t, result_sha1);
+
+	if (result >= 0) /* Merge resulted (trivially) in result_sha1 */
+		/* Update default notes ref with new commit */
+		update_ref(msg.buf, default_notes_ref(), result_sha1, NULL,
+			   0, DIE_ON_ERR);
+	else { /* Merge has unresolved conflicts */
+		/* Update .git/NOTES_MERGE_PARTIAL with partial merge result */
+		update_ref(msg.buf, "NOTES_MERGE_PARTIAL", result_sha1, NULL,
+			   0, DIE_ON_ERR);
+		/* Store ref-to-be-updated into .git/NOTES_MERGE_REF */
+		if (create_symref("NOTES_MERGE_REF", default_notes_ref(), NULL))
+			die("Failed to store link to current notes ref (%s)",
+			    default_notes_ref());
+		printf("Automatic notes merge failed. Fix conflicts in %s and "
+		       "commit the result with 'git notes merge --commit', or "
+		       "abort the merge with 'git notes merge --abort'.\n",
+		       git_path(NOTES_MERGE_WORKTREE));
+	}
+
+	free_notes(t);
+	strbuf_release(&remote_ref);
+	strbuf_release(&msg);
+	return result < 0; /* return non-zero on conflicts */
+}
+
 static int remove_cmd(int argc, const char **argv, const char *prefix)
 {
 	struct option options[] = {
@@ -827,6 +1013,21 @@ static int prune(int argc, const char **argv, const char *prefix)
 	return 0;
 }
 
+static int get_ref(int argc, const char **argv, const char *prefix)
+{
+	struct option options[] = { OPT_END() };
+	argc = parse_options(argc, argv, prefix, options,
+			     git_notes_get_ref_usage, 0);
+
+	if (argc) {
+		error("too many parameters");
+		usage_with_options(git_notes_get_ref_usage, options);
+	}
+
+	puts(default_notes_ref());
+	return 0;
+}
+
 int cmd_notes(int argc, const char **argv, const char *prefix)
 {
 	int result;
@@ -843,13 +1044,8 @@ int cmd_notes(int argc, const char **argv, const char *prefix)
 
 	if (override_notes_ref) {
 		struct strbuf sb = STRBUF_INIT;
-		if (!prefixcmp(override_notes_ref, "refs/notes/"))
-			/* we're happy */;
-		else if (!prefixcmp(override_notes_ref, "notes/"))
-			strbuf_addstr(&sb, "refs/");
-		else
-			strbuf_addstr(&sb, "refs/notes/");
 		strbuf_addstr(&sb, override_notes_ref);
+		expand_notes_ref(&sb);
 		setenv("GIT_NOTES_REF", sb.buf, 1);
 		strbuf_release(&sb);
 	}
@@ -864,10 +1060,14 @@ int cmd_notes(int argc, const char **argv, const char *prefix)
 		result = append_edit(argc, argv, prefix);
 	else if (!strcmp(argv[0], "show"))
 		result = show(argc, argv, prefix);
+	else if (!strcmp(argv[0], "merge"))
+		result = merge(argc, argv, prefix);
 	else if (!strcmp(argv[0], "remove"))
 		result = remove_cmd(argc, argv, prefix);
 	else if (!strcmp(argv[0], "prune"))
 		result = prune(argc, argv, prefix);
+	else if (!strcmp(argv[0], "get-ref"))
+		result = get_ref(argc, argv, prefix);
 	else {
 		result = error("Unknown subcommand: %s", argv[0]);
 		usage_with_options(git_notes_usage, options);
diff --git a/notes-cache.c b/notes-cache.c
index dee6d62e72..4c8984ede1 100644
--- a/notes-cache.c
+++ b/notes-cache.c
@@ -89,6 +89,5 @@ int notes_cache_put(struct notes_cache *c, unsigned char key_sha1[20],
 
 	if (write_sha1_file(data, size, "blob", value_sha1) < 0)
 		return -1;
-	add_note(&c->tree, key_sha1, value_sha1, NULL);
-	return 0;
+	return add_note(&c->tree, key_sha1, value_sha1, NULL);
 }
diff --git a/notes-merge.c b/notes-merge.c
new file mode 100644
index 0000000000..71c4d45fcd
--- /dev/null
+++ b/notes-merge.c
@@ -0,0 +1,737 @@
+#include "cache.h"
+#include "commit.h"
+#include "refs.h"
+#include "diff.h"
+#include "diffcore.h"
+#include "xdiff-interface.h"
+#include "ll-merge.h"
+#include "dir.h"
+#include "notes.h"
+#include "notes-merge.h"
+#include "strbuf.h"
+
+struct notes_merge_pair {
+	unsigned char obj[20], base[20], local[20], remote[20];
+};
+
+void init_notes_merge_options(struct notes_merge_options *o)
+{
+	memset(o, 0, sizeof(struct notes_merge_options));
+	strbuf_init(&(o->commit_msg), 0);
+	o->verbosity = NOTES_MERGE_VERBOSITY_DEFAULT;
+}
+
+#define OUTPUT(o, v, ...) \
+	do { \
+		if ((o)->verbosity >= (v)) { \
+			printf(__VA_ARGS__); \
+			puts(""); \
+		} \
+	} while (0)
+
+static int path_to_sha1(const char *path, unsigned char *sha1)
+{
+	char hex_sha1[40];
+	int i = 0;
+	while (*path && i < 40) {
+		if (*path != '/')
+			hex_sha1[i++] = *path;
+		path++;
+	}
+	if (*path || i != 40)
+		return -1;
+	return get_sha1_hex(hex_sha1, sha1);
+}
+
+static int verify_notes_filepair(struct diff_filepair *p, unsigned char *sha1)
+{
+	switch (p->status) {
+	case DIFF_STATUS_MODIFIED:
+		assert(p->one->mode == p->two->mode);
+		assert(!is_null_sha1(p->one->sha1));
+		assert(!is_null_sha1(p->two->sha1));
+		break;
+	case DIFF_STATUS_ADDED:
+		assert(is_null_sha1(p->one->sha1));
+		break;
+	case DIFF_STATUS_DELETED:
+		assert(is_null_sha1(p->two->sha1));
+		break;
+	default:
+		return -1;
+	}
+	assert(!strcmp(p->one->path, p->two->path));
+	return path_to_sha1(p->one->path, sha1);
+}
+
+static struct notes_merge_pair *find_notes_merge_pair_pos(
+		struct notes_merge_pair *list, int len, unsigned char *obj,
+		int insert_new, int *occupied)
+{
+	/*
+	 * Both diff_tree_remote() and diff_tree_local() tend to process
+	 * merge_pairs in ascending order. Therefore, cache last returned
+	 * index, and search sequentially from there until the appropriate
+	 * position is found.
+	 *
+	 * Since inserts only happen from diff_tree_remote() (which mainly
+	 * _appends_), we don't care that inserting into the middle of the
+	 * list is expensive (using memmove()).
+	 */
+	static int last_index;
+	int i = last_index < len ? last_index : len - 1;
+	int prev_cmp = 0, cmp = -1;
+	while (i >= 0 && i < len) {
+		cmp = hashcmp(obj, list[i].obj);
+		if (!cmp) /* obj belongs @ i */
+			break;
+		else if (cmp < 0 && prev_cmp <= 0) /* obj belongs < i */
+			i--;
+		else if (cmp < 0) /* obj belongs between i-1 and i */
+			break;
+		else if (cmp > 0 && prev_cmp >= 0) /* obj belongs > i */
+			i++;
+		else /* if (cmp > 0) */ { /* obj belongs between i and i+1 */
+			i++;
+			break;
+		}
+		prev_cmp = cmp;
+	}
+	if (i < 0)
+		i = 0;
+	/* obj belongs at, or immediately preceding, index i (0 <= i <= len) */
+
+	if (!cmp)
+		*occupied = 1;
+	else {
+		*occupied = 0;
+		if (insert_new && i < len) {
+			memmove(list + i + 1, list + i,
+				(len - i) * sizeof(struct notes_merge_pair));
+			memset(list + i, 0, sizeof(struct notes_merge_pair));
+		}
+	}
+	last_index = i;
+	return list + i;
+}
+
+static unsigned char uninitialized[20] =
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" \
+	"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff";
+
+static struct notes_merge_pair *diff_tree_remote(struct notes_merge_options *o,
+						 const unsigned char *base,
+						 const unsigned char *remote,
+						 int *num_changes)
+{
+	struct diff_options opt;
+	struct notes_merge_pair *changes;
+	int i, len = 0;
+
+	trace_printf("\tdiff_tree_remote(base = %.7s, remote = %.7s)\n",
+	       sha1_to_hex(base), sha1_to_hex(remote));
+
+	diff_setup(&opt);
+	DIFF_OPT_SET(&opt, RECURSIVE);
+	opt.output_format = DIFF_FORMAT_NO_OUTPUT;
+	if (diff_setup_done(&opt) < 0)
+		die("diff_setup_done failed");
+	diff_tree_sha1(base, remote, "", &opt);
+	diffcore_std(&opt);
+
+	changes = xcalloc(diff_queued_diff.nr, sizeof(struct notes_merge_pair));
+
+	for (i = 0; i < diff_queued_diff.nr; i++) {
+		struct diff_filepair *p = diff_queued_diff.queue[i];
+		struct notes_merge_pair *mp;
+		int occupied;
+		unsigned char obj[20];
+
+		if (verify_notes_filepair(p, obj)) {
+			trace_printf("\t\tCannot merge entry '%s' (%c): "
+			       "%.7s -> %.7s. Skipping!\n", p->one->path,
+			       p->status, sha1_to_hex(p->one->sha1),
+			       sha1_to_hex(p->two->sha1));
+			continue;
+		}
+		mp = find_notes_merge_pair_pos(changes, len, obj, 1, &occupied);
+		if (occupied) {
+			/* We've found an addition/deletion pair */
+			assert(!hashcmp(mp->obj, obj));
+			if (is_null_sha1(p->one->sha1)) { /* addition */
+				assert(is_null_sha1(mp->remote));
+				hashcpy(mp->remote, p->two->sha1);
+			} else if (is_null_sha1(p->two->sha1)) { /* deletion */
+				assert(is_null_sha1(mp->base));
+				hashcpy(mp->base, p->one->sha1);
+			} else
+				assert(!"Invalid existing change recorded");
+		} else {
+			hashcpy(mp->obj, obj);
+			hashcpy(mp->base, p->one->sha1);
+			hashcpy(mp->local, uninitialized);
+			hashcpy(mp->remote, p->two->sha1);
+			len++;
+		}
+		trace_printf("\t\tStored remote change for %s: %.7s -> %.7s\n",
+		       sha1_to_hex(mp->obj), sha1_to_hex(mp->base),
+		       sha1_to_hex(mp->remote));
+	}
+	diff_flush(&opt);
+	diff_tree_release_paths(&opt);
+
+	*num_changes = len;
+	return changes;
+}
+
+static void diff_tree_local(struct notes_merge_options *o,
+			    struct notes_merge_pair *changes, int len,
+			    const unsigned char *base,
+			    const unsigned char *local)
+{
+	struct diff_options opt;
+	int i;
+
+	trace_printf("\tdiff_tree_local(len = %i, base = %.7s, local = %.7s)\n",
+	       len, sha1_to_hex(base), sha1_to_hex(local));
+
+	diff_setup(&opt);
+	DIFF_OPT_SET(&opt, RECURSIVE);
+	opt.output_format = DIFF_FORMAT_NO_OUTPUT;
+	if (diff_setup_done(&opt) < 0)
+		die("diff_setup_done failed");
+	diff_tree_sha1(base, local, "", &opt);
+	diffcore_std(&opt);
+
+	for (i = 0; i < diff_queued_diff.nr; i++) {
+		struct diff_filepair *p = diff_queued_diff.queue[i];
+		struct notes_merge_pair *mp;
+		int match;
+		unsigned char obj[20];
+
+		if (verify_notes_filepair(p, obj)) {
+			trace_printf("\t\tCannot merge entry '%s' (%c): "
+			       "%.7s -> %.7s. Skipping!\n", p->one->path,
+			       p->status, sha1_to_hex(p->one->sha1),
+			       sha1_to_hex(p->two->sha1));
+			continue;
+		}
+		mp = find_notes_merge_pair_pos(changes, len, obj, 0, &match);
+		if (!match) {
+			trace_printf("\t\tIgnoring local-only change for %s: "
+			       "%.7s -> %.7s\n", sha1_to_hex(obj),
+			       sha1_to_hex(p->one->sha1),
+			       sha1_to_hex(p->two->sha1));
+			continue;
+		}
+
+		assert(!hashcmp(mp->obj, obj));
+		if (is_null_sha1(p->two->sha1)) { /* deletion */
+			/*
+			 * Either this is a true deletion (1), or it is part
+			 * of an A/D pair (2), or D/A pair (3):
+			 *
+			 * (1) mp->local is uninitialized; set it to null_sha1
+			 * (2) mp->local is not uninitialized; don't touch it
+			 * (3) mp->local is uninitialized; set it to null_sha1
+			 *     (will be overwritten by following addition)
+			 */
+			if (!hashcmp(mp->local, uninitialized))
+				hashclr(mp->local);
+		} else if (is_null_sha1(p->one->sha1)) { /* addition */
+			/*
+			 * Either this is a true addition (1), or it is part
+			 * of an A/D pair (2), or D/A pair (3):
+			 *
+			 * (1) mp->local is uninitialized; set to p->two->sha1
+			 * (2) mp->local is uninitialized; set to p->two->sha1
+			 * (3) mp->local is null_sha1;     set to p->two->sha1
+			 */
+			assert(is_null_sha1(mp->local) ||
+			       !hashcmp(mp->local, uninitialized));
+			hashcpy(mp->local, p->two->sha1);
+		} else { /* modification */
+			/*
+			 * This is a true modification. p->one->sha1 shall
+			 * match mp->base, and mp->local shall be uninitialized.
+			 * Set mp->local to p->two->sha1.
+			 */
+			assert(!hashcmp(p->one->sha1, mp->base));
+			assert(!hashcmp(mp->local, uninitialized));
+			hashcpy(mp->local, p->two->sha1);
+		}
+		trace_printf("\t\tStored local change for %s: %.7s -> %.7s\n",
+		       sha1_to_hex(mp->obj), sha1_to_hex(mp->base),
+		       sha1_to_hex(mp->local));
+	}
+	diff_flush(&opt);
+	diff_tree_release_paths(&opt);
+}
+
+static void check_notes_merge_worktree(struct notes_merge_options *o)
+{
+	if (!o->has_worktree) {
+		/*
+		 * Must establish NOTES_MERGE_WORKTREE.
+		 * Abort if NOTES_MERGE_WORKTREE already exists
+		 */
+		if (file_exists(git_path(NOTES_MERGE_WORKTREE))) {
+			if (advice_resolve_conflict)
+				die("You have not concluded your previous "
+				    "notes merge (%s exists).\nPlease, use "
+				    "'git notes merge --commit' or 'git notes "
+				    "merge --abort' to commit/abort the "
+				    "previous merge before you start a new "
+				    "notes merge.", git_path("NOTES_MERGE_*"));
+			else
+				die("You have not concluded your notes merge "
+				    "(%s exists).", git_path("NOTES_MERGE_*"));
+		}
+
+		if (safe_create_leading_directories(git_path(
+				NOTES_MERGE_WORKTREE "/.test")))
+			die_errno("unable to create directory %s",
+				  git_path(NOTES_MERGE_WORKTREE));
+		o->has_worktree = 1;
+	} else if (!file_exists(git_path(NOTES_MERGE_WORKTREE)))
+		/* NOTES_MERGE_WORKTREE should already be established */
+		die("missing '%s'. This should not happen",
+		    git_path(NOTES_MERGE_WORKTREE));
+}
+
+static void write_buf_to_worktree(const unsigned char *obj,
+				  const char *buf, unsigned long size)
+{
+	int fd;
+	char *path = git_path(NOTES_MERGE_WORKTREE "/%s", sha1_to_hex(obj));
+	if (safe_create_leading_directories(path))
+		die_errno("unable to create directory for '%s'", path);
+	if (file_exists(path))
+		die("found existing file at '%s'", path);
+
+	fd = open(path, O_WRONLY | O_TRUNC | O_CREAT, 0666);
+	if (fd < 0)
+		die_errno("failed to open '%s'", path);
+
+	while (size > 0) {
+		long ret = write_in_full(fd, buf, size);
+		if (ret < 0) {
+			/* Ignore epipe */
+			if (errno == EPIPE)
+				break;
+			die_errno("notes-merge");
+		} else if (!ret) {
+			die("notes-merge: disk full?");
+		}
+		size -= ret;
+		buf += ret;
+	}
+
+	close(fd);
+}
+
+static void write_note_to_worktree(const unsigned char *obj,
+				   const unsigned char *note)
+{
+	enum object_type type;
+	unsigned long size;
+	void *buf = read_sha1_file(note, &type, &size);
+
+	if (!buf)
+		die("cannot read note %s for object %s",
+		    sha1_to_hex(note), sha1_to_hex(obj));
+	if (type != OBJ_BLOB)
+		die("blob expected in note %s for object %s",
+		    sha1_to_hex(note), sha1_to_hex(obj));
+	write_buf_to_worktree(obj, buf, size);
+	free(buf);
+}
+
+static int ll_merge_in_worktree(struct notes_merge_options *o,
+				struct notes_merge_pair *p)
+{
+	mmbuffer_t result_buf;
+	mmfile_t base, local, remote;
+	int status;
+
+	read_mmblob(&base, p->base);
+	read_mmblob(&local, p->local);
+	read_mmblob(&remote, p->remote);
+
+	status = ll_merge(&result_buf, sha1_to_hex(p->obj), &base, NULL,
+			  &local, o->local_ref, &remote, o->remote_ref, 0);
+
+	free(base.ptr);
+	free(local.ptr);
+	free(remote.ptr);
+
+	if ((status < 0) || !result_buf.ptr)
+		die("Failed to execute internal merge");
+
+	write_buf_to_worktree(p->obj, result_buf.ptr, result_buf.size);
+	free(result_buf.ptr);
+
+	return status;
+}
+
+static int merge_one_change_manual(struct notes_merge_options *o,
+				   struct notes_merge_pair *p,
+				   struct notes_tree *t)
+{
+	const char *lref = o->local_ref ? o->local_ref : "local version";
+	const char *rref = o->remote_ref ? o->remote_ref : "remote version";
+
+	trace_printf("\t\t\tmerge_one_change_manual(obj = %.7s, base = %.7s, "
+	       "local = %.7s, remote = %.7s)\n",
+	       sha1_to_hex(p->obj), sha1_to_hex(p->base),
+	       sha1_to_hex(p->local), sha1_to_hex(p->remote));
+
+	/* add "Conflicts:" section to commit message first time through */
+	if (!o->has_worktree)
+		strbuf_addstr(&(o->commit_msg), "\n\nConflicts:\n");
+
+	strbuf_addf(&(o->commit_msg), "\t%s\n", sha1_to_hex(p->obj));
+
+	OUTPUT(o, 2, "Auto-merging notes for %s", sha1_to_hex(p->obj));
+	check_notes_merge_worktree(o);
+	if (is_null_sha1(p->local)) {
+		/* D/F conflict, checkout p->remote */
+		assert(!is_null_sha1(p->remote));
+		OUTPUT(o, 1, "CONFLICT (delete/modify): Notes for object %s "
+		       "deleted in %s and modified in %s. Version from %s "
+		       "left in tree.", sha1_to_hex(p->obj), lref, rref, rref);
+		write_note_to_worktree(p->obj, p->remote);
+	} else if (is_null_sha1(p->remote)) {
+		/* D/F conflict, checkout p->local */
+		assert(!is_null_sha1(p->local));
+		OUTPUT(o, 1, "CONFLICT (delete/modify): Notes for object %s "
+		       "deleted in %s and modified in %s. Version from %s "
+		       "left in tree.", sha1_to_hex(p->obj), rref, lref, lref);
+		write_note_to_worktree(p->obj, p->local);
+	} else {
+		/* "regular" conflict, checkout result of ll_merge() */
+		const char *reason = "content";
+		if (is_null_sha1(p->base))
+			reason = "add/add";
+		assert(!is_null_sha1(p->local));
+		assert(!is_null_sha1(p->remote));
+		OUTPUT(o, 1, "CONFLICT (%s): Merge conflict in notes for "
+		       "object %s", reason, sha1_to_hex(p->obj));
+		ll_merge_in_worktree(o, p);
+	}
+
+	trace_printf("\t\t\tremoving from partial merge result\n");
+	remove_note(t, p->obj);
+
+	return 1;
+}
+
+static int merge_one_change(struct notes_merge_options *o,
+			    struct notes_merge_pair *p, struct notes_tree *t)
+{
+	/*
+	 * Return 0 if change is successfully resolved (stored in notes_tree).
+	 * Return 1 is change results in a conflict (NOT stored in notes_tree,
+	 * but instead written to NOTES_MERGE_WORKTREE with conflict markers).
+	 */
+	switch (o->strategy) {
+	case NOTES_MERGE_RESOLVE_MANUAL:
+		return merge_one_change_manual(o, p, t);
+	case NOTES_MERGE_RESOLVE_OURS:
+		OUTPUT(o, 2, "Using local notes for %s", sha1_to_hex(p->obj));
+		/* nothing to do */
+		return 0;
+	case NOTES_MERGE_RESOLVE_THEIRS:
+		OUTPUT(o, 2, "Using remote notes for %s", sha1_to_hex(p->obj));
+		if (add_note(t, p->obj, p->remote, combine_notes_overwrite))
+			die("BUG: combine_notes_overwrite failed");
+		return 0;
+	case NOTES_MERGE_RESOLVE_UNION:
+		OUTPUT(o, 2, "Concatenating local and remote notes for %s",
+		       sha1_to_hex(p->obj));
+		if (add_note(t, p->obj, p->remote, combine_notes_concatenate))
+			die("failed to concatenate notes "
+			    "(combine_notes_concatenate)");
+		return 0;
+	case NOTES_MERGE_RESOLVE_CAT_SORT_UNIQ:
+		OUTPUT(o, 2, "Concatenating unique lines in local and remote "
+		       "notes for %s", sha1_to_hex(p->obj));
+		if (add_note(t, p->obj, p->remote, combine_notes_cat_sort_uniq))
+			die("failed to concatenate notes "
+			    "(combine_notes_cat_sort_uniq)");
+		return 0;
+	}
+	die("Unknown strategy (%i).", o->strategy);
+}
+
+static int merge_changes(struct notes_merge_options *o,
+			 struct notes_merge_pair *changes, int *num_changes,
+			 struct notes_tree *t)
+{
+	int i, conflicts = 0;
+
+	trace_printf("\tmerge_changes(num_changes = %i)\n", *num_changes);
+	for (i = 0; i < *num_changes; i++) {
+		struct notes_merge_pair *p = changes + i;
+		trace_printf("\t\t%.7s: %.7s -> %.7s/%.7s\n",
+		       sha1_to_hex(p->obj), sha1_to_hex(p->base),
+		       sha1_to_hex(p->local), sha1_to_hex(p->remote));
+
+		if (!hashcmp(p->base, p->remote)) {
+			/* no remote change; nothing to do */
+			trace_printf("\t\t\tskipping (no remote change)\n");
+		} else if (!hashcmp(p->local, p->remote)) {
+			/* same change in local and remote; nothing to do */
+			trace_printf("\t\t\tskipping (local == remote)\n");
+		} else if (!hashcmp(p->local, uninitialized) ||
+			   !hashcmp(p->local, p->base)) {
+			/* no local change; adopt remote change */
+			trace_printf("\t\t\tno local change, adopted remote\n");
+			if (add_note(t, p->obj, p->remote,
+				     combine_notes_overwrite))
+				die("BUG: combine_notes_overwrite failed");
+		} else {
+			/* need file-level merge between local and remote */
+			trace_printf("\t\t\tneed content-level merge\n");
+			conflicts += merge_one_change(o, p, t);
+		}
+	}
+
+	return conflicts;
+}
+
+static int merge_from_diffs(struct notes_merge_options *o,
+			    const unsigned char *base,
+			    const unsigned char *local,
+			    const unsigned char *remote, struct notes_tree *t)
+{
+	struct notes_merge_pair *changes;
+	int num_changes, conflicts;
+
+	trace_printf("\tmerge_from_diffs(base = %.7s, local = %.7s, "
+	       "remote = %.7s)\n", sha1_to_hex(base), sha1_to_hex(local),
+	       sha1_to_hex(remote));
+
+	changes = diff_tree_remote(o, base, remote, &num_changes);
+	diff_tree_local(o, changes, num_changes, base, local);
+
+	conflicts = merge_changes(o, changes, &num_changes, t);
+	free(changes);
+
+	OUTPUT(o, 4, "Merge result: %i unmerged notes and a %s notes tree",
+	       conflicts, t->dirty ? "dirty" : "clean");
+
+	return conflicts ? -1 : 1;
+}
+
+void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
+			 const char *msg, unsigned char *result_sha1)
+{
+	unsigned char tree_sha1[20];
+
+	assert(t->initialized);
+
+	if (write_notes_tree(t, tree_sha1))
+		die("Failed to write notes tree to database");
+
+	if (!parents) {
+		/* Deduce parent commit from t->ref */
+		unsigned char parent_sha1[20];
+		if (!read_ref(t->ref, parent_sha1)) {
+			struct commit *parent = lookup_commit(parent_sha1);
+			if (!parent || parse_commit(parent))
+				die("Failed to find/parse commit %s", t->ref);
+			commit_list_insert(parent, &parents);
+		}
+		/* else: t->ref points to nothing, assume root/orphan commit */
+	}
+
+	if (commit_tree(msg, tree_sha1, parents, result_sha1, NULL))
+		die("Failed to commit notes tree to database");
+}
+
+int notes_merge(struct notes_merge_options *o,
+		struct notes_tree *local_tree,
+		unsigned char *result_sha1)
+{
+	unsigned char local_sha1[20], remote_sha1[20];
+	struct commit *local, *remote;
+	struct commit_list *bases = NULL;
+	const unsigned char *base_sha1, *base_tree_sha1;
+	int result = 0;
+
+	assert(o->local_ref && o->remote_ref);
+	assert(!strcmp(o->local_ref, local_tree->ref));
+	hashclr(result_sha1);
+
+	trace_printf("notes_merge(o->local_ref = %s, o->remote_ref = %s)\n",
+	       o->local_ref, o->remote_ref);
+
+	/* Dereference o->local_ref into local_sha1 */
+	if (!resolve_ref(o->local_ref, local_sha1, 0, NULL))
+		die("Failed to resolve local notes ref '%s'", o->local_ref);
+	else if (!check_ref_format(o->local_ref) && is_null_sha1(local_sha1))
+		local = NULL; /* local_sha1 == null_sha1 indicates unborn ref */
+	else if (!(local = lookup_commit_reference(local_sha1)))
+		die("Could not parse local commit %s (%s)",
+		    sha1_to_hex(local_sha1), o->local_ref);
+	trace_printf("\tlocal commit: %.7s\n", sha1_to_hex(local_sha1));
+
+	/* Dereference o->remote_ref into remote_sha1 */
+	if (get_sha1(o->remote_ref, remote_sha1)) {
+		/*
+		 * Failed to get remote_sha1. If o->remote_ref looks like an
+		 * unborn ref, perform the merge using an empty notes tree.
+		 */
+		if (!check_ref_format(o->remote_ref)) {
+			hashclr(remote_sha1);
+			remote = NULL;
+		} else {
+			die("Failed to resolve remote notes ref '%s'",
+			    o->remote_ref);
+		}
+	} else if (!(remote = lookup_commit_reference(remote_sha1))) {
+		die("Could not parse remote commit %s (%s)",
+		    sha1_to_hex(remote_sha1), o->remote_ref);
+	}
+	trace_printf("\tremote commit: %.7s\n", sha1_to_hex(remote_sha1));
+
+	if (!local && !remote)
+		die("Cannot merge empty notes ref (%s) into empty notes ref "
+		    "(%s)", o->remote_ref, o->local_ref);
+	if (!local) {
+		/* result == remote commit */
+		hashcpy(result_sha1, remote_sha1);
+		goto found_result;
+	}
+	if (!remote) {
+		/* result == local commit */
+		hashcpy(result_sha1, local_sha1);
+		goto found_result;
+	}
+	assert(local && remote);
+
+	/* Find merge bases */
+	bases = get_merge_bases(local, remote, 1);
+	if (!bases) {
+		base_sha1 = null_sha1;
+		base_tree_sha1 = (unsigned char *)EMPTY_TREE_SHA1_BIN;
+		OUTPUT(o, 4, "No merge base found; doing history-less merge");
+	} else if (!bases->next) {
+		base_sha1 = bases->item->object.sha1;
+		base_tree_sha1 = bases->item->tree->object.sha1;
+		OUTPUT(o, 4, "One merge base found (%.7s)",
+		       sha1_to_hex(base_sha1));
+	} else {
+		/* TODO: How to handle multiple merge-bases? */
+		base_sha1 = bases->item->object.sha1;
+		base_tree_sha1 = bases->item->tree->object.sha1;
+		OUTPUT(o, 3, "Multiple merge bases found. Using the first "
+		       "(%.7s)", sha1_to_hex(base_sha1));
+	}
+
+	OUTPUT(o, 4, "Merging remote commit %.7s into local commit %.7s with "
+	       "merge-base %.7s", sha1_to_hex(remote->object.sha1),
+	       sha1_to_hex(local->object.sha1), sha1_to_hex(base_sha1));
+
+	if (!hashcmp(remote->object.sha1, base_sha1)) {
+		/* Already merged; result == local commit */
+		OUTPUT(o, 2, "Already up-to-date!");
+		hashcpy(result_sha1, local->object.sha1);
+		goto found_result;
+	}
+	if (!hashcmp(local->object.sha1, base_sha1)) {
+		/* Fast-forward; result == remote commit */
+		OUTPUT(o, 2, "Fast-forward");
+		hashcpy(result_sha1, remote->object.sha1);
+		goto found_result;
+	}
+
+	result = merge_from_diffs(o, base_tree_sha1, local->tree->object.sha1,
+				  remote->tree->object.sha1, local_tree);
+
+	if (result != 0) { /* non-trivial merge (with or without conflicts) */
+		/* Commit (partial) result */
+		struct commit_list *parents = NULL;
+		commit_list_insert(remote, &parents); /* LIFO order */
+		commit_list_insert(local, &parents);
+		create_notes_commit(local_tree, parents, o->commit_msg.buf,
+				    result_sha1);
+	}
+
+found_result:
+	free_commit_list(bases);
+	strbuf_release(&(o->commit_msg));
+	trace_printf("notes_merge(): result = %i, result_sha1 = %.7s\n",
+	       result, sha1_to_hex(result_sha1));
+	return result;
+}
+
+int notes_merge_commit(struct notes_merge_options *o,
+		       struct notes_tree *partial_tree,
+		       struct commit *partial_commit,
+		       unsigned char *result_sha1)
+{
+	/*
+	 * Iterate through files in .git/NOTES_MERGE_WORKTREE and add all
+	 * found notes to 'partial_tree'. Write the updates notes tree to
+	 * the DB, and commit the resulting tree object while reusing the
+	 * commit message and parents from 'partial_commit'.
+	 * Finally store the new commit object SHA1 into 'result_sha1'.
+	 */
+	struct dir_struct dir;
+	const char *path = git_path(NOTES_MERGE_WORKTREE "/");
+	int path_len = strlen(path), i;
+	const char *msg = strstr(partial_commit->buffer, "\n\n");
+
+	OUTPUT(o, 3, "Committing notes in notes merge worktree at %.*s",
+	       path_len - 1, path);
+
+	if (!msg || msg[2] == '\0')
+		die("partial notes commit has empty message");
+	msg += 2;
+
+	memset(&dir, 0, sizeof(dir));
+	read_directory(&dir, path, path_len, NULL);
+	for (i = 0; i < dir.nr; i++) {
+		struct dir_entry *ent = dir.entries[i];
+		struct stat st;
+		const char *relpath = ent->name + path_len;
+		unsigned char obj_sha1[20], blob_sha1[20];
+
+		if (ent->len - path_len != 40 || get_sha1_hex(relpath, obj_sha1)) {
+			OUTPUT(o, 3, "Skipping non-SHA1 entry '%s'", ent->name);
+			continue;
+		}
+
+		/* write file as blob, and add to partial_tree */
+		if (stat(ent->name, &st))
+			die_errno("Failed to stat '%s'", ent->name);
+		if (index_path(blob_sha1, ent->name, &st, 1))
+			die("Failed to write blob object from '%s'", ent->name);
+		if (add_note(partial_tree, obj_sha1, blob_sha1, NULL))
+			die("Failed to add resolved note '%s' to notes tree",
+			    ent->name);
+		OUTPUT(o, 4, "Added resolved note for object %s: %s",
+		       sha1_to_hex(obj_sha1), sha1_to_hex(blob_sha1));
+	}
+
+	create_notes_commit(partial_tree, partial_commit->parents, msg,
+			    result_sha1);
+	OUTPUT(o, 4, "Finalized notes merge commit: %s",
+	       sha1_to_hex(result_sha1));
+	return 0;
+}
+
+int notes_merge_abort(struct notes_merge_options *o)
+{
+	/* Remove .git/NOTES_MERGE_WORKTREE directory and all files within */
+	struct strbuf buf = STRBUF_INIT;
+	int ret;
+
+	strbuf_addstr(&buf, git_path(NOTES_MERGE_WORKTREE));
+	OUTPUT(o, 3, "Removing notes merge worktree at %s", buf.buf);
+	ret = remove_dir_recursively(&buf, 0);
+	strbuf_release(&buf);
+	return ret;
+}
diff --git a/notes-merge.h b/notes-merge.h
new file mode 100644
index 0000000000..168a6724cd
--- /dev/null
+++ b/notes-merge.h
@@ -0,0 +1,98 @@
+#ifndef NOTES_MERGE_H
+#define NOTES_MERGE_H
+
+#define NOTES_MERGE_WORKTREE "NOTES_MERGE_WORKTREE"
+
+enum notes_merge_verbosity {
+	NOTES_MERGE_VERBOSITY_DEFAULT = 2,
+	NOTES_MERGE_VERBOSITY_MAX = 5
+};
+
+struct notes_merge_options {
+	const char *local_ref;
+	const char *remote_ref;
+	struct strbuf commit_msg;
+	int verbosity;
+	enum {
+		NOTES_MERGE_RESOLVE_MANUAL = 0,
+		NOTES_MERGE_RESOLVE_OURS,
+		NOTES_MERGE_RESOLVE_THEIRS,
+		NOTES_MERGE_RESOLVE_UNION,
+		NOTES_MERGE_RESOLVE_CAT_SORT_UNIQ
+	} strategy;
+	unsigned has_worktree:1;
+};
+
+void init_notes_merge_options(struct notes_merge_options *o);
+
+/*
+ * Create new notes commit from the given notes tree
+ *
+ * Properties of the created commit:
+ * - tree: the result of converting t to a tree object with write_notes_tree().
+ * - parents: the given parents OR (if NULL) the commit referenced by t->ref.
+ * - author/committer: the default determined by commmit_tree().
+ * - commit message: msg
+ *
+ * The resulting commit SHA1 is stored in result_sha1.
+ */
+void create_notes_commit(struct notes_tree *t, struct commit_list *parents,
+			 const char *msg, unsigned char *result_sha1);
+
+/*
+ * Merge notes from o->remote_ref into o->local_ref
+ *
+ * The given notes_tree 'local_tree' must be the notes_tree referenced by the
+ * o->local_ref. This is the notes_tree in which the object-level merge is
+ * performed.
+ *
+ * The commits given by the two refs are merged, producing one of the following
+ * outcomes:
+ *
+ * 1. The merge trivially results in an existing commit (e.g. fast-forward or
+ *    already-up-to-date). 'local_tree' is untouched, the SHA1 of the result
+ *    is written into 'result_sha1' and 0 is returned.
+ * 2. The merge successfully completes, producing a merge commit. local_tree
+ *    contains the updated notes tree, the SHA1 of the resulting commit is
+ *    written into 'result_sha1', and 1 is returned.
+ * 3. The merge results in conflicts. This is similar to #2 in that the
+ *    partial merge result (i.e. merge result minus the unmerged entries)
+ *    are stored in 'local_tree', and the SHA1 or the resulting commit
+ *    (to be amended when the conflicts have been resolved) is written into
+ *    'result_sha1'. The unmerged entries are written into the
+ *    .git/NOTES_MERGE_WORKTREE directory with conflict markers.
+ *    -1 is returned.
+ *
+ * Both o->local_ref and o->remote_ref must be given (non-NULL), but either ref
+ * (although not both) may refer to a non-existing notes ref, in which case
+ * that notes ref is interpreted as an empty notes tree, and the merge
+ * trivially results in what the other ref points to.
+ */
+int notes_merge(struct notes_merge_options *o,
+		struct notes_tree *local_tree,
+		unsigned char *result_sha1);
+
+/*
+ * Finalize conflict resolution from an earlier notes_merge()
+ *
+ * The given notes tree 'partial_tree' must be the notes_tree corresponding to
+ * the given 'partial_commit', the partial result commit created by a previous
+ * call to notes_merge().
+ *
+ * This function will add the (now resolved) notes in .git/NOTES_MERGE_WORKTREE
+ * to 'partial_tree', and create a final notes merge commit, the SHA1 of which
+ * will be stored in 'result_sha1'.
+ */
+int notes_merge_commit(struct notes_merge_options *o,
+		       struct notes_tree *partial_tree,
+		       struct commit *partial_commit,
+		       unsigned char *result_sha1);
+
+/*
+ * Abort conflict resolution from an earlier notes_merge()
+ *
+ * Removes the notes merge worktree in .git/NOTES_MERGE_WORKTREE.
+ */
+int notes_merge_abort(struct notes_merge_options *o);
+
+#endif
diff --git a/notes.c b/notes.c
index 70d00135eb..a013c1bc63 100644
--- a/notes.c
+++ b/notes.c
@@ -150,86 +150,6 @@ static struct leaf_node *note_tree_find(struct notes_tree *t,
 }
 
 /*
- * To insert a leaf_node:
- * Search to the tree location appropriate for the given leaf_node's key:
- * - If location is unused (NULL), store the tweaked pointer directly there
- * - If location holds a note entry that matches the note-to-be-inserted, then
- *   combine the two notes (by calling the given combine_notes function).
- * - If location holds a note entry that matches the subtree-to-be-inserted,
- *   then unpack the subtree-to-be-inserted into the location.
- * - If location holds a matching subtree entry, unpack the subtree at that
- *   location, and restart the insert operation from that level.
- * - Else, create a new int_node, holding both the node-at-location and the
- *   node-to-be-inserted, and store the new int_node into the location.
- */
-static void note_tree_insert(struct notes_tree *t, struct int_node *tree,
-		unsigned char n, struct leaf_node *entry, unsigned char type,
-		combine_notes_fn combine_notes)
-{
-	struct int_node *new_node;
-	struct leaf_node *l;
-	void **p = note_tree_search(t, &tree, &n, entry->key_sha1);
-
-	assert(GET_PTR_TYPE(entry) == 0); /* no type bits set */
-	l = (struct leaf_node *) CLR_PTR_TYPE(*p);
-	switch (GET_PTR_TYPE(*p)) {
-	case PTR_TYPE_NULL:
-		assert(!*p);
-		*p = SET_PTR_TYPE(entry, type);
-		return;
-	case PTR_TYPE_NOTE:
-		switch (type) {
-		case PTR_TYPE_NOTE:
-			if (!hashcmp(l->key_sha1, entry->key_sha1)) {
-				/* skip concatenation if l == entry */
-				if (!hashcmp(l->val_sha1, entry->val_sha1))
-					return;
-
-				if (combine_notes(l->val_sha1, entry->val_sha1))
-					die("failed to combine notes %s and %s"
-					    " for object %s",
-					    sha1_to_hex(l->val_sha1),
-					    sha1_to_hex(entry->val_sha1),
-					    sha1_to_hex(l->key_sha1));
-				free(entry);
-				return;
-			}
-			break;
-		case PTR_TYPE_SUBTREE:
-			if (!SUBTREE_SHA1_PREFIXCMP(l->key_sha1,
-						    entry->key_sha1)) {
-				/* unpack 'entry' */
-				load_subtree(t, entry, tree, n);
-				free(entry);
-				return;
-			}
-			break;
-		}
-		break;
-	case PTR_TYPE_SUBTREE:
-		if (!SUBTREE_SHA1_PREFIXCMP(entry->key_sha1, l->key_sha1)) {
-			/* unpack 'l' and restart insert */
-			*p = NULL;
-			load_subtree(t, l, tree, n);
-			free(l);
-			note_tree_insert(t, tree, n, entry, type,
-					 combine_notes);
-			return;
-		}
-		break;
-	}
-
-	/* non-matching leaf_node */
-	assert(GET_PTR_TYPE(*p) == PTR_TYPE_NOTE ||
-	       GET_PTR_TYPE(*p) == PTR_TYPE_SUBTREE);
-	new_node = (struct int_node *) xcalloc(sizeof(struct int_node), 1);
-	note_tree_insert(t, new_node, n + 1, l, GET_PTR_TYPE(*p),
-			 combine_notes);
-	*p = SET_PTR_TYPE(new_node, PTR_TYPE_INTERNAL);
-	note_tree_insert(t, new_node, n + 1, entry, type, combine_notes);
-}
-
-/*
  * How to consolidate an int_node:
  * If there are > 1 non-NULL entries, give up and return non-zero.
  * Otherwise replace the int_node at the given index in the given parent node
@@ -305,6 +225,93 @@ static void note_tree_remove(struct notes_tree *t,
 		i--;
 }
 
+/*
+ * To insert a leaf_node:
+ * Search to the tree location appropriate for the given leaf_node's key:
+ * - If location is unused (NULL), store the tweaked pointer directly there
+ * - If location holds a note entry that matches the note-to-be-inserted, then
+ *   combine the two notes (by calling the given combine_notes function).
+ * - If location holds a note entry that matches the subtree-to-be-inserted,
+ *   then unpack the subtree-to-be-inserted into the location.
+ * - If location holds a matching subtree entry, unpack the subtree at that
+ *   location, and restart the insert operation from that level.
+ * - Else, create a new int_node, holding both the node-at-location and the
+ *   node-to-be-inserted, and store the new int_node into the location.
+ */
+static int note_tree_insert(struct notes_tree *t, struct int_node *tree,
+		unsigned char n, struct leaf_node *entry, unsigned char type,
+		combine_notes_fn combine_notes)
+{
+	struct int_node *new_node;
+	struct leaf_node *l;
+	void **p = note_tree_search(t, &tree, &n, entry->key_sha1);
+	int ret = 0;
+
+	assert(GET_PTR_TYPE(entry) == 0); /* no type bits set */
+	l = (struct leaf_node *) CLR_PTR_TYPE(*p);
+	switch (GET_PTR_TYPE(*p)) {
+	case PTR_TYPE_NULL:
+		assert(!*p);
+		if (is_null_sha1(entry->val_sha1))
+			free(entry);
+		else
+			*p = SET_PTR_TYPE(entry, type);
+		return 0;
+	case PTR_TYPE_NOTE:
+		switch (type) {
+		case PTR_TYPE_NOTE:
+			if (!hashcmp(l->key_sha1, entry->key_sha1)) {
+				/* skip concatenation if l == entry */
+				if (!hashcmp(l->val_sha1, entry->val_sha1))
+					return 0;
+
+				ret = combine_notes(l->val_sha1,
+						    entry->val_sha1);
+				if (!ret && is_null_sha1(l->val_sha1))
+					note_tree_remove(t, tree, n, entry);
+				free(entry);
+				return ret;
+			}
+			break;
+		case PTR_TYPE_SUBTREE:
+			if (!SUBTREE_SHA1_PREFIXCMP(l->key_sha1,
+						    entry->key_sha1)) {
+				/* unpack 'entry' */
+				load_subtree(t, entry, tree, n);
+				free(entry);
+				return 0;
+			}
+			break;
+		}
+		break;
+	case PTR_TYPE_SUBTREE:
+		if (!SUBTREE_SHA1_PREFIXCMP(entry->key_sha1, l->key_sha1)) {
+			/* unpack 'l' and restart insert */
+			*p = NULL;
+			load_subtree(t, l, tree, n);
+			free(l);
+			return note_tree_insert(t, tree, n, entry, type,
+						combine_notes);
+		}
+		break;
+	}
+
+	/* non-matching leaf_node */
+	assert(GET_PTR_TYPE(*p) == PTR_TYPE_NOTE ||
+	       GET_PTR_TYPE(*p) == PTR_TYPE_SUBTREE);
+	if (is_null_sha1(entry->val_sha1)) { /* skip insertion of empty note */
+		free(entry);
+		return 0;
+	}
+	new_node = (struct int_node *) xcalloc(sizeof(struct int_node), 1);
+	ret = note_tree_insert(t, new_node, n + 1, l, GET_PTR_TYPE(*p),
+			       combine_notes);
+	if (ret)
+		return ret;
+	*p = SET_PTR_TYPE(new_node, PTR_TYPE_INTERNAL);
+	return note_tree_insert(t, new_node, n + 1, entry, type, combine_notes);
+}
+
 /* Free the entire notes data contained in the given tree */
 static void note_tree_free(struct int_node *tree)
 {
@@ -445,8 +452,12 @@ static void load_subtree(struct notes_tree *t, struct leaf_node *subtree,
 				l->key_sha1[19] = (unsigned char) len;
 				type = PTR_TYPE_SUBTREE;
 			}
-			note_tree_insert(t, node, n, l, type,
-					 combine_notes_concatenate);
+			if (note_tree_insert(t, node, n, l, type,
+					     combine_notes_concatenate))
+				die("Failed to load %s %s into notes tree "
+				    "from %s",
+				    type == PTR_TYPE_NOTE ? "note" : "subtree",
+				    sha1_to_hex(l->key_sha1), t->ref);
 		}
 		continue;
 
@@ -804,16 +815,17 @@ int combine_notes_concatenate(unsigned char *cur_sha1,
 		return 0;
 	}
 
-	/* we will separate the notes by a newline anyway */
+	/* we will separate the notes by two newlines anyway */
 	if (cur_msg[cur_len - 1] == '\n')
 		cur_len--;
 
 	/* concatenate cur_msg and new_msg into buf */
-	buf_len = cur_len + 1 + new_len;
+	buf_len = cur_len + 2 + new_len;
 	buf = (char *) xmalloc(buf_len);
 	memcpy(buf, cur_msg, cur_len);
 	buf[cur_len] = '\n';
-	memcpy(buf + cur_len + 1, new_msg, new_len);
+	buf[cur_len + 1] = '\n';
+	memcpy(buf + cur_len + 2, new_msg, new_len);
 	free(cur_msg);
 	free(new_msg);
 
@@ -836,6 +848,82 @@ int combine_notes_ignore(unsigned char *cur_sha1,
 	return 0;
 }
 
+static int string_list_add_note_lines(struct string_list *sort_uniq_list,
+				      const unsigned char *sha1)
+{
+	char *data;
+	unsigned long len;
+	enum object_type t;
+	struct strbuf buf = STRBUF_INIT;
+	struct strbuf **lines = NULL;
+	int i, list_index;
+
+	if (is_null_sha1(sha1))
+		return 0;
+
+	/* read_sha1_file NUL-terminates */
+	data = read_sha1_file(sha1, &t, &len);
+	if (t != OBJ_BLOB || !data || !len) {
+		free(data);
+		return t != OBJ_BLOB || !data;
+	}
+
+	strbuf_attach(&buf, data, len, len + 1);
+	lines = strbuf_split(&buf, '\n');
+
+	for (i = 0; lines[i]; i++) {
+		if (lines[i]->buf[lines[i]->len - 1] == '\n')
+			strbuf_setlen(lines[i], lines[i]->len - 1);
+		if (!lines[i]->len)
+			continue; /* skip empty lines */
+		list_index = string_list_find_insert_index(sort_uniq_list,
+							   lines[i]->buf, 0);
+		if (list_index < 0)
+			continue; /* skip duplicate lines */
+		string_list_insert_at_index(sort_uniq_list, list_index,
+					    lines[i]->buf);
+	}
+
+	strbuf_list_free(lines);
+	strbuf_release(&buf);
+	return 0;
+}
+
+static int string_list_join_lines_helper(struct string_list_item *item,
+					 void *cb_data)
+{
+	struct strbuf *buf = cb_data;
+	strbuf_addstr(buf, item->string);
+	strbuf_addch(buf, '\n');
+	return 0;
+}
+
+int combine_notes_cat_sort_uniq(unsigned char *cur_sha1,
+		const unsigned char *new_sha1)
+{
+	struct string_list sort_uniq_list = { NULL, 0, 0, 1 };
+	struct strbuf buf = STRBUF_INIT;
+	int ret = 1;
+
+	/* read both note blob objects into unique_lines */
+	if (string_list_add_note_lines(&sort_uniq_list, cur_sha1))
+		goto out;
+	if (string_list_add_note_lines(&sort_uniq_list, new_sha1))
+		goto out;
+
+	/* create a new blob object from sort_uniq_list */
+	if (for_each_string_list(&sort_uniq_list,
+				 string_list_join_lines_helper, &buf))
+		goto out;
+
+	ret = write_sha1_file(buf.buf, buf.len, blob_type, cur_sha1);
+
+out:
+	strbuf_release(&buf);
+	string_list_clear(&sort_uniq_list, 0);
+	return ret;
+}
+
 static int string_list_add_one_ref(const char *path, const unsigned char *sha1,
 				   int flag, void *cb)
 {
@@ -893,7 +981,7 @@ static int notes_display_config(const char *k, const char *v, void *cb)
 	return 0;
 }
 
-static const char *default_notes_ref(void)
+const char *default_notes_ref(void)
 {
 	const char *notes_ref = NULL;
 	if (!notes_ref)
@@ -935,7 +1023,7 @@ void init_notes(struct notes_tree *t, const char *notes_ref,
 		return;
 	if (get_tree_entry(object_sha1, "", sha1, &mode))
 		die("Failed to read notes tree referenced by %s (%s)",
-		    notes_ref, object_sha1);
+		    notes_ref, sha1_to_hex(object_sha1));
 
 	hashclr(root_tree.key_sha1);
 	hashcpy(root_tree.val_sha1, sha1);
@@ -989,7 +1077,7 @@ void init_display_notes(struct display_notes_opt *opt)
 	string_list_clear(&display_notes_refs, 0);
 }
 
-void add_note(struct notes_tree *t, const unsigned char *object_sha1,
+int add_note(struct notes_tree *t, const unsigned char *object_sha1,
 		const unsigned char *note_sha1, combine_notes_fn combine_notes)
 {
 	struct leaf_node *l;
@@ -1003,7 +1091,7 @@ void add_note(struct notes_tree *t, const unsigned char *object_sha1,
 	l = (struct leaf_node *) xmalloc(sizeof(struct leaf_node));
 	hashcpy(l->key_sha1, object_sha1);
 	hashcpy(l->val_sha1, note_sha1);
-	note_tree_insert(t, t->root, 0, l, PTR_TYPE_NOTE, combine_notes);
+	return note_tree_insert(t, t->root, 0, l, PTR_TYPE_NOTE, combine_notes);
 }
 
 int remove_note(struct notes_tree *t, const unsigned char *object_sha1)
@@ -1182,7 +1270,7 @@ void format_display_notes(const unsigned char *object_sha1,
 
 int copy_note(struct notes_tree *t,
 	      const unsigned char *from_obj, const unsigned char *to_obj,
-	      int force, combine_notes_fn combine_fn)
+	      int force, combine_notes_fn combine_notes)
 {
 	const unsigned char *note = get_note(t, from_obj);
 	const unsigned char *existing_note = get_note(t, to_obj);
@@ -1191,9 +1279,9 @@ int copy_note(struct notes_tree *t,
 		return 1;
 
 	if (note)
-		add_note(t, to_obj, note, combine_fn);
+		return add_note(t, to_obj, note, combine_notes);
 	else if (existing_note)
-		add_note(t, to_obj, null_sha1, combine_fn);
+		return add_note(t, to_obj, null_sha1, combine_notes);
 
 	return 0;
 }
diff --git a/notes.h b/notes.h
index 5106761534..83bd6e0ec0 100644
--- a/notes.h
+++ b/notes.h
@@ -12,7 +12,10 @@
  * resulting SHA1 into the first SHA1 argument (cur_sha1). A non-zero return
  * value indicates failure.
  *
- * The two given SHA1s must both be non-NULL and different from each other.
+ * The two given SHA1s shall both be non-NULL and different from each other.
+ * Either of them (but not both) may be == null_sha1, which indicates an
+ * empty/non-existent note. If the resulting SHA1 (cur_sha1) is == null_sha1,
+ * the note will be removed from the notes tree.
  *
  * The default combine_notes function (you get this when passing NULL) is
  * combine_notes_concatenate(), which appends the contents of the new note to
@@ -24,6 +27,7 @@ typedef int (*combine_notes_fn)(unsigned char *cur_sha1, const unsigned char *ne
 int combine_notes_concatenate(unsigned char *cur_sha1, const unsigned char *new_sha1);
 int combine_notes_overwrite(unsigned char *cur_sha1, const unsigned char *new_sha1);
 int combine_notes_ignore(unsigned char *cur_sha1, const unsigned char *new_sha1);
+int combine_notes_cat_sort_uniq(unsigned char *cur_sha1, const unsigned char *new_sha1);
 
 /*
  * Notes tree object
@@ -44,6 +48,20 @@ extern struct notes_tree {
 } default_notes_tree;
 
 /*
+ * Return the default notes ref.
+ *
+ * The default notes ref is the notes ref that is used when notes_ref == NULL
+ * is passed to init_notes().
+ *
+ * This the first of the following to be defined:
+ * 1. The '--ref' option to 'git notes', if given
+ * 2. The $GIT_NOTES_REF environment variable, if set
+ * 3. The value of the core.notesRef config variable, if set
+ * 4. GIT_NOTES_DEFAULT_REF (i.e. "refs/notes/commits")
+ */
+const char *default_notes_ref(void);
+
+/*
  * Flags controlling behaviour of notes tree initialization
  *
  * Default behaviour is to initialize the notes tree from the tree object
@@ -76,11 +94,24 @@ void init_notes(struct notes_tree *t, const char *notes_ref,
 /*
  * Add the given note object to the given notes_tree structure
  *
+ * If there already exists a note for the given object_sha1, the given
+ * combine_notes function is invoked to break the tie. If not given (i.e.
+ * combine_notes == NULL), the default combine_notes function for the given
+ * notes_tree is used.
+ *
+ * Passing note_sha1 == null_sha1 indicates the addition of an
+ * empty/non-existent note. This is a (potentially expensive) no-op unless
+ * there already exists a note for the given object_sha1, AND combining that
+ * note with the empty note (using the given combine_notes function) results
+ * in a new/changed note.
+ *
+ * Returns zero on success; non-zero means combine_notes failed.
+ *
  * IMPORTANT: The changes made by add_note() to the given notes_tree structure
  * are not persistent until a subsequent call to write_notes_tree() returns
  * zero.
  */
-void add_note(struct notes_tree *t, const unsigned char *object_sha1,
+int add_note(struct notes_tree *t, const unsigned char *object_sha1,
 		const unsigned char *note_sha1, combine_notes_fn combine_notes);
 
 /*
@@ -105,11 +136,18 @@ const unsigned char *get_note(struct notes_tree *t,
 /*
  * Copy a note from one object to another in the given notes_tree.
  *
- * Fails if the to_obj already has a note unless 'force' is true.
+ * Returns 1 if the to_obj already has a note and 'force' is false. Otherwise,
+ * returns non-zero if 'force' is true, but the given combine_notes function
+ * failed to combine from_obj's note with to_obj's existing note.
+ * Returns zero on success.
+ *
+ * IMPORTANT: The changes made by copy_note() to the given notes_tree structure
+ * are not persistent until a subsequent call to write_notes_tree() returns
+ * zero.
  */
 int copy_note(struct notes_tree *t,
 	      const unsigned char *from_obj, const unsigned char *to_obj,
-	      int force, combine_notes_fn combine_fn);
+	      int force, combine_notes_fn combine_notes);
 
 /*
  * Flags controlling behaviour of for_each_note()
@@ -151,6 +189,7 @@ int copy_note(struct notes_tree *t,
  * notes tree) from within the callback:
  * - add_note()
  * - remove_note()
+ * - copy_note()
  * - free_notes()
  */
 typedef int each_note_fn(const unsigned char *object_sha1,
diff --git a/t/t3301-notes.sh b/t/t3301-notes.sh
index 7e84ab9790..dc2e04a016 100755
--- a/t/t3301-notes.sh
+++ b/t/t3301-notes.sh
@@ -962,6 +962,7 @@ Date:   Thu Apr 7 15:27:13 2005 -0700
 
 Notes (other):
     a fresh note
+$whitespace
     another fresh note
 EOF
 
@@ -983,8 +984,11 @@ Date:   Thu Apr 7 15:27:13 2005 -0700
 
 Notes (other):
     a fresh note
+$whitespace
     another fresh note
+$whitespace
     append 1
+$whitespace
     append 2
 EOF
 
@@ -1061,4 +1065,23 @@ test_expect_success 'git notes copy diagnoses too many or too few parameters' '
 	test_must_fail git notes copy one two three
 '
 
+test_expect_success 'git notes get-ref (no overrides)' '
+	git config --unset core.notesRef &&
+	unset GIT_NOTES_REF &&
+	test "$(git notes get-ref)" = "refs/notes/commits"
+'
+
+test_expect_success 'git notes get-ref (core.notesRef)' '
+	git config core.notesRef refs/notes/foo &&
+	test "$(git notes get-ref)" = "refs/notes/foo"
+'
+
+test_expect_success 'git notes get-ref (GIT_NOTES_REF)' '
+	test "$(GIT_NOTES_REF=refs/notes/bar git notes get-ref)" = "refs/notes/bar"
+'
+
+test_expect_success 'git notes get-ref (--ref)' '
+	test "$(GIT_NOTES_REF=refs/notes/bar git notes --ref=baz get-ref)" = "refs/notes/baz"
+'
+
 test_done
diff --git a/t/t3303-notes-subtrees.sh b/t/t3303-notes-subtrees.sh
index 75ec18778e..704aee81ef 100755
--- a/t/t3303-notes-subtrees.sh
+++ b/t/t3303-notes-subtrees.sh
@@ -168,15 +168,16 @@ INPUT_END
 }
 
 verify_concatenated_notes () {
-    git log | grep "^    " > output &&
-    i=$number_of_commits &&
-    while [ $i -gt 0 ]; do
-        echo "    commit #$i" &&
-        echo "    first note for commit #$i" &&
-        echo "    second note for commit #$i" &&
-        i=$(($i-1));
-    done > expect &&
-    test_cmp expect output
+	git log | grep "^    " > output &&
+	i=$number_of_commits &&
+	while [ $i -gt 0 ]; do
+		echo "    commit #$i" &&
+		echo "    first note for commit #$i" &&
+		echo "    " &&
+		echo "    second note for commit #$i" &&
+		i=$(($i-1));
+	done > expect &&
+	test_cmp expect output
 }
 
 test_expect_success 'test notes in no fanout concatenated with 2/38-fanout' 'test_concatenated_notes "s|^..|&/|" ""'
diff --git a/t/t3308-notes-merge.sh b/t/t3308-notes-merge.sh
new file mode 100755
index 0000000000..24d82b49bb
--- /dev/null
+++ b/t/t3308-notes-merge.sh
@@ -0,0 +1,368 @@
+#!/bin/sh
+#
+# Copyright (c) 2010 Johan Herland
+#
+
+test_description='Test merging of notes trees'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	test_commit 1st &&
+	test_commit 2nd &&
+	test_commit 3rd &&
+	test_commit 4th &&
+	test_commit 5th &&
+	# Create notes on 4 first commits
+	git config core.notesRef refs/notes/x &&
+	git notes add -m "Notes on 1st commit" 1st &&
+	git notes add -m "Notes on 2nd commit" 2nd &&
+	git notes add -m "Notes on 3rd commit" 3rd &&
+	git notes add -m "Notes on 4th commit" 4th
+'
+
+commit_sha1=$(git rev-parse 1st^{commit})
+commit_sha2=$(git rev-parse 2nd^{commit})
+commit_sha3=$(git rev-parse 3rd^{commit})
+commit_sha4=$(git rev-parse 4th^{commit})
+commit_sha5=$(git rev-parse 5th^{commit})
+
+verify_notes () {
+	notes_ref="$1"
+	git -c core.notesRef="refs/notes/$notes_ref" notes |
+		sort >"output_notes_$notes_ref" &&
+	test_cmp "expect_notes_$notes_ref" "output_notes_$notes_ref" &&
+	git -c core.notesRef="refs/notes/$notes_ref" log --format="%H %s%n%N" \
+		>"output_log_$notes_ref" &&
+	test_cmp "expect_log_$notes_ref" "output_log_$notes_ref"
+}
+
+cat <<EOF | sort >expect_notes_x
+5e93d24084d32e1cb61f7070505b9d2530cca987 $commit_sha4
+8366731eeee53787d2bdf8fc1eff7d94757e8da0 $commit_sha3
+eede89064cd42441590d6afec6c37b321ada3389 $commit_sha2
+daa55ffad6cb99bf64226532147ffcaf5ce8bdd1 $commit_sha1
+EOF
+
+cat >expect_log_x <<EOF
+$commit_sha5 5th
+
+$commit_sha4 4th
+Notes on 4th commit
+
+$commit_sha3 3rd
+Notes on 3rd commit
+
+$commit_sha2 2nd
+Notes on 2nd commit
+
+$commit_sha1 1st
+Notes on 1st commit
+
+EOF
+
+test_expect_success 'verify initial notes (x)' '
+	verify_notes x
+'
+
+cp expect_notes_x expect_notes_y
+cp expect_log_x expect_log_y
+
+test_expect_success 'fail to merge empty notes ref into empty notes ref (z => y)' '
+	test_must_fail git -c "core.notesRef=refs/notes/y" notes merge z
+'
+
+test_expect_success 'fail to merge into various non-notes refs' '
+	test_must_fail git -c "core.notesRef=refs/notes" notes merge x &&
+	test_must_fail git -c "core.notesRef=refs/notes/" notes merge x &&
+	mkdir -p .git/refs/notes/dir &&
+	test_must_fail git -c "core.notesRef=refs/notes/dir" notes merge x &&
+	test_must_fail git -c "core.notesRef=refs/notes/dir/" notes merge x &&
+	test_must_fail git -c "core.notesRef=refs/heads/master" notes merge x &&
+	test_must_fail git -c "core.notesRef=refs/notes/y:" notes merge x &&
+	test_must_fail git -c "core.notesRef=refs/notes/y:foo" notes merge x &&
+	test_must_fail git -c "core.notesRef=refs/notes/foo^{bar" notes merge x
+'
+
+test_expect_success 'fail to merge various non-note-trees' '
+	git config core.notesRef refs/notes/y &&
+	test_must_fail git notes merge refs/notes &&
+	test_must_fail git notes merge refs/notes/ &&
+	test_must_fail git notes merge refs/notes/dir &&
+	test_must_fail git notes merge refs/notes/dir/ &&
+	test_must_fail git notes merge refs/heads/master &&
+	test_must_fail git notes merge x: &&
+	test_must_fail git notes merge x:foo &&
+	test_must_fail git notes merge foo^{bar
+'
+
+test_expect_success 'merge notes into empty notes ref (x => y)' '
+	git config core.notesRef refs/notes/y &&
+	git notes merge x &&
+	verify_notes y &&
+	# x and y should point to the same notes commit
+	test "$(git rev-parse refs/notes/x)" = "$(git rev-parse refs/notes/y)"
+'
+
+test_expect_success 'merge empty notes ref (z => y)' '
+	git notes merge z &&
+	# y should not change (still == x)
+	test "$(git rev-parse refs/notes/x)" = "$(git rev-parse refs/notes/y)"
+'
+
+test_expect_success 'change notes on other notes ref (y)' '
+	# Not touching notes to 1st commit
+	git notes remove 2nd &&
+	git notes append -m "More notes on 3rd commit" 3rd &&
+	git notes add -f -m "New notes on 4th commit" 4th &&
+	git notes add -m "Notes on 5th commit" 5th
+'
+
+test_expect_success 'merge previous notes commit (y^ => y) => No-op' '
+	pre_state="$(git rev-parse refs/notes/y)" &&
+	git notes merge y^ &&
+	# y should not move
+	test "$pre_state" = "$(git rev-parse refs/notes/y)"
+'
+
+cat <<EOF | sort >expect_notes_y
+0f2efbd00262f2fd41dfae33df8765618eeacd99 $commit_sha5
+dec2502dac3ea161543f71930044deff93fa945c $commit_sha4
+4069cdb399fd45463ec6eef8e051a16a03592d91 $commit_sha3
+daa55ffad6cb99bf64226532147ffcaf5ce8bdd1 $commit_sha1
+EOF
+
+cat >expect_log_y <<EOF
+$commit_sha5 5th
+Notes on 5th commit
+
+$commit_sha4 4th
+New notes on 4th commit
+
+$commit_sha3 3rd
+Notes on 3rd commit
+
+More notes on 3rd commit
+
+$commit_sha2 2nd
+
+$commit_sha1 1st
+Notes on 1st commit
+
+EOF
+
+test_expect_success 'verify changed notes on other notes ref (y)' '
+	verify_notes y
+'
+
+test_expect_success 'verify unchanged notes on original notes ref (x)' '
+	verify_notes x
+'
+
+test_expect_success 'merge original notes (x) into changed notes (y) => No-op' '
+	git notes merge -vvv x &&
+	verify_notes y &&
+	verify_notes x
+'
+
+cp expect_notes_y expect_notes_x
+cp expect_log_y expect_log_x
+
+test_expect_success 'merge changed (y) into original (x) => Fast-forward' '
+	git config core.notesRef refs/notes/x &&
+	git notes merge y &&
+	verify_notes x &&
+	verify_notes y &&
+	# x and y should point to same the notes commit
+	test "$(git rev-parse refs/notes/x)" = "$(git rev-parse refs/notes/y)"
+'
+
+test_expect_success 'merge empty notes ref (z => y)' '
+	# Prepare empty (but valid) notes ref (z)
+	git config core.notesRef refs/notes/z &&
+	git notes add -m "foo" &&
+	git notes remove &&
+	git notes >output_notes_z &&
+	test_cmp /dev/null output_notes_z &&
+	# Do the merge (z => y)
+	git config core.notesRef refs/notes/y &&
+	git notes merge z &&
+	verify_notes y &&
+	# y should no longer point to the same notes commit as x
+	test "$(git rev-parse refs/notes/x)" != "$(git rev-parse refs/notes/y)"
+'
+
+cat <<EOF | sort >expect_notes_y
+0f2efbd00262f2fd41dfae33df8765618eeacd99 $commit_sha5
+dec2502dac3ea161543f71930044deff93fa945c $commit_sha4
+4069cdb399fd45463ec6eef8e051a16a03592d91 $commit_sha3
+d000d30e6ddcfce3a8122c403226a2ce2fd04d9d $commit_sha2
+43add6bd0c8c0bc871ac7991e0f5573cfba27804 $commit_sha1
+EOF
+
+cat >expect_log_y <<EOF
+$commit_sha5 5th
+Notes on 5th commit
+
+$commit_sha4 4th
+New notes on 4th commit
+
+$commit_sha3 3rd
+Notes on 3rd commit
+
+More notes on 3rd commit
+
+$commit_sha2 2nd
+New notes on 2nd commit
+
+$commit_sha1 1st
+Notes on 1st commit
+
+More notes on 1st commit
+
+EOF
+
+test_expect_success 'change notes on other notes ref (y)' '
+	# Append to 1st commit notes
+	git notes append -m "More notes on 1st commit" 1st &&
+	# Add new notes to 2nd commit
+	git notes add -m "New notes on 2nd commit" 2nd &&
+	verify_notes y
+'
+
+cat <<EOF | sort >expect_notes_x
+0f2efbd00262f2fd41dfae33df8765618eeacd99 $commit_sha5
+1f257a3a90328557c452f0817d6cc50c89d315d4 $commit_sha4
+daa55ffad6cb99bf64226532147ffcaf5ce8bdd1 $commit_sha1
+EOF
+
+cat >expect_log_x <<EOF
+$commit_sha5 5th
+Notes on 5th commit
+
+$commit_sha4 4th
+New notes on 4th commit
+
+More notes on 4th commit
+
+$commit_sha3 3rd
+
+$commit_sha2 2nd
+
+$commit_sha1 1st
+Notes on 1st commit
+
+EOF
+
+test_expect_success 'change notes on notes ref (x)' '
+	git config core.notesRef refs/notes/x &&
+	git notes remove 3rd &&
+	git notes append -m "More notes on 4th commit" 4th &&
+	verify_notes x
+'
+
+cat <<EOF | sort >expect_notes_x
+0f2efbd00262f2fd41dfae33df8765618eeacd99 $commit_sha5
+1f257a3a90328557c452f0817d6cc50c89d315d4 $commit_sha4
+d000d30e6ddcfce3a8122c403226a2ce2fd04d9d $commit_sha2
+43add6bd0c8c0bc871ac7991e0f5573cfba27804 $commit_sha1
+EOF
+
+cat >expect_log_x <<EOF
+$commit_sha5 5th
+Notes on 5th commit
+
+$commit_sha4 4th
+New notes on 4th commit
+
+More notes on 4th commit
+
+$commit_sha3 3rd
+
+$commit_sha2 2nd
+New notes on 2nd commit
+
+$commit_sha1 1st
+Notes on 1st commit
+
+More notes on 1st commit
+
+EOF
+
+test_expect_success 'merge y into x => Non-conflicting 3-way merge' '
+	git notes merge y &&
+	verify_notes x &&
+	verify_notes y
+'
+
+cat <<EOF | sort >expect_notes_w
+05a4927951bcef347f51486575b878b2b60137f2 $commit_sha3
+d000d30e6ddcfce3a8122c403226a2ce2fd04d9d $commit_sha2
+EOF
+
+cat >expect_log_w <<EOF
+$commit_sha5 5th
+
+$commit_sha4 4th
+
+$commit_sha3 3rd
+New notes on 3rd commit
+
+$commit_sha2 2nd
+New notes on 2nd commit
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'create notes on new, separate notes ref (w)' '
+	git config core.notesRef refs/notes/w &&
+	# Add same note as refs/notes/y on 2nd commit
+	git notes add -m "New notes on 2nd commit" 2nd &&
+	# Add new note on 3rd commit (non-conflicting)
+	git notes add -m "New notes on 3rd commit" 3rd &&
+	# Verify state of notes on new, separate notes ref (w)
+	verify_notes w
+'
+
+cat <<EOF | sort >expect_notes_x
+0f2efbd00262f2fd41dfae33df8765618eeacd99 $commit_sha5
+1f257a3a90328557c452f0817d6cc50c89d315d4 $commit_sha4
+05a4927951bcef347f51486575b878b2b60137f2 $commit_sha3
+d000d30e6ddcfce3a8122c403226a2ce2fd04d9d $commit_sha2
+43add6bd0c8c0bc871ac7991e0f5573cfba27804 $commit_sha1
+EOF
+
+cat >expect_log_x <<EOF
+$commit_sha5 5th
+Notes on 5th commit
+
+$commit_sha4 4th
+New notes on 4th commit
+
+More notes on 4th commit
+
+$commit_sha3 3rd
+New notes on 3rd commit
+
+$commit_sha2 2nd
+New notes on 2nd commit
+
+$commit_sha1 1st
+Notes on 1st commit
+
+More notes on 1st commit
+
+EOF
+
+test_expect_success 'merge w into x => Non-conflicting history-less merge' '
+	git config core.notesRef refs/notes/x &&
+	git notes merge w &&
+	# Verify new state of notes on other notes ref (x)
+	verify_notes x &&
+	# Also verify that nothing changed on other notes refs (y and w)
+	verify_notes y &&
+	verify_notes w
+'
+
+test_done
diff --git a/t/t3309-notes-merge-auto-resolve.sh b/t/t3309-notes-merge-auto-resolve.sh
new file mode 100755
index 0000000000..461fd84755
--- /dev/null
+++ b/t/t3309-notes-merge-auto-resolve.sh
@@ -0,0 +1,647 @@
+#!/bin/sh
+#
+# Copyright (c) 2010 Johan Herland
+#
+
+test_description='Test notes merging with auto-resolving strategies'
+
+. ./test-lib.sh
+
+# Set up a notes merge scenario with all kinds of potential conflicts
+test_expect_success 'setup commits' '
+	test_commit 1st &&
+	test_commit 2nd &&
+	test_commit 3rd &&
+	test_commit 4th &&
+	test_commit 5th &&
+	test_commit 6th &&
+	test_commit 7th &&
+	test_commit 8th &&
+	test_commit 9th &&
+	test_commit 10th &&
+	test_commit 11th &&
+	test_commit 12th &&
+	test_commit 13th &&
+	test_commit 14th &&
+	test_commit 15th
+'
+
+commit_sha1=$(git rev-parse 1st^{commit})
+commit_sha2=$(git rev-parse 2nd^{commit})
+commit_sha3=$(git rev-parse 3rd^{commit})
+commit_sha4=$(git rev-parse 4th^{commit})
+commit_sha5=$(git rev-parse 5th^{commit})
+commit_sha6=$(git rev-parse 6th^{commit})
+commit_sha7=$(git rev-parse 7th^{commit})
+commit_sha8=$(git rev-parse 8th^{commit})
+commit_sha9=$(git rev-parse 9th^{commit})
+commit_sha10=$(git rev-parse 10th^{commit})
+commit_sha11=$(git rev-parse 11th^{commit})
+commit_sha12=$(git rev-parse 12th^{commit})
+commit_sha13=$(git rev-parse 13th^{commit})
+commit_sha14=$(git rev-parse 14th^{commit})
+commit_sha15=$(git rev-parse 15th^{commit})
+
+verify_notes () {
+	notes_ref="$1"
+	suffix="$2"
+	git -c core.notesRef="refs/notes/$notes_ref" notes |
+		sort >"output_notes_$suffix" &&
+	test_cmp "expect_notes_$suffix" "output_notes_$suffix" &&
+	git -c core.notesRef="refs/notes/$notes_ref" log --format="%H %s%n%N" \
+		>"output_log_$suffix" &&
+	test_cmp "expect_log_$suffix" "output_log_$suffix"
+}
+
+test_expect_success 'setup merge base (x)' '
+	git config core.notesRef refs/notes/x &&
+	git notes add -m "x notes on 6th commit" 6th &&
+	git notes add -m "x notes on 7th commit" 7th &&
+	git notes add -m "x notes on 8th commit" 8th &&
+	git notes add -m "x notes on 9th commit" 9th &&
+	git notes add -m "x notes on 10th commit" 10th &&
+	git notes add -m "x notes on 11th commit" 11th &&
+	git notes add -m "x notes on 12th commit" 12th &&
+	git notes add -m "x notes on 13th commit" 13th &&
+	git notes add -m "x notes on 14th commit" 14th &&
+	git notes add -m "x notes on 15th commit" 15th
+'
+
+cat <<EOF | sort >expect_notes_x
+457a85d6c814ea208550f15fcc48f804ac8dc023 $commit_sha15
+b0c95b954301d69da2bc3723f4cb1680d355937c $commit_sha14
+5d30216a129eeffa97d9694ffe8c74317a560315 $commit_sha13
+dd161bc149470fd890dd4ab52a4cbd79bbd18c36 $commit_sha12
+7abbc45126d680336fb24294f013a7cdfa3ed545 $commit_sha11
+b8d03e173f67f6505a76f6e00cf93440200dd9be $commit_sha10
+20c613c835011c48a5abe29170a2402ca6354910 $commit_sha9
+a3daf8a1e4e5dc3409a303ad8481d57bfea7f5d6 $commit_sha8
+897003322b53bc6ca098e9324ee508362347e734 $commit_sha7
+11d97fdebfa5ceee540a3da07bce6fa0222bc082 $commit_sha6
+EOF
+
+cat >expect_log_x <<EOF
+$commit_sha15 15th
+x notes on 15th commit
+
+$commit_sha14 14th
+x notes on 14th commit
+
+$commit_sha13 13th
+x notes on 13th commit
+
+$commit_sha12 12th
+x notes on 12th commit
+
+$commit_sha11 11th
+x notes on 11th commit
+
+$commit_sha10 10th
+x notes on 10th commit
+
+$commit_sha9 9th
+x notes on 9th commit
+
+$commit_sha8 8th
+x notes on 8th commit
+
+$commit_sha7 7th
+x notes on 7th commit
+
+$commit_sha6 6th
+x notes on 6th commit
+
+$commit_sha5 5th
+
+$commit_sha4 4th
+
+$commit_sha3 3rd
+
+$commit_sha2 2nd
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'verify state of merge base (x)' 'verify_notes x x'
+
+test_expect_success 'setup local branch (y)' '
+	git update-ref refs/notes/y refs/notes/x &&
+	git config core.notesRef refs/notes/y &&
+	git notes add -f -m "y notes on 3rd commit" 3rd &&
+	git notes add -f -m "y notes on 4th commit" 4th &&
+	git notes add -f -m "y notes on 5th commit" 5th &&
+	git notes remove 6th &&
+	git notes remove 7th &&
+	git notes remove 8th &&
+	git notes add -f -m "y notes on 12th commit" 12th &&
+	git notes add -f -m "y notes on 13th commit" 13th &&
+	git notes add -f -m "y notes on 14th commit" 14th &&
+	git notes add -f -m "y notes on 15th commit" 15th
+'
+
+cat <<EOF | sort >expect_notes_y
+68b8630d25516028bed862719855b3d6768d7833 $commit_sha15
+5de7ea7ad4f47e7ff91989fb82234634730f75df $commit_sha14
+3a631fdb6f41b05b55d8f4baf20728ba8f6fccbc $commit_sha13
+a66055fa82f7a03fe0c02a6aba3287a85abf7c62 $commit_sha12
+7abbc45126d680336fb24294f013a7cdfa3ed545 $commit_sha11
+b8d03e173f67f6505a76f6e00cf93440200dd9be $commit_sha10
+20c613c835011c48a5abe29170a2402ca6354910 $commit_sha9
+154508c7a0bcad82b6fe4b472bc4c26b3bf0825b $commit_sha5
+e2bfd06a37dd2031684a59a6e2b033e212239c78 $commit_sha4
+5772f42408c0dd6f097a7ca2d24de0e78d1c46b1 $commit_sha3
+EOF
+
+cat >expect_log_y <<EOF
+$commit_sha15 15th
+y notes on 15th commit
+
+$commit_sha14 14th
+y notes on 14th commit
+
+$commit_sha13 13th
+y notes on 13th commit
+
+$commit_sha12 12th
+y notes on 12th commit
+
+$commit_sha11 11th
+x notes on 11th commit
+
+$commit_sha10 10th
+x notes on 10th commit
+
+$commit_sha9 9th
+x notes on 9th commit
+
+$commit_sha8 8th
+
+$commit_sha7 7th
+
+$commit_sha6 6th
+
+$commit_sha5 5th
+y notes on 5th commit
+
+$commit_sha4 4th
+y notes on 4th commit
+
+$commit_sha3 3rd
+y notes on 3rd commit
+
+$commit_sha2 2nd
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'verify state of local branch (y)' 'verify_notes y y'
+
+test_expect_success 'setup remote branch (z)' '
+	git update-ref refs/notes/z refs/notes/x &&
+	git config core.notesRef refs/notes/z &&
+	git notes add -f -m "z notes on 2nd commit" 2nd &&
+	git notes add -f -m "y notes on 4th commit" 4th &&
+	git notes add -f -m "z notes on 5th commit" 5th &&
+	git notes remove 6th &&
+	git notes add -f -m "z notes on 8th commit" 8th &&
+	git notes remove 9th &&
+	git notes add -f -m "z notes on 11th commit" 11th &&
+	git notes remove 12th &&
+	git notes add -f -m "y notes on 14th commit" 14th &&
+	git notes add -f -m "z notes on 15th commit" 15th
+'
+
+cat <<EOF | sort >expect_notes_z
+9b4b2c61f0615412da3c10f98ff85b57c04ec765 $commit_sha15
+5de7ea7ad4f47e7ff91989fb82234634730f75df $commit_sha14
+5d30216a129eeffa97d9694ffe8c74317a560315 $commit_sha13
+7e3c53503a3db8dd996cb62e37c66e070b44b54d $commit_sha11
+b8d03e173f67f6505a76f6e00cf93440200dd9be $commit_sha10
+851e1638784a884c7dd26c5d41f3340f6387413a $commit_sha8
+897003322b53bc6ca098e9324ee508362347e734 $commit_sha7
+99fc34adfc400b95c67b013115e37e31aa9a6d23 $commit_sha5
+e2bfd06a37dd2031684a59a6e2b033e212239c78 $commit_sha4
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+EOF
+
+cat >expect_log_z <<EOF
+$commit_sha15 15th
+z notes on 15th commit
+
+$commit_sha14 14th
+y notes on 14th commit
+
+$commit_sha13 13th
+x notes on 13th commit
+
+$commit_sha12 12th
+
+$commit_sha11 11th
+z notes on 11th commit
+
+$commit_sha10 10th
+x notes on 10th commit
+
+$commit_sha9 9th
+
+$commit_sha8 8th
+z notes on 8th commit
+
+$commit_sha7 7th
+x notes on 7th commit
+
+$commit_sha6 6th
+
+$commit_sha5 5th
+z notes on 5th commit
+
+$commit_sha4 4th
+y notes on 4th commit
+
+$commit_sha3 3rd
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'verify state of remote branch (z)' 'verify_notes z z'
+
+# At this point, before merging z into y, we have the following status:
+#
+# commit | base/x  | local/y | remote/z | diff from x to y/z         | result
+# -------|---------|---------|----------|----------------------------|-------
+# 1st    | [none]  | [none]  | [none]   | unchanged / unchanged      | [none]
+# 2nd    | [none]  | [none]  | 283b482  | unchanged / added          | 283b482
+# 3rd    | [none]  | 5772f42 | [none]   | added     / unchanged      | 5772f42
+# 4th    | [none]  | e2bfd06 | e2bfd06  | added     / added (same)   | e2bfd06
+# 5th    | [none]  | 154508c | 99fc34a  | added     / added (diff)   | ???
+# 6th    | 11d97fd | [none]  | [none]   | removed   / removed        | [none]
+# 7th    | 8970033 | [none]  | 8970033  | removed   / unchanged      | [none]
+# 8th    | a3daf8a | [none]  | 851e163  | removed   / changed        | ???
+# 9th    | 20c613c | 20c613c | [none]   | unchanged / removed        | [none]
+# 10th   | b8d03e1 | b8d03e1 | b8d03e1  | unchanged / unchanged      | b8d03e1
+# 11th   | 7abbc45 | 7abbc45 | 7e3c535  | unchanged / changed        | 7e3c535
+# 12th   | dd161bc | a66055f | [none]   | changed   / removed        | ???
+# 13th   | 5d30216 | 3a631fd | 5d30216  | changed   / unchanged      | 3a631fd
+# 14th   | b0c95b9 | 5de7ea7 | 5de7ea7  | changed   / changed (same) | 5de7ea7
+# 15th   | 457a85d | 68b8630 | 9b4b2c6  | changed   / changed (diff) | ???
+
+test_expect_success 'merge z into y with invalid strategy => Fail/No changes' '
+	git config core.notesRef refs/notes/y &&
+	test_must_fail git notes merge --strategy=foo z &&
+	# Verify no changes (y)
+	verify_notes y y
+'
+
+cat <<EOF | sort >expect_notes_ours
+68b8630d25516028bed862719855b3d6768d7833 $commit_sha15
+5de7ea7ad4f47e7ff91989fb82234634730f75df $commit_sha14
+3a631fdb6f41b05b55d8f4baf20728ba8f6fccbc $commit_sha13
+a66055fa82f7a03fe0c02a6aba3287a85abf7c62 $commit_sha12
+7e3c53503a3db8dd996cb62e37c66e070b44b54d $commit_sha11
+b8d03e173f67f6505a76f6e00cf93440200dd9be $commit_sha10
+154508c7a0bcad82b6fe4b472bc4c26b3bf0825b $commit_sha5
+e2bfd06a37dd2031684a59a6e2b033e212239c78 $commit_sha4
+5772f42408c0dd6f097a7ca2d24de0e78d1c46b1 $commit_sha3
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+EOF
+
+cat >expect_log_ours <<EOF
+$commit_sha15 15th
+y notes on 15th commit
+
+$commit_sha14 14th
+y notes on 14th commit
+
+$commit_sha13 13th
+y notes on 13th commit
+
+$commit_sha12 12th
+y notes on 12th commit
+
+$commit_sha11 11th
+z notes on 11th commit
+
+$commit_sha10 10th
+x notes on 10th commit
+
+$commit_sha9 9th
+
+$commit_sha8 8th
+
+$commit_sha7 7th
+
+$commit_sha6 6th
+
+$commit_sha5 5th
+y notes on 5th commit
+
+$commit_sha4 4th
+y notes on 4th commit
+
+$commit_sha3 3rd
+y notes on 3rd commit
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'merge z into y with "ours" strategy => Non-conflicting 3-way merge' '
+	git notes merge --strategy=ours z &&
+	verify_notes y ours
+'
+
+test_expect_success 'reset to pre-merge state (y)' '
+	git update-ref refs/notes/y refs/notes/y^1 &&
+	# Verify pre-merge state
+	verify_notes y y
+'
+
+cat <<EOF | sort >expect_notes_theirs
+9b4b2c61f0615412da3c10f98ff85b57c04ec765 $commit_sha15
+5de7ea7ad4f47e7ff91989fb82234634730f75df $commit_sha14
+3a631fdb6f41b05b55d8f4baf20728ba8f6fccbc $commit_sha13
+7e3c53503a3db8dd996cb62e37c66e070b44b54d $commit_sha11
+b8d03e173f67f6505a76f6e00cf93440200dd9be $commit_sha10
+851e1638784a884c7dd26c5d41f3340f6387413a $commit_sha8
+99fc34adfc400b95c67b013115e37e31aa9a6d23 $commit_sha5
+e2bfd06a37dd2031684a59a6e2b033e212239c78 $commit_sha4
+5772f42408c0dd6f097a7ca2d24de0e78d1c46b1 $commit_sha3
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+EOF
+
+cat >expect_log_theirs <<EOF
+$commit_sha15 15th
+z notes on 15th commit
+
+$commit_sha14 14th
+y notes on 14th commit
+
+$commit_sha13 13th
+y notes on 13th commit
+
+$commit_sha12 12th
+
+$commit_sha11 11th
+z notes on 11th commit
+
+$commit_sha10 10th
+x notes on 10th commit
+
+$commit_sha9 9th
+
+$commit_sha8 8th
+z notes on 8th commit
+
+$commit_sha7 7th
+
+$commit_sha6 6th
+
+$commit_sha5 5th
+z notes on 5th commit
+
+$commit_sha4 4th
+y notes on 4th commit
+
+$commit_sha3 3rd
+y notes on 3rd commit
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'merge z into y with "theirs" strategy => Non-conflicting 3-way merge' '
+	git notes merge --strategy=theirs z &&
+	verify_notes y theirs
+'
+
+test_expect_success 'reset to pre-merge state (y)' '
+	git update-ref refs/notes/y refs/notes/y^1 &&
+	# Verify pre-merge state
+	verify_notes y y
+'
+
+cat <<EOF | sort >expect_notes_union
+7c4e546efd0fe939f876beb262ece02797880b54 $commit_sha15
+5de7ea7ad4f47e7ff91989fb82234634730f75df $commit_sha14
+3a631fdb6f41b05b55d8f4baf20728ba8f6fccbc $commit_sha13
+a66055fa82f7a03fe0c02a6aba3287a85abf7c62 $commit_sha12
+7e3c53503a3db8dd996cb62e37c66e070b44b54d $commit_sha11
+b8d03e173f67f6505a76f6e00cf93440200dd9be $commit_sha10
+851e1638784a884c7dd26c5d41f3340f6387413a $commit_sha8
+6c841cc36ea496027290967ca96bd2bef54dbb47 $commit_sha5
+e2bfd06a37dd2031684a59a6e2b033e212239c78 $commit_sha4
+5772f42408c0dd6f097a7ca2d24de0e78d1c46b1 $commit_sha3
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+EOF
+
+cat >expect_log_union <<EOF
+$commit_sha15 15th
+y notes on 15th commit
+
+z notes on 15th commit
+
+$commit_sha14 14th
+y notes on 14th commit
+
+$commit_sha13 13th
+y notes on 13th commit
+
+$commit_sha12 12th
+y notes on 12th commit
+
+$commit_sha11 11th
+z notes on 11th commit
+
+$commit_sha10 10th
+x notes on 10th commit
+
+$commit_sha9 9th
+
+$commit_sha8 8th
+z notes on 8th commit
+
+$commit_sha7 7th
+
+$commit_sha6 6th
+
+$commit_sha5 5th
+y notes on 5th commit
+
+z notes on 5th commit
+
+$commit_sha4 4th
+y notes on 4th commit
+
+$commit_sha3 3rd
+y notes on 3rd commit
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'merge z into y with "union" strategy => Non-conflicting 3-way merge' '
+	git notes merge --strategy=union z &&
+	verify_notes y union
+'
+
+test_expect_success 'reset to pre-merge state (y)' '
+	git update-ref refs/notes/y refs/notes/y^1 &&
+	# Verify pre-merge state
+	verify_notes y y
+'
+
+cat <<EOF | sort >expect_notes_union2
+d682107b8bf7a7aea1e537a8d5cb6a12b60135f1 $commit_sha15
+5de7ea7ad4f47e7ff91989fb82234634730f75df $commit_sha14
+3a631fdb6f41b05b55d8f4baf20728ba8f6fccbc $commit_sha13
+a66055fa82f7a03fe0c02a6aba3287a85abf7c62 $commit_sha12
+7e3c53503a3db8dd996cb62e37c66e070b44b54d $commit_sha11
+b8d03e173f67f6505a76f6e00cf93440200dd9be $commit_sha10
+851e1638784a884c7dd26c5d41f3340f6387413a $commit_sha8
+357b6ca14c7afd59b7f8b8aaaa6b8b723771135b $commit_sha5
+e2bfd06a37dd2031684a59a6e2b033e212239c78 $commit_sha4
+5772f42408c0dd6f097a7ca2d24de0e78d1c46b1 $commit_sha3
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+EOF
+
+cat >expect_log_union2 <<EOF
+$commit_sha15 15th
+z notes on 15th commit
+
+y notes on 15th commit
+
+$commit_sha14 14th
+y notes on 14th commit
+
+$commit_sha13 13th
+y notes on 13th commit
+
+$commit_sha12 12th
+y notes on 12th commit
+
+$commit_sha11 11th
+z notes on 11th commit
+
+$commit_sha10 10th
+x notes on 10th commit
+
+$commit_sha9 9th
+
+$commit_sha8 8th
+z notes on 8th commit
+
+$commit_sha7 7th
+
+$commit_sha6 6th
+
+$commit_sha5 5th
+z notes on 5th commit
+
+y notes on 5th commit
+
+$commit_sha4 4th
+y notes on 4th commit
+
+$commit_sha3 3rd
+y notes on 3rd commit
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'merge y into z with "union" strategy => Non-conflicting 3-way merge' '
+	git config core.notesRef refs/notes/z &&
+	git notes merge --strategy=union y &&
+	verify_notes z union2
+'
+
+test_expect_success 'reset to pre-merge state (z)' '
+	git update-ref refs/notes/z refs/notes/z^1 &&
+	# Verify pre-merge state
+	verify_notes z z
+'
+
+cat <<EOF | sort >expect_notes_cat_sort_uniq
+6be90240b5f54594203e25d9f2f64b7567175aee $commit_sha15
+5de7ea7ad4f47e7ff91989fb82234634730f75df $commit_sha14
+3a631fdb6f41b05b55d8f4baf20728ba8f6fccbc $commit_sha13
+a66055fa82f7a03fe0c02a6aba3287a85abf7c62 $commit_sha12
+7e3c53503a3db8dd996cb62e37c66e070b44b54d $commit_sha11
+b8d03e173f67f6505a76f6e00cf93440200dd9be $commit_sha10
+851e1638784a884c7dd26c5d41f3340f6387413a $commit_sha8
+660311d7f78dc53db12ac373a43fca7465381a7e $commit_sha5
+e2bfd06a37dd2031684a59a6e2b033e212239c78 $commit_sha4
+5772f42408c0dd6f097a7ca2d24de0e78d1c46b1 $commit_sha3
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+EOF
+
+cat >expect_log_cat_sort_uniq <<EOF
+$commit_sha15 15th
+y notes on 15th commit
+z notes on 15th commit
+
+$commit_sha14 14th
+y notes on 14th commit
+
+$commit_sha13 13th
+y notes on 13th commit
+
+$commit_sha12 12th
+y notes on 12th commit
+
+$commit_sha11 11th
+z notes on 11th commit
+
+$commit_sha10 10th
+x notes on 10th commit
+
+$commit_sha9 9th
+
+$commit_sha8 8th
+z notes on 8th commit
+
+$commit_sha7 7th
+
+$commit_sha6 6th
+
+$commit_sha5 5th
+y notes on 5th commit
+z notes on 5th commit
+
+$commit_sha4 4th
+y notes on 4th commit
+
+$commit_sha3 3rd
+y notes on 3rd commit
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'merge y into z with "cat_sort_uniq" strategy => Non-conflicting 3-way merge' '
+	git notes merge --strategy=cat_sort_uniq y &&
+	verify_notes z cat_sort_uniq
+'
+
+test_done
diff --git a/t/t3310-notes-merge-manual-resolve.sh b/t/t3310-notes-merge-manual-resolve.sh
new file mode 100755
index 0000000000..4ec4d11450
--- /dev/null
+++ b/t/t3310-notes-merge-manual-resolve.sh
@@ -0,0 +1,556 @@
+#!/bin/sh
+#
+# Copyright (c) 2010 Johan Herland
+#
+
+test_description='Test notes merging with manual conflict resolution'
+
+. ./test-lib.sh
+
+# Set up a notes merge scenario with different kinds of conflicts
+test_expect_success 'setup commits' '
+	test_commit 1st &&
+	test_commit 2nd &&
+	test_commit 3rd &&
+	test_commit 4th &&
+	test_commit 5th
+'
+
+commit_sha1=$(git rev-parse 1st^{commit})
+commit_sha2=$(git rev-parse 2nd^{commit})
+commit_sha3=$(git rev-parse 3rd^{commit})
+commit_sha4=$(git rev-parse 4th^{commit})
+commit_sha5=$(git rev-parse 5th^{commit})
+
+verify_notes () {
+	notes_ref="$1"
+	git -c core.notesRef="refs/notes/$notes_ref" notes |
+		sort >"output_notes_$notes_ref" &&
+	test_cmp "expect_notes_$notes_ref" "output_notes_$notes_ref" &&
+	git -c core.notesRef="refs/notes/$notes_ref" log --format="%H %s%n%N" \
+		>"output_log_$notes_ref" &&
+	test_cmp "expect_log_$notes_ref" "output_log_$notes_ref"
+}
+
+cat <<EOF | sort >expect_notes_x
+6e8e3febca3c2bb896704335cc4d0c34cb2f8715 $commit_sha4
+e5388c10860456ee60673025345fe2e153eb8cf8 $commit_sha3
+ceefa674873670e7ecd131814d909723cce2b669 $commit_sha2
+EOF
+
+cat >expect_log_x <<EOF
+$commit_sha5 5th
+
+$commit_sha4 4th
+x notes on 4th commit
+
+$commit_sha3 3rd
+x notes on 3rd commit
+
+$commit_sha2 2nd
+x notes on 2nd commit
+
+$commit_sha1 1st
+
+EOF
+
+test_expect_success 'setup merge base (x)' '
+	git config core.notesRef refs/notes/x &&
+	git notes add -m "x notes on 2nd commit" 2nd &&
+	git notes add -m "x notes on 3rd commit" 3rd &&
+	git notes add -m "x notes on 4th commit" 4th &&
+	verify_notes x
+'
+
+cat <<EOF | sort >expect_notes_y
+e2bfd06a37dd2031684a59a6e2b033e212239c78 $commit_sha4
+5772f42408c0dd6f097a7ca2d24de0e78d1c46b1 $commit_sha3
+b0a6021ec006d07e80e9b20ec9b444cbd9d560d3 $commit_sha1
+EOF
+
+cat >expect_log_y <<EOF
+$commit_sha5 5th
+
+$commit_sha4 4th
+y notes on 4th commit
+
+$commit_sha3 3rd
+y notes on 3rd commit
+
+$commit_sha2 2nd
+
+$commit_sha1 1st
+y notes on 1st commit
+
+EOF
+
+test_expect_success 'setup local branch (y)' '
+	git update-ref refs/notes/y refs/notes/x &&
+	git config core.notesRef refs/notes/y &&
+	git notes add -f -m "y notes on 1st commit" 1st &&
+	git notes remove 2nd &&
+	git notes add -f -m "y notes on 3rd commit" 3rd &&
+	git notes add -f -m "y notes on 4th commit" 4th &&
+	verify_notes y
+'
+
+cat <<EOF | sort >expect_notes_z
+cff59c793c20bb49a4e01bc06fb06bad642e0d54 $commit_sha4
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+0a81da8956346e19bcb27a906f04af327e03e31b $commit_sha1
+EOF
+
+cat >expect_log_z <<EOF
+$commit_sha5 5th
+
+$commit_sha4 4th
+z notes on 4th commit
+
+$commit_sha3 3rd
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+z notes on 1st commit
+
+EOF
+
+test_expect_success 'setup remote branch (z)' '
+	git update-ref refs/notes/z refs/notes/x &&
+	git config core.notesRef refs/notes/z &&
+	git notes add -f -m "z notes on 1st commit" 1st &&
+	git notes add -f -m "z notes on 2nd commit" 2nd &&
+	git notes remove 3rd &&
+	git notes add -f -m "z notes on 4th commit" 4th &&
+	verify_notes z
+'
+
+# At this point, before merging z into y, we have the following status:
+#
+# commit | base/x  | local/y | remote/z | diff from x to y/z
+# -------|---------|---------|----------|---------------------------
+# 1st    | [none]  | b0a6021 | 0a81da8  | added     / added (diff)
+# 2nd    | ceefa67 | [none]  | 283b482  | removed   / changed
+# 3rd    | e5388c1 | 5772f42 | [none]   | changed   / removed
+# 4th    | 6e8e3fe | e2bfd06 | cff59c7  | changed   / changed (diff)
+# 5th    | [none]  | [none]  | [none]   | [none]
+
+cat <<EOF | sort >expect_conflicts
+$commit_sha1
+$commit_sha2
+$commit_sha3
+$commit_sha4
+EOF
+
+cat >expect_conflict_$commit_sha1 <<EOF
+<<<<<<< refs/notes/m
+y notes on 1st commit
+=======
+z notes on 1st commit
+>>>>>>> refs/notes/z
+EOF
+
+cat >expect_conflict_$commit_sha2 <<EOF
+z notes on 2nd commit
+EOF
+
+cat >expect_conflict_$commit_sha3 <<EOF
+y notes on 3rd commit
+EOF
+
+cat >expect_conflict_$commit_sha4 <<EOF
+<<<<<<< refs/notes/m
+y notes on 4th commit
+=======
+z notes on 4th commit
+>>>>>>> refs/notes/z
+EOF
+
+cp expect_notes_y expect_notes_m
+cp expect_log_y expect_log_m
+
+git rev-parse refs/notes/y > pre_merge_y
+git rev-parse refs/notes/z > pre_merge_z
+
+test_expect_success 'merge z into m (== y) with default ("manual") resolver => Conflicting 3-way merge' '
+	git update-ref refs/notes/m refs/notes/y &&
+	git config core.notesRef refs/notes/m &&
+	test_must_fail git notes merge z >output &&
+	# Output should point to where to resolve conflicts
+	grep -q "\\.git/NOTES_MERGE_WORKTREE" output &&
+	# Inspect merge conflicts
+	ls .git/NOTES_MERGE_WORKTREE >output_conflicts &&
+	test_cmp expect_conflicts output_conflicts &&
+	( for f in $(cat expect_conflicts); do
+		test_cmp "expect_conflict_$f" ".git/NOTES_MERGE_WORKTREE/$f" ||
+		exit 1
+	done ) &&
+	# Verify that current notes tree (pre-merge) has not changed (m == y)
+	verify_notes y &&
+	verify_notes m &&
+	test "$(git rev-parse refs/notes/m)" = "$(cat pre_merge_y)"
+'
+
+cat <<EOF | sort >expect_notes_z
+00494adecf2d9635a02fa431308d67993f853968 $commit_sha4
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+0a81da8956346e19bcb27a906f04af327e03e31b $commit_sha1
+EOF
+
+cat >expect_log_z <<EOF
+$commit_sha5 5th
+
+$commit_sha4 4th
+z notes on 4th commit
+
+More z notes on 4th commit
+
+$commit_sha3 3rd
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+z notes on 1st commit
+
+EOF
+
+test_expect_success 'change notes in z' '
+	git notes --ref z append -m "More z notes on 4th commit" 4th &&
+	verify_notes z
+'
+
+test_expect_success 'cannot do merge w/conflicts when previous merge is unfinished' '
+	test -d .git/NOTES_MERGE_WORKTREE &&
+	test_must_fail git notes merge z >output 2>&1 &&
+	# Output should indicate what is wrong
+	grep -q "\\.git/NOTES_MERGE_\\* exists" output
+'
+
+# Setup non-conflicting merge between x and new notes ref w
+
+cat <<EOF | sort >expect_notes_w
+ceefa674873670e7ecd131814d909723cce2b669 $commit_sha2
+f75d1df88cbfe4258d49852f26cfc83f2ad4494b $commit_sha1
+EOF
+
+cat >expect_log_w <<EOF
+$commit_sha5 5th
+
+$commit_sha4 4th
+
+$commit_sha3 3rd
+
+$commit_sha2 2nd
+x notes on 2nd commit
+
+$commit_sha1 1st
+w notes on 1st commit
+
+EOF
+
+test_expect_success 'setup unrelated notes ref (w)' '
+	git config core.notesRef refs/notes/w &&
+	git notes add -m "w notes on 1st commit" 1st &&
+	git notes add -m "x notes on 2nd commit" 2nd &&
+	verify_notes w
+'
+
+cat <<EOF | sort >expect_notes_w
+6e8e3febca3c2bb896704335cc4d0c34cb2f8715 $commit_sha4
+e5388c10860456ee60673025345fe2e153eb8cf8 $commit_sha3
+ceefa674873670e7ecd131814d909723cce2b669 $commit_sha2
+f75d1df88cbfe4258d49852f26cfc83f2ad4494b $commit_sha1
+EOF
+
+cat >expect_log_w <<EOF
+$commit_sha5 5th
+
+$commit_sha4 4th
+x notes on 4th commit
+
+$commit_sha3 3rd
+x notes on 3rd commit
+
+$commit_sha2 2nd
+x notes on 2nd commit
+
+$commit_sha1 1st
+w notes on 1st commit
+
+EOF
+
+test_expect_success 'can do merge without conflicts even if previous merge is unfinished (x => w)' '
+	test -d .git/NOTES_MERGE_WORKTREE &&
+	git notes merge x &&
+	verify_notes w &&
+	# Verify that other notes refs has not changed (x and y)
+	verify_notes x &&
+	verify_notes y
+'
+
+cat <<EOF | sort >expect_notes_m
+021faa20e931fb48986ffc6282b4bb05553ac946 $commit_sha4
+5772f42408c0dd6f097a7ca2d24de0e78d1c46b1 $commit_sha3
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+0a59e787e6d688aa6309e56e8c1b89431a0fc1c1 $commit_sha1
+EOF
+
+cat >expect_log_m <<EOF
+$commit_sha5 5th
+
+$commit_sha4 4th
+y and z notes on 4th commit
+
+$commit_sha3 3rd
+y notes on 3rd commit
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+y and z notes on 1st commit
+
+EOF
+
+test_expect_success 'finalize conflicting merge (z => m)' '
+	# Resolve conflicts and finalize merge
+	cat >.git/NOTES_MERGE_WORKTREE/$commit_sha1 <<EOF &&
+y and z notes on 1st commit
+EOF
+	cat >.git/NOTES_MERGE_WORKTREE/$commit_sha4 <<EOF &&
+y and z notes on 4th commit
+EOF
+	git notes merge --commit &&
+	# No .git/NOTES_MERGE_* files left
+	test_must_fail ls .git/NOTES_MERGE_* >output 2>/dev/null &&
+	test_cmp /dev/null output &&
+	# Merge commit has pre-merge y and pre-merge z as parents
+	test "$(git rev-parse refs/notes/m^1)" = "$(cat pre_merge_y)" &&
+	test "$(git rev-parse refs/notes/m^2)" = "$(cat pre_merge_z)" &&
+	# Merge commit mentions the notes refs merged
+	git log -1 --format=%B refs/notes/m > merge_commit_msg &&
+	grep -q refs/notes/m merge_commit_msg &&
+	grep -q refs/notes/z merge_commit_msg &&
+	# Merge commit mentions conflicting notes
+	grep -q "Conflicts" merge_commit_msg &&
+	( for sha1 in $(cat expect_conflicts); do
+		grep -q "$sha1" merge_commit_msg ||
+		exit 1
+	done ) &&
+	# Verify contents of merge result
+	verify_notes m &&
+	# Verify that other notes refs has not changed (w, x, y and z)
+	verify_notes w &&
+	verify_notes x &&
+	verify_notes y &&
+	verify_notes z
+'
+
+cat >expect_conflict_$commit_sha4 <<EOF
+<<<<<<< refs/notes/m
+y notes on 4th commit
+=======
+z notes on 4th commit
+
+More z notes on 4th commit
+>>>>>>> refs/notes/z
+EOF
+
+cp expect_notes_y expect_notes_m
+cp expect_log_y expect_log_m
+
+git rev-parse refs/notes/y > pre_merge_y
+git rev-parse refs/notes/z > pre_merge_z
+
+test_expect_success 'redo merge of z into m (== y) with default ("manual") resolver => Conflicting 3-way merge' '
+	git update-ref refs/notes/m refs/notes/y &&
+	git config core.notesRef refs/notes/m &&
+	test_must_fail git notes merge z >output &&
+	# Output should point to where to resolve conflicts
+	grep -q "\\.git/NOTES_MERGE_WORKTREE" output &&
+	# Inspect merge conflicts
+	ls .git/NOTES_MERGE_WORKTREE >output_conflicts &&
+	test_cmp expect_conflicts output_conflicts &&
+	( for f in $(cat expect_conflicts); do
+		test_cmp "expect_conflict_$f" ".git/NOTES_MERGE_WORKTREE/$f" ||
+		exit 1
+	done ) &&
+	# Verify that current notes tree (pre-merge) has not changed (m == y)
+	verify_notes y &&
+	verify_notes m &&
+	test "$(git rev-parse refs/notes/m)" = "$(cat pre_merge_y)"
+'
+
+test_expect_success 'abort notes merge' '
+	git notes merge --abort &&
+	# No .git/NOTES_MERGE_* files left
+	test_must_fail ls .git/NOTES_MERGE_* >output 2>/dev/null &&
+	test_cmp /dev/null output &&
+	# m has not moved (still == y)
+	test "$(git rev-parse refs/notes/m)" = "$(cat pre_merge_y)"
+	# Verify that other notes refs has not changed (w, x, y and z)
+	verify_notes w &&
+	verify_notes x &&
+	verify_notes y &&
+	verify_notes z
+'
+
+git rev-parse refs/notes/y > pre_merge_y
+git rev-parse refs/notes/z > pre_merge_z
+
+test_expect_success 'redo merge of z into m (== y) with default ("manual") resolver => Conflicting 3-way merge' '
+	test_must_fail git notes merge z >output &&
+	# Output should point to where to resolve conflicts
+	grep -q "\\.git/NOTES_MERGE_WORKTREE" output &&
+	# Inspect merge conflicts
+	ls .git/NOTES_MERGE_WORKTREE >output_conflicts &&
+	test_cmp expect_conflicts output_conflicts &&
+	( for f in $(cat expect_conflicts); do
+		test_cmp "expect_conflict_$f" ".git/NOTES_MERGE_WORKTREE/$f" ||
+		exit 1
+	done ) &&
+	# Verify that current notes tree (pre-merge) has not changed (m == y)
+	verify_notes y &&
+	verify_notes m &&
+	test "$(git rev-parse refs/notes/m)" = "$(cat pre_merge_y)"
+'
+
+cat <<EOF | sort >expect_notes_m
+304dfb4325cf243025b9957486eb605a9b51c199 $commit_sha5
+283b48219aee9a4105f6cab337e789065c82c2b9 $commit_sha2
+0a59e787e6d688aa6309e56e8c1b89431a0fc1c1 $commit_sha1
+EOF
+
+cat >expect_log_m <<EOF
+$commit_sha5 5th
+new note on 5th commit
+
+$commit_sha4 4th
+
+$commit_sha3 3rd
+
+$commit_sha2 2nd
+z notes on 2nd commit
+
+$commit_sha1 1st
+y and z notes on 1st commit
+
+EOF
+
+test_expect_success 'add + remove notes in finalized merge (z => m)' '
+	# Resolve one conflict
+	cat >.git/NOTES_MERGE_WORKTREE/$commit_sha1 <<EOF &&
+y and z notes on 1st commit
+EOF
+	# Remove another conflict
+	rm .git/NOTES_MERGE_WORKTREE/$commit_sha4 &&
+	# Remove a D/F conflict
+	rm .git/NOTES_MERGE_WORKTREE/$commit_sha3 &&
+	# Add a new note
+	echo "new note on 5th commit" > .git/NOTES_MERGE_WORKTREE/$commit_sha5 &&
+	# Finalize merge
+	git notes merge --commit &&
+	# No .git/NOTES_MERGE_* files left
+	test_must_fail ls .git/NOTES_MERGE_* >output 2>/dev/null &&
+	test_cmp /dev/null output &&
+	# Merge commit has pre-merge y and pre-merge z as parents
+	test "$(git rev-parse refs/notes/m^1)" = "$(cat pre_merge_y)" &&
+	test "$(git rev-parse refs/notes/m^2)" = "$(cat pre_merge_z)" &&
+	# Merge commit mentions the notes refs merged
+	git log -1 --format=%B refs/notes/m > merge_commit_msg &&
+	grep -q refs/notes/m merge_commit_msg &&
+	grep -q refs/notes/z merge_commit_msg &&
+	# Merge commit mentions conflicting notes
+	grep -q "Conflicts" merge_commit_msg &&
+	( for sha1 in $(cat expect_conflicts); do
+		grep -q "$sha1" merge_commit_msg ||
+		exit 1
+	done ) &&
+	# Verify contents of merge result
+	verify_notes m &&
+	# Verify that other notes refs has not changed (w, x, y and z)
+	verify_notes w &&
+	verify_notes x &&
+	verify_notes y &&
+	verify_notes z
+'
+
+cp expect_notes_y expect_notes_m
+cp expect_log_y expect_log_m
+
+test_expect_success 'redo merge of z into m (== y) with default ("manual") resolver => Conflicting 3-way merge' '
+	git update-ref refs/notes/m refs/notes/y &&
+	test_must_fail git notes merge z >output &&
+	# Output should point to where to resolve conflicts
+	grep -q "\\.git/NOTES_MERGE_WORKTREE" output &&
+	# Inspect merge conflicts
+	ls .git/NOTES_MERGE_WORKTREE >output_conflicts &&
+	test_cmp expect_conflicts output_conflicts &&
+	( for f in $(cat expect_conflicts); do
+		test_cmp "expect_conflict_$f" ".git/NOTES_MERGE_WORKTREE/$f" ||
+		exit 1
+	done ) &&
+	# Verify that current notes tree (pre-merge) has not changed (m == y)
+	verify_notes y &&
+	verify_notes m &&
+	test "$(git rev-parse refs/notes/m)" = "$(cat pre_merge_y)"
+'
+
+cp expect_notes_w expect_notes_m
+cp expect_log_w expect_log_m
+
+test_expect_success 'reset notes ref m to somewhere else (w)' '
+	git update-ref refs/notes/m refs/notes/w &&
+	verify_notes m &&
+	test "$(git rev-parse refs/notes/m)" = "$(git rev-parse refs/notes/w)"
+'
+
+test_expect_success 'fail to finalize conflicting merge if underlying ref has moved in the meantime (m != NOTES_MERGE_PARTIAL^1)' '
+	# Resolve conflicts
+	cat >.git/NOTES_MERGE_WORKTREE/$commit_sha1 <<EOF &&
+y and z notes on 1st commit
+EOF
+	cat >.git/NOTES_MERGE_WORKTREE/$commit_sha4 <<EOF &&
+y and z notes on 4th commit
+EOF
+	# Fail to finalize merge
+	test_must_fail git notes merge --commit >output 2>&1 &&
+	# .git/NOTES_MERGE_* must remain
+	test -f .git/NOTES_MERGE_PARTIAL &&
+	test -f .git/NOTES_MERGE_REF &&
+	test -f .git/NOTES_MERGE_WORKTREE/$commit_sha1 &&
+	test -f .git/NOTES_MERGE_WORKTREE/$commit_sha2 &&
+	test -f .git/NOTES_MERGE_WORKTREE/$commit_sha3 &&
+	test -f .git/NOTES_MERGE_WORKTREE/$commit_sha4 &&
+	# Refs are unchanged
+	test "$(git rev-parse refs/notes/m)" = "$(git rev-parse refs/notes/w)"
+	test "$(git rev-parse refs/notes/y)" = "$(git rev-parse NOTES_MERGE_PARTIAL^1)"
+	test "$(git rev-parse refs/notes/m)" != "$(git rev-parse NOTES_MERGE_PARTIAL^1)"
+	# Mention refs/notes/m, and its current and expected value in output
+	grep -q "refs/notes/m" output &&
+	grep -q "$(git rev-parse refs/notes/m)" output &&
+	grep -q "$(git rev-parse NOTES_MERGE_PARTIAL^1)" output &&
+	# Verify that other notes refs has not changed (w, x, y and z)
+	verify_notes w &&
+	verify_notes x &&
+	verify_notes y &&
+	verify_notes z
+'
+
+test_expect_success 'resolve situation by aborting the notes merge' '
+	git notes merge --abort &&
+	# No .git/NOTES_MERGE_* files left
+	test_must_fail ls .git/NOTES_MERGE_* >output 2>/dev/null &&
+	test_cmp /dev/null output &&
+	# m has not moved (still == w)
+	test "$(git rev-parse refs/notes/m)" = "$(git rev-parse refs/notes/w)"
+	# Verify that other notes refs has not changed (w, x, y and z)
+	verify_notes w &&
+	verify_notes x &&
+	verify_notes y &&
+	verify_notes z
+'
+
+test_done
diff --git a/t/t3311-notes-merge-fanout.sh b/t/t3311-notes-merge-fanout.sh
new file mode 100755
index 0000000000..93516ef67c
--- /dev/null
+++ b/t/t3311-notes-merge-fanout.sh
@@ -0,0 +1,436 @@
+#!/bin/sh
+#
+# Copyright (c) 2010 Johan Herland
+#
+
+test_description='Test notes merging at various fanout levels'
+
+. ./test-lib.sh
+
+verify_notes () {
+	notes_ref="$1"
+	commit="$2"
+	if test -f "expect_notes_$notes_ref"
+	then
+		git -c core.notesRef="refs/notes/$notes_ref" notes |
+			sort >"output_notes_$notes_ref" &&
+		test_cmp "expect_notes_$notes_ref" "output_notes_$notes_ref" ||
+			return 1
+	fi &&
+	git -c core.notesRef="refs/notes/$notes_ref" log --format="%H %s%n%N" \
+		"$commit" >"output_log_$notes_ref" &&
+	test_cmp "expect_log_$notes_ref" "output_log_$notes_ref"
+}
+
+verify_fanout () {
+	notes_ref="$1"
+	# Expect entire notes tree to have a fanout == 1
+	git rev-parse --quiet --verify "refs/notes/$notes_ref" >/dev/null &&
+	git ls-tree -r --name-only "refs/notes/$notes_ref" |
+	while read path
+	do
+		case "$path" in
+		??/??????????????????????????????????????)
+			: true
+			;;
+		*)
+			echo "Invalid path \"$path\"" &&
+			return 1
+			;;
+		esac
+	done
+}
+
+verify_no_fanout () {
+	notes_ref="$1"
+	# Expect entire notes tree to have a fanout == 0
+	git rev-parse --quiet --verify "refs/notes/$notes_ref" >/dev/null &&
+	git ls-tree -r --name-only "refs/notes/$notes_ref" |
+	while read path
+	do
+		case "$path" in
+		????????????????????????????????????????)
+			: true
+			;;
+		*)
+			echo "Invalid path \"$path\"" &&
+			return 1
+			;;
+		esac
+	done
+}
+
+# Set up a notes merge scenario with different kinds of conflicts
+test_expect_success 'setup a few initial commits with notes (notes ref: x)' '
+	git config core.notesRef refs/notes/x &&
+	for i in 1 2 3 4 5
+	do
+		test_commit "commit$i" >/dev/null &&
+		git notes add -m "notes for commit$i" || return 1
+	done
+'
+
+commit_sha1=$(git rev-parse commit1^{commit})
+commit_sha2=$(git rev-parse commit2^{commit})
+commit_sha3=$(git rev-parse commit3^{commit})
+commit_sha4=$(git rev-parse commit4^{commit})
+commit_sha5=$(git rev-parse commit5^{commit})
+
+cat <<EOF | sort >expect_notes_x
+aed91155c7a72c2188e781fdf40e0f3761b299db $commit_sha5
+99fab268f9d7ee7b011e091a436c78def8eeee69 $commit_sha4
+953c20ae26c7aa0b428c20693fe38bc687f9d1a9 $commit_sha3
+6358796131b8916eaa2dde6902642942a1cb37e1 $commit_sha2
+b02d459c32f0e68f2fe0981033bb34f38776ba47 $commit_sha1
+EOF
+
+cat >expect_log_x <<EOF
+$commit_sha5 commit5
+notes for commit5
+
+$commit_sha4 commit4
+notes for commit4
+
+$commit_sha3 commit3
+notes for commit3
+
+$commit_sha2 commit2
+notes for commit2
+
+$commit_sha1 commit1
+notes for commit1
+
+EOF
+
+test_expect_success 'sanity check (x)' '
+	verify_notes x commit5 &&
+	verify_no_fanout x
+'
+
+num=300
+
+cp expect_log_x expect_log_y
+
+test_expect_success 'Add a few hundred commits w/notes to trigger fanout (x -> y)' '
+	git update-ref refs/notes/y refs/notes/x &&
+	git config core.notesRef refs/notes/y &&
+	i=5 &&
+	while test $i -lt $num
+	do
+		i=$(($i + 1)) &&
+		test_commit "commit$i" >/dev/null &&
+		git notes add -m "notes for commit$i" || return 1
+	done &&
+	test "$(git rev-parse refs/notes/y)" != "$(git rev-parse refs/notes/x)" &&
+	# Expected number of commits and notes
+	test $(git rev-list HEAD | wc -l) = $num &&
+	test $(git notes list | wc -l) = $num &&
+	# 5 first notes unchanged
+	verify_notes y commit5
+'
+
+test_expect_success 'notes tree has fanout (y)' 'verify_fanout y'
+
+test_expect_success 'No-op merge (already included) (x => y)' '
+	git update-ref refs/notes/m refs/notes/y &&
+	git config core.notesRef refs/notes/m &&
+	git notes merge x &&
+	test "$(git rev-parse refs/notes/m)" = "$(git rev-parse refs/notes/y)"
+'
+
+test_expect_success 'Fast-forward merge (y => x)' '
+	git update-ref refs/notes/m refs/notes/x &&
+	git notes merge y &&
+	test "$(git rev-parse refs/notes/m)" = "$(git rev-parse refs/notes/y)"
+'
+
+cat <<EOF | sort >expect_notes_z
+9f506ee70e20379d7f78204c77b334f43d77410d $commit_sha3
+23a47d6ea7d589895faf800752054818e1e7627b $commit_sha2
+b02d459c32f0e68f2fe0981033bb34f38776ba47 $commit_sha1
+EOF
+
+cat >expect_log_z <<EOF
+$commit_sha5 commit5
+
+$commit_sha4 commit4
+
+$commit_sha3 commit3
+notes for commit3
+
+appended notes for commit3
+
+$commit_sha2 commit2
+new notes for commit2
+
+$commit_sha1 commit1
+notes for commit1
+
+EOF
+
+test_expect_success 'change some of the initial 5 notes (x -> z)' '
+	git update-ref refs/notes/z refs/notes/x &&
+	git config core.notesRef refs/notes/z &&
+	git notes add -f -m "new notes for commit2" commit2 &&
+	git notes append -m "appended notes for commit3" commit3 &&
+	git notes remove commit4 &&
+	git notes remove commit5 &&
+	verify_notes z commit5
+'
+
+test_expect_success 'notes tree has no fanout (z)' 'verify_no_fanout z'
+
+cp expect_log_z expect_log_m
+
+test_expect_success 'successful merge without conflicts (y => z)' '
+	git update-ref refs/notes/m refs/notes/z &&
+	git config core.notesRef refs/notes/m &&
+	git notes merge y &&
+	verify_notes m commit5 &&
+	# x/y/z unchanged
+	verify_notes x commit5 &&
+	verify_notes y commit5 &&
+	verify_notes z commit5
+'
+
+test_expect_success 'notes tree still has fanout after merge (m)' 'verify_fanout m'
+
+cat >expect_log_w <<EOF
+$commit_sha5 commit5
+
+$commit_sha4 commit4
+other notes for commit4
+
+$commit_sha3 commit3
+other notes for commit3
+
+$commit_sha2 commit2
+notes for commit2
+
+$commit_sha1 commit1
+other notes for commit1
+
+EOF
+
+test_expect_success 'introduce conflicting changes (y -> w)' '
+	git update-ref refs/notes/w refs/notes/y &&
+	git config core.notesRef refs/notes/w &&
+	git notes add -f -m "other notes for commit1" commit1 &&
+	git notes add -f -m "other notes for commit3" commit3 &&
+	git notes add -f -m "other notes for commit4" commit4 &&
+	git notes remove commit5 &&
+	verify_notes w commit5
+'
+
+cat >expect_log_m <<EOF
+$commit_sha5 commit5
+
+$commit_sha4 commit4
+other notes for commit4
+
+$commit_sha3 commit3
+other notes for commit3
+
+$commit_sha2 commit2
+new notes for commit2
+
+$commit_sha1 commit1
+other notes for commit1
+
+EOF
+
+test_expect_success 'successful merge using "ours" strategy (z => w)' '
+	git update-ref refs/notes/m refs/notes/w &&
+	git config core.notesRef refs/notes/m &&
+	git notes merge -s ours z &&
+	verify_notes m commit5 &&
+	# w/x/y/z unchanged
+	verify_notes w commit5 &&
+	verify_notes x commit5 &&
+	verify_notes y commit5 &&
+	verify_notes z commit5
+'
+
+test_expect_success 'notes tree still has fanout after merge (m)' 'verify_fanout m'
+
+cat >expect_log_m <<EOF
+$commit_sha5 commit5
+
+$commit_sha4 commit4
+
+$commit_sha3 commit3
+notes for commit3
+
+appended notes for commit3
+
+$commit_sha2 commit2
+new notes for commit2
+
+$commit_sha1 commit1
+other notes for commit1
+
+EOF
+
+test_expect_success 'successful merge using "theirs" strategy (z => w)' '
+	git update-ref refs/notes/m refs/notes/w &&
+	git notes merge -s theirs z &&
+	verify_notes m commit5 &&
+	# w/x/y/z unchanged
+	verify_notes w commit5 &&
+	verify_notes x commit5 &&
+	verify_notes y commit5 &&
+	verify_notes z commit5
+'
+
+test_expect_success 'notes tree still has fanout after merge (m)' 'verify_fanout m'
+
+cat >expect_log_m <<EOF
+$commit_sha5 commit5
+
+$commit_sha4 commit4
+other notes for commit4
+
+$commit_sha3 commit3
+other notes for commit3
+
+notes for commit3
+
+appended notes for commit3
+
+$commit_sha2 commit2
+new notes for commit2
+
+$commit_sha1 commit1
+other notes for commit1
+
+EOF
+
+test_expect_success 'successful merge using "union" strategy (z => w)' '
+	git update-ref refs/notes/m refs/notes/w &&
+	git notes merge -s union z &&
+	verify_notes m commit5 &&
+	# w/x/y/z unchanged
+	verify_notes w commit5 &&
+	verify_notes x commit5 &&
+	verify_notes y commit5 &&
+	verify_notes z commit5
+'
+
+test_expect_success 'notes tree still has fanout after merge (m)' 'verify_fanout m'
+
+cat >expect_log_m <<EOF
+$commit_sha5 commit5
+
+$commit_sha4 commit4
+other notes for commit4
+
+$commit_sha3 commit3
+appended notes for commit3
+notes for commit3
+other notes for commit3
+
+$commit_sha2 commit2
+new notes for commit2
+
+$commit_sha1 commit1
+other notes for commit1
+
+EOF
+
+test_expect_success 'successful merge using "cat_sort_uniq" strategy (z => w)' '
+	git update-ref refs/notes/m refs/notes/w &&
+	git notes merge -s cat_sort_uniq z &&
+	verify_notes m commit5 &&
+	# w/x/y/z unchanged
+	verify_notes w commit5 &&
+	verify_notes x commit5 &&
+	verify_notes y commit5 &&
+	verify_notes z commit5
+'
+
+test_expect_success 'notes tree still has fanout after merge (m)' 'verify_fanout m'
+
+# We're merging z into w. Here are the conflicts we expect:
+#
+# commit | x -> w    | x -> z    | conflict?
+# -------|-----------|-----------|----------
+# 1      | changed   | unchanged | no, use w
+# 2      | unchanged | changed   | no, use z
+# 3      | changed   | changed   | yes (w, then z in conflict markers)
+# 4      | changed   | deleted   | yes (w)
+# 5      | deleted   | deleted   | no, deleted
+
+test_expect_success 'fails to merge using "manual" strategy (z => w)' '
+	git update-ref refs/notes/m refs/notes/w &&
+	test_must_fail git notes merge z
+'
+
+test_expect_success 'notes tree still has fanout after merge (m)' 'verify_fanout m'
+
+cat <<EOF | sort >expect_conflicts
+$commit_sha3
+$commit_sha4
+EOF
+
+cat >expect_conflict_$commit_sha3 <<EOF
+<<<<<<< refs/notes/m
+other notes for commit3
+=======
+notes for commit3
+
+appended notes for commit3
+>>>>>>> refs/notes/z
+EOF
+
+cat >expect_conflict_$commit_sha4 <<EOF
+other notes for commit4
+EOF
+
+test_expect_success 'verify conflict entries (with no fanout)' '
+	ls .git/NOTES_MERGE_WORKTREE >output_conflicts &&
+	test_cmp expect_conflicts output_conflicts &&
+	( for f in $(cat expect_conflicts); do
+		test_cmp "expect_conflict_$f" ".git/NOTES_MERGE_WORKTREE/$f" ||
+		exit 1
+	done ) &&
+	# Verify that current notes tree (pre-merge) has not changed (m == w)
+	test "$(git rev-parse refs/notes/m)" = "$(git rev-parse refs/notes/w)"
+'
+
+cat >expect_log_m <<EOF
+$commit_sha5 commit5
+
+$commit_sha4 commit4
+other notes for commit4
+
+$commit_sha3 commit3
+other notes for commit3
+
+appended notes for commit3
+
+$commit_sha2 commit2
+new notes for commit2
+
+$commit_sha1 commit1
+other notes for commit1
+
+EOF
+
+test_expect_success 'resolve and finalize merge (z => w)' '
+	cat >.git/NOTES_MERGE_WORKTREE/$commit_sha3 <<EOF &&
+other notes for commit3
+
+appended notes for commit3
+EOF
+	git notes merge --commit &&
+	verify_notes m commit5 &&
+	# w/x/y/z unchanged
+	verify_notes w commit5 &&
+	verify_notes x commit5 &&
+	verify_notes y commit5 &&
+	verify_notes z commit5
+'
+
+test_expect_success 'notes tree still has fanout after merge (m)' 'verify_fanout m'
+
+test_done
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index 5cb7e70d54..d3a3bd2679 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -647,6 +647,7 @@ test_expect_success 'rebase -i can copy notes' '
 
 cat >expect <<EOF
 an earlier note
+
 a note
 EOF
 
diff --git a/t/t7609-merge-abort.sh b/t/t7609-merge-abort.sh
new file mode 100755
index 0000000000..61890bc892
--- /dev/null
+++ b/t/t7609-merge-abort.sh
@@ -0,0 +1,313 @@
+#!/bin/sh
+
+test_description='test aborting in-progress merges
+
+Set up repo with conflicting and non-conflicting branches:
+
+There are three files foo/bar/baz, and the following graph illustrates the
+content of these files in each commit:
+
+# foo/bar/baz --- foo/bar/bazz     <-- master
+#             \
+#              --- foo/barf/bazf   <-- conflict_branch
+#               \
+#                --- foo/bart/baz  <-- clean_branch
+
+Next, test git merge --abort with the following variables:
+- before/after successful merge (should fail when not in merge context)
+- with/without conflicts
+- clean/dirty index before merge
+- clean/dirty worktree before merge
+- dirty index before merge matches contents on remote branch
+- changed/unchanged worktree after merge
+- changed/unchanged index after merge
+'
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	# Create the above repo
+	echo foo > foo &&
+	echo bar > bar &&
+	echo baz > baz &&
+	git add foo bar baz &&
+	git commit -m initial &&
+	echo bazz > baz &&
+	git commit -a -m "second" &&
+	git checkout -b conflict_branch HEAD^ &&
+	echo barf > bar &&
+	echo bazf > baz &&
+	git commit -a -m "conflict" &&
+	git checkout -b clean_branch HEAD^ &&
+	echo bart > bar &&
+	git commit -a -m "clean" &&
+	git checkout master
+'
+
+pre_merge_head="$(git rev-parse HEAD)"
+
+test_expect_success 'fails without MERGE_HEAD (unstarted merge)' '
+	test_must_fail git merge --abort 2>output &&
+	grep -q MERGE_HEAD output &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)"
+'
+
+test_expect_success 'fails without MERGE_HEAD (completed merge)' '
+	git merge clean_branch &&
+	test ! -f .git/MERGE_HEAD &&
+	# Merge successfully completed
+	post_merge_head="$(git rev-parse HEAD)" &&
+	test_must_fail git merge --abort 2>output &&
+	grep -q MERGE_HEAD output &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$post_merge_head" = "$(git rev-parse HEAD)"
+'
+
+test_expect_success 'Forget previous merge' '
+	git reset --hard "$pre_merge_head"
+'
+
+test_expect_success 'Abort after --no-commit' '
+	# Redo merge, but stop before creating merge commit
+	git merge --no-commit clean_branch &&
+	test -f .git/MERGE_HEAD &&
+	# Abort non-conflicting merge
+	git merge --abort &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff)" &&
+	test -z "$(git diff --staged)"
+'
+
+test_expect_success 'Abort after conflicts' '
+	# Create conflicting merge
+	test_must_fail git merge conflict_branch &&
+	test -f .git/MERGE_HEAD &&
+	# Abort conflicting merge
+	git merge --abort &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff)" &&
+	test -z "$(git diff --staged)"
+'
+
+test_expect_success 'Clean merge with dirty index fails' '
+	echo xyzzy >> foo &&
+	git add foo &&
+	git diff --staged > expect &&
+	test_must_fail git merge clean_branch &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff)" &&
+	git diff --staged > actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Conflicting merge with dirty index fails' '
+	test_must_fail git merge conflict_branch &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff)" &&
+	git diff --staged > actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Reset index (but preserve worktree changes)' '
+	git reset "$pre_merge_head" &&
+	git diff > actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Abort clean merge with non-conflicting dirty worktree' '
+	git merge --no-commit clean_branch &&
+	test -f .git/MERGE_HEAD &&
+	# Abort merge
+	git merge --abort &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff --staged)" &&
+	git diff > actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Abort conflicting merge with non-conflicting dirty worktree' '
+	test_must_fail git merge conflict_branch &&
+	test -f .git/MERGE_HEAD &&
+	# Abort merge
+	git merge --abort &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff --staged)" &&
+	git diff > actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Reset worktree changes' '
+	git reset --hard "$pre_merge_head"
+'
+
+test_expect_success 'Fail clean merge with conflicting dirty worktree' '
+	echo xyzzy >> bar &&
+	git diff > expect &&
+	test_must_fail git merge --no-commit clean_branch &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff --staged)" &&
+	git diff > actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Fail conflicting merge with conflicting dirty worktree' '
+	test_must_fail git merge conflict_branch &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff --staged)" &&
+	git diff > actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Reset worktree changes' '
+	git reset --hard "$pre_merge_head"
+'
+
+test_expect_success 'Fail clean merge with matching dirty worktree' '
+	echo bart > bar &&
+	git diff > expect &&
+	test_must_fail git merge --no-commit clean_branch &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff --staged)" &&
+	git diff > actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Abort clean merge with matching dirty index' '
+	git add bar &&
+	git diff --staged > expect &&
+	git merge --no-commit clean_branch &&
+	test -f .git/MERGE_HEAD &&
+	### When aborting the merge, git will discard all staged changes,
+	### including those that were staged pre-merge. In other words,
+	### --abort will LOSE any staged changes (the staged changes that
+	### are lost must match the merge result, or the merge would not
+	### have been allowed to start). Change expectations accordingly:
+	rm expect &&
+	touch expect &&
+	# Abort merge
+	git merge --abort &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	git diff --staged > actual &&
+	test_cmp expect actual &&
+	test -z "$(git diff)"
+'
+
+test_expect_success 'Reset worktree changes' '
+	git reset --hard "$pre_merge_head"
+'
+
+test_expect_success 'Fail conflicting merge with matching dirty worktree' '
+	echo barf > bar &&
+	git diff > expect &&
+	test_must_fail git merge conflict_branch &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	test -z "$(git diff --staged)" &&
+	git diff > actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'Abort conflicting merge with matching dirty index' '
+	git add bar &&
+	git diff --staged > expect &&
+	test_must_fail git merge conflict_branch &&
+	test -f .git/MERGE_HEAD &&
+	### When aborting the merge, git will discard all staged changes,
+	### including those that were staged pre-merge. In other words,
+	### --abort will LOSE any staged changes (the staged changes that
+	### are lost must match the merge result, or the merge would not
+	### have been allowed to start). Change expectations accordingly:
+	rm expect &&
+	touch expect &&
+	# Abort merge
+	git merge --abort &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	git diff --staged > actual &&
+	test_cmp expect actual &&
+	test -z "$(git diff)"
+'
+
+test_expect_success 'Reset worktree changes' '
+	git reset --hard "$pre_merge_head"
+'
+
+test_expect_success 'Abort merge with pre- and post-merge worktree changes' '
+	# Pre-merge worktree changes
+	echo xyzzy > foo &&
+	echo barf > bar &&
+	git add bar &&
+	git diff > expect &&
+	git diff --staged > expect-staged &&
+	# Perform merge
+	test_must_fail git merge conflict_branch &&
+	test -f .git/MERGE_HEAD &&
+	# Post-merge worktree changes
+	echo yzxxz > foo &&
+	echo blech > baz &&
+	### When aborting the merge, git will discard staged changes (bar)
+	### and unmerged changes (baz). Other changes that are neither
+	### staged nor marked as unmerged (foo), will be preserved. For
+	### these changed, git cannot tell pre-merge changes apart from
+	### post-merge changes, so the post-merge changes will be
+	### preserved. Change expectations accordingly:
+	git diff -- foo > expect &&
+	rm expect-staged &&
+	touch expect-staged &&
+	# Abort merge
+	git merge --abort &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	git diff > actual &&
+	test_cmp expect actual &&
+	git diff --staged > actual-staged &&
+	test_cmp expect-staged actual-staged
+'
+
+test_expect_success 'Reset worktree changes' '
+	git reset --hard "$pre_merge_head"
+'
+
+test_expect_success 'Abort merge with pre- and post-merge index changes' '
+	# Pre-merge worktree changes
+	echo xyzzy > foo &&
+	echo barf > bar &&
+	git add bar &&
+	git diff > expect &&
+	git diff --staged > expect-staged &&
+	# Perform merge
+	test_must_fail git merge conflict_branch &&
+	test -f .git/MERGE_HEAD &&
+	# Post-merge worktree changes
+	echo yzxxz > foo &&
+	echo blech > baz &&
+	git add foo bar &&
+	### When aborting the merge, git will discard all staged changes
+	### (foo, bar and baz), and no changes will be preserved. Whether
+	### the changes were staged pre- or post-merge does not matter
+	### (except for not preventing starting the merge).
+	### Change expectations accordingly:
+	rm expect expect-staged &&
+	touch expect &&
+	touch expect-staged &&
+	# Abort merge
+	git merge --abort &&
+	test ! -f .git/MERGE_HEAD &&
+	test "$pre_merge_head" = "$(git rev-parse HEAD)" &&
+	git diff > actual &&
+	test_cmp expect actual &&
+	git diff --staged > actual-staged &&
+	test_cmp expect-staged actual-staged
+'
+
+test_done
diff --git a/t/t9301-fast-import-notes.sh b/t/t9301-fast-import-notes.sh
index a5c99d8507..7cf8cd8a2f 100755
--- a/t/t9301-fast-import-notes.sh
+++ b/t/t9301-fast-import-notes.sh
@@ -255,13 +255,18 @@ EOF
 
 INPUT_END
 
+whitespace="    "
+
 cat >expect <<EXPECT_END
     fourth commit
     pre-prefix of note for fourth commit
+$whitespace
     prefix of note for fourth commit
+$whitespace
     third note for fourth commit
     third commit
     prefix of note for third commit
+$whitespace
     third note for third commit
     second commit
     third note for second commit