Merge branch 'jc/cache-unmerge'

* jc/cache-unmerge:
  rerere forget path: forget recorded resolution
  rerere: refactor rerere logic to make it independent from I/O
  rerere: remove silly 1024-byte line limit
  resolve-undo: teach "update-index --unresolve" to use resolve-undo info
  resolve-undo: "checkout -m path" uses resolve-undo information
  resolve-undo: allow plumbing to clear the information
  resolve-undo: basic tests
  resolve-undo: record resolved conflicts in a new index extension section
  builtin-merge.c: use standard active_cache macros

Conflicts:
	builtin-ls-files.c
	builtin-merge.c
	builtin-rerere.c

14 files changed, 652 insertions, 47 deletions

diff --git a/Makefile b/Makefile
index cf0723ea12..ad890ec925 100644
--- a/Makefile
+++ b/Makefile
@@ -497,6 +497,7 @@ LIB_H += reflog-walk.h
 LIB_H += refs.h
 LIB_H += remote.h
 LIB_H += rerere.h
+LIB_H += resolve-undo.h
 LIB_H += revision.h
 LIB_H += run-command.h
 LIB_H += sha1-lookup.h
@@ -592,6 +593,7 @@ LIB_OBJS += refs.o
 LIB_OBJS += remote.o
 LIB_OBJS += replace_object.o
 LIB_OBJS += rerere.o
+LIB_OBJS += resolve-undo.o
 LIB_OBJS += revision.o
 LIB_OBJS += run-command.o
 LIB_OBJS += server-info.o
diff --git a/builtin-checkout.c b/builtin-checkout.c
index e44e238c39..d0b1a728cb 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -17,6 +17,7 @@
 #include "blob.h"
 #include "xdiff-interface.h"
 #include "ll-merge.h"
+#include "resolve-undo.h"
 
 static const char * const checkout_usage[] = {
 	"git checkout [options] <branch>",
@@ -234,6 +235,10 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	if (report_path_error(ps_matched, pathspec, 0))
 		return 1;
 
+	/* "checkout -m path" to recreate conflicted state */
+	if (opts->merge)
+		unmerge_cache(pathspec);
+
 	/* Any unmerged paths? */
 	for (pos = 0; pos < active_nr; pos++) {
 		struct cache_entry *ce = active_cache[pos];
@@ -370,6 +375,7 @@ static int merge_working_tree(struct checkout_opts *opts,
 	if (read_cache_preload(NULL) < 0)
 		return error("corrupt index file");
 
+	resolve_undo_clear();
 	if (opts->force) {
 		ret = reset_tree(new->commit->tree, opts, 1);
 		if (ret)
diff --git a/builtin-ls-files.c b/builtin-ls-files.c
index 738215768e..b065061392 100644
--- a/builtin-ls-files.c
+++ b/builtin-ls-files.c
@@ -11,6 +11,8 @@
 #include "builtin.h"
 #include "tree.h"
 #include "parse-options.h"
+#include "resolve-undo.h"
+#include "string-list.h"
 
 static int abbrev;
 static int show_deleted;
@@ -18,6 +20,7 @@ static int show_cached;
 static int show_others;
 static int show_stage;
 static int show_unmerged;
+static int show_resolve_undo;
 static int show_modified;
 static int show_killed;
 static int show_valid_bit;
@@ -38,6 +41,7 @@ static const char *tag_other = "";
 static const char *tag_killed = "";
 static const char *tag_modified = "";
 static const char *tag_skip_worktree = "";
+static const char *tag_resolve_undo = "";
 
 static void show_dir_entry(const char *tag, struct dir_entry *ent)
 {
@@ -156,6 +160,38 @@ static void show_ce_entry(const char *tag, struct cache_entry *ce)
 	write_name_quoted(ce->name + offset, stdout, line_terminator);
 }
 
+static int show_one_ru(struct string_list_item *item, void *cbdata)
+{
+	int offset = prefix_offset;
+	const char *path = item->string;
+	struct resolve_undo_info *ui = item->util;
+	int i, len;
+
+	len = strlen(path);
+	if (len < prefix_len)
+		return 0; /* outside of the prefix */
+	if (!match_pathspec(pathspec, path, len, prefix_len, ps_matched))
+		return 0; /* uninterested */
+	for (i = 0; i < 3; i++) {
+		if (!ui->mode[i])
+			continue;
+		printf("%s%06o %s %d\t", tag_resolve_undo, ui->mode[i],
+		       abbrev
+		       ? find_unique_abbrev(ui->sha1[i], abbrev)
+		       : sha1_to_hex(ui->sha1[i]),
+		       i + 1);
+		write_name_quoted(path + offset, stdout, line_terminator);
+	}
+	return 0;
+}
+
+static void show_ru_info(const char *prefix)
+{
+	if (!the_index.resolve_undo)
+		return;
+	for_each_string_list(show_one_ru, the_index.resolve_undo, NULL);
+}
+
 static void show_files(struct dir_struct *dir, const char *prefix)
 {
 	int i;
@@ -458,6 +494,8 @@ int cmd_ls_files(int argc, const char **argv, const char *prefix)
 			DIR_HIDE_EMPTY_DIRECTORIES),
 		OPT_BOOLEAN('u', "unmerged", &show_unmerged,
 			"show unmerged files in the output"),
+		OPT_BOOLEAN(0, "resolve-undo", &show_resolve_undo,
+			    "show resolve-undo information"),
 		{ OPTION_CALLBACK, 'x', "exclude", &dir.exclude_list[EXC_CMDL], "pattern",
 			"skip files matching pattern",
 			0, option_parse_exclude },
@@ -498,6 +536,7 @@ int cmd_ls_files(int argc, const char **argv, const char *prefix)
 		tag_other = "? ";
 		tag_killed = "K ";
 		tag_skip_worktree = "S ";
+		tag_resolve_undo = "U ";
 	}
 	if (show_modified || show_others || show_deleted || (dir.flags & DIR_SHOW_IGNORED) || show_killed)
 		require_work_tree = 1;
@@ -536,7 +575,7 @@ int cmd_ls_files(int argc, const char **argv, const char *prefix)
 
 	/* With no flags, we default to showing the cached files */
 	if (!(show_stage | show_deleted | show_others | show_unmerged |
-	      show_killed | show_modified))
+	      show_killed | show_modified | show_resolve_undo))
 		show_cached = 1;
 
 	if (prefix)
@@ -551,6 +590,8 @@ int cmd_ls_files(int argc, const char **argv, const char *prefix)
 		overlay_tree_on_cache(with_tree, prefix);
 	}
 	show_files(&dir, prefix);
+	if (show_resolve_undo)
+		show_ru_info(prefix);
 
 	if (ps_matched) {
 		int bad;
diff --git a/builtin-merge.c b/builtin-merge.c
index 6f1311414b..9f60ffa2cd 100644
--- a/builtin-merge.c
+++ b/builtin-merge.c
@@ -24,6 +24,7 @@
 #include "rerere.h"
 #include "help.h"
 #include "merge-recursive.h"
+#include "resolve-undo.h"
 
 #define DEFAULT_TWOHEAD (1<<0)
 #define DEFAULT_OCTOPUS (1<<1)
@@ -606,6 +607,7 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 		discard_cache();
 		if (read_cache() < 0)
 			die("failed to read the cache");
+		resolve_undo_clear();
 		return ret;
 	}
 }
@@ -620,11 +622,10 @@ static void count_diff_files(struct diff_queue_struct *q,
 
 static int count_unmerged_entries(void)
 {
-	const struct index_state *state = &the_index;
 	int i, ret = 0;
 
-	for (i = 0; i < state->cache_nr; i++)
-		if (ce_stage(state->cache[i]))
+	for (i = 0; i < active_nr; i++)
+		if (ce_stage(active_cache[i]))
 			ret++;
 
 	return ret;
@@ -864,6 +865,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			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.
diff --git a/builtin-read-tree.c b/builtin-read-tree.c
index 50413ca17d..5fda9905fc 100644
--- a/builtin-read-tree.c
+++ b/builtin-read-tree.c
@@ -13,6 +13,7 @@
 #include "dir.h"
 #include "builtin.h"
 #include "parse-options.h"
+#include "resolve-undo.h"
 
 static int nr_trees;
 static struct tree *trees[MAX_UNPACK_TREES];
@@ -124,6 +125,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 			die("You need to resolve your current index first");
 		stage = opts.merge = 1;
 	}
+	resolve_undo_clear();
 
 	for (i = 0; i < argc; i++) {
 		const char *arg = argv[i];
diff --git a/builtin-rerere.c b/builtin-rerere.c
index 5028138898..25f507a2f1 100644
--- a/builtin-rerere.c
+++ b/builtin-rerere.c
@@ -120,6 +120,9 @@ int cmd_rerere(int argc, const char **argv, const char *prefix)
 	if (argc < 2)
 		return rerere(flags);
 
+	if (!strcmp(argv[1], "forget"))
+		return rerere_forget(argv + 2);
+
 	fd = setup_rerere(&merge_rr, flags);
 	if (fd < 0)
 		return 0;
diff --git a/builtin-update-index.c b/builtin-update-index.c
index a64a752990..3ab214d24e 100644
--- a/builtin-update-index.c
+++ b/builtin-update-index.c
@@ -9,6 +9,7 @@
 #include "tree-walk.h"
 #include "builtin.h"
 #include "refs.h"
+#include "resolve-undo.h"
 
 /*
  * Default to not allowing changes to the list of files. The
@@ -440,7 +441,18 @@ static int unresolve_one(const char *path)
 
 	/* See if there is such entry in the index. */
 	pos = cache_name_pos(path, namelen);
-	if (pos < 0) {
+	if (0 <= pos) {
+		/* already merged */
+		pos = unmerge_cache_entry_at(pos);
+		if (pos < active_nr) {
+			struct cache_entry *ce = active_cache[pos];
+			if (ce_stage(ce) &&
+			    ce_namelen(ce) == namelen &&
+			    !memcmp(ce->name, path, namelen))
+				return 0;
+		}
+		/* no resolve-undo information; fall back */
+	} else {
 		/* If there isn't, either it is unmerged, or
 		 * resolved as "removed" by mistake.  We do not
 		 * want to do anything in the former case.
@@ -719,6 +731,10 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 				verbose = 1;
 				continue;
 			}
+			if (!strcmp(path, "--clear-resolve-undo")) {
+				resolve_undo_clear();
+				continue;
+			}
 			if (!strcmp(path, "-h") || !strcmp(path, "--help"))
 				usage(update_index_usage);
 			die("unknown option %s", path);
diff --git a/cache.h b/cache.h
index bebe1a891d..5abcb09fcf 100644
--- a/cache.h
+++ b/cache.h
@@ -288,6 +288,7 @@ static inline int ce_to_dtype(const struct cache_entry *ce)
 struct index_state {
 	struct cache_entry **cache;
 	unsigned int cache_nr, cache_alloc, cache_changed;
+	struct string_list *resolve_undo;
 	struct cache_tree *cache_tree;
 	struct cache_time timestamp;
 	void *alloc;
@@ -342,6 +343,9 @@ static inline void remove_name_hash(struct cache_entry *ce)
 #define ce_modified(ce, st, options) ie_modified(&the_index, (ce), (st), (options))
 #define cache_name_exists(name, namelen, igncase) index_name_exists(&the_index, (name), (namelen), (igncase))
 #define cache_name_is_other(name, namelen) index_name_is_other(&the_index, (name), (namelen))
+#define resolve_undo_clear() resolve_undo_clear_index(&the_index)
+#define unmerge_cache_entry_at(at) unmerge_index_entry_at(&the_index, at)
+#define unmerge_cache(pathspec) unmerge_index(&the_index, pathspec)
 #endif
 
 enum object_type {
diff --git a/read-cache.c b/read-cache.c
index f4512967b8..edd995943d 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -14,6 +14,7 @@
 #include "diffcore.h"
 #include "revision.h"
 #include "blob.h"
+#include "resolve-undo.h"
 
 static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int really);
 
@@ -28,6 +29,7 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int reall
 
 #define CACHE_EXT(s) ( (s[0]<<24)|(s[1]<<16)|(s[2]<<8)|(s[3]) )
 #define CACHE_EXT_TREE 0x54524545	/* "TREE" */
+#define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUN" */
 
 struct index_state the_index;
 
@@ -456,6 +458,7 @@ int remove_index_entry_at(struct index_state *istate, int pos)
 {
 	struct cache_entry *ce = istate->cache[pos];
 
+	record_resolve_undo(istate, ce);
 	remove_name_hash(ce);
 	istate->cache_changed = 1;
 	istate->cache_nr--;
@@ -1183,6 +1186,9 @@ static int read_index_extension(struct index_state *istate,
 	case CACHE_EXT_TREE:
 		istate->cache_tree = cache_tree_read(data, sz);
 		break;
+	case CACHE_EXT_RESOLVE_UNDO:
+		istate->resolve_undo = resolve_undo_read(data, sz);
+		break;
 	default:
 		if (*ext < 'A' || 'Z' < *ext)
 			return error("index uses %.4s extension, which we do not understand",
@@ -1362,6 +1368,7 @@ int is_index_unborn(struct index_state *istate)
 
 int discard_index(struct index_state *istate)
 {
+	resolve_undo_clear_index(istate);
 	istate->cache_nr = 0;
 	istate->cache_changed = 0;
 	istate->timestamp.sec = 0;
@@ -1587,6 +1594,17 @@ int write_index(struct index_state *istate, int newfd)
 		if (err)
 			return -1;
 	}
+	if (istate->resolve_undo) {
+		struct strbuf sb = STRBUF_INIT;
+
+		resolve_undo_write(&sb, istate->resolve_undo);
+		err = write_index_ext_header(&c, newfd, CACHE_EXT_RESOLVE_UNDO,
+					     sb.len) < 0
+			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
+		strbuf_release(&sb);
+		if (err)
+			return -1;
+	}
 
 	if (ce_flush(&c, newfd) || fstat(newfd, &st))
 		return -1;
diff --git a/rerere.c b/rerere.c
index e0ac5bcaed..70d0f7afff 100644
--- a/rerere.c
+++ b/rerere.c
@@ -3,6 +3,9 @@
 #include "rerere.h"
 #include "xdiff/xdiff.h"
 #include "xdiff-interface.h"
+#include "dir.h"
+#include "resolve-undo.h"
+#include "ll-merge.h"
 
 /* if rerere_enabled == -1, fall back to detection of .git/rr-cache */
 static int rerere_enabled = -1;
@@ -83,61 +86,74 @@ static inline void ferr_puts(const char *s, FILE *fp, int *err)
 	ferr_write(s, strlen(s), fp, err);
 }
 
-static int handle_file(const char *path,
-	 unsigned char *sha1, const char *output)
+struct rerere_io {
+	int (*getline)(struct strbuf *, struct rerere_io *);
+	FILE *output;
+	int wrerror;
+	/* some more stuff */
+};
+
+static void rerere_io_putstr(const char *str, struct rerere_io *io)
+{
+	if (io->output)
+		ferr_puts(str, io->output, &io->wrerror);
+}
+
+static void rerere_io_putmem(const char *mem, size_t sz, struct rerere_io *io)
+{
+	if (io->output)
+		ferr_write(mem, sz, io->output, &io->wrerror);
+}
+
+struct rerere_io_file {
+	struct rerere_io io;
+	FILE *input;
+};
+
+static int rerere_file_getline(struct strbuf *sb, struct rerere_io *io_)
+{
+	struct rerere_io_file *io = (struct rerere_io_file *)io_;
+	return strbuf_getwholeline(sb, io->input, '\n');
+}
+
+static int handle_path(unsigned char *sha1, struct rerere_io *io)
 {
 	git_SHA_CTX ctx;
-	char buf[1024];
 	int hunk_no = 0;
 	enum {
 		RR_CONTEXT = 0, RR_SIDE_1, RR_SIDE_2, RR_ORIGINAL,
 	} hunk = RR_CONTEXT;
 	struct strbuf one = STRBUF_INIT, two = STRBUF_INIT;
-	FILE *f = fopen(path, "r");
-	FILE *out = NULL;
-	int wrerror = 0;
-
-	if (!f)
-		return error("Could not open %s", path);
-
-	if (output) {
-		out = fopen(output, "w");
-		if (!out) {
-			fclose(f);
-			return error("Could not write %s", output);
-		}
-	}
+	struct strbuf buf = STRBUF_INIT;
 
 	if (sha1)
 		git_SHA1_Init(&ctx);
 
-	while (fgets(buf, sizeof(buf), f)) {
-		if (!prefixcmp(buf, "<<<<<<< ")) {
+	while (!io->getline(&buf, io)) {
+		if (!prefixcmp(buf.buf, "<<<<<<< ")) {
 			if (hunk != RR_CONTEXT)
 				goto bad;
 			hunk = RR_SIDE_1;
-		} else if (!prefixcmp(buf, "|||||||") && isspace(buf[7])) {
+		} else if (!prefixcmp(buf.buf, "|||||||") && isspace(buf.buf[7])) {
 			if (hunk != RR_SIDE_1)
 				goto bad;
 			hunk = RR_ORIGINAL;
-		} else if (!prefixcmp(buf, "=======") && isspace(buf[7])) {
+		} else if (!prefixcmp(buf.buf, "=======") && isspace(buf.buf[7])) {
 			if (hunk != RR_SIDE_1 && hunk != RR_ORIGINAL)
 				goto bad;
 			hunk = RR_SIDE_2;
-		} else if (!prefixcmp(buf, ">>>>>>> ")) {
+		} else if (!prefixcmp(buf.buf, ">>>>>>> ")) {
 			if (hunk != RR_SIDE_2)
 				goto bad;
 			if (strbuf_cmp(&one, &two) > 0)
 				strbuf_swap(&one, &two);
 			hunk_no++;
 			hunk = RR_CONTEXT;
-			if (out) {
-				ferr_puts("<<<<<<<\n", out, &wrerror);
-				ferr_write(one.buf, one.len, out, &wrerror);
-				ferr_puts("=======\n", out, &wrerror);
-				ferr_write(two.buf, two.len, out, &wrerror);
-				ferr_puts(">>>>>>>\n", out, &wrerror);
-			}
+			rerere_io_putstr("<<<<<<<\n", io);
+			rerere_io_putmem(one.buf, one.len, io);
+			rerere_io_putstr("=======\n", io);
+			rerere_io_putmem(two.buf, two.len, io);
+			rerere_io_putstr(">>>>>>>\n", io);
 			if (sha1) {
 				git_SHA1_Update(&ctx, one.buf ? one.buf : "",
 					    one.len + 1);
@@ -147,13 +163,13 @@ static int handle_file(const char *path,
 			strbuf_reset(&one);
 			strbuf_reset(&two);
 		} else if (hunk == RR_SIDE_1)
-			strbuf_addstr(&one, buf);
+			strbuf_addstr(&one, buf.buf);
 		else if (hunk == RR_ORIGINAL)
 			; /* discard */
 		else if (hunk == RR_SIDE_2)
-			strbuf_addstr(&two, buf);
-		else if (out)
-			ferr_puts(buf, out, &wrerror);
+			strbuf_addstr(&two, buf.buf);
+		else
+			rerere_io_putstr(buf.buf, io);
 		continue;
 	bad:
 		hunk = 99; /* force error exit */
@@ -161,23 +177,133 @@ static int handle_file(const char *path,
 	}
 	strbuf_release(&one);
 	strbuf_release(&two);
+	strbuf_release(&buf);
 
-	fclose(f);
-	if (wrerror)
-		error("There were errors while writing %s (%s)",
-		      path, strerror(wrerror));
-	if (out && fclose(out))
-		wrerror = error("Failed to flush %s: %s",
-				path, strerror(errno));
 	if (sha1)
 		git_SHA1_Final(sha1, &ctx);
-	if (hunk != RR_CONTEXT) {
+	if (hunk != RR_CONTEXT)
+		return -1;
+	return hunk_no;
+}
+
+static int handle_file(const char *path, unsigned char *sha1, const char *output)
+{
+	int hunk_no = 0;
+	struct rerere_io_file io;
+
+	memset(&io, 0, sizeof(io));
+	io.io.getline = rerere_file_getline;
+	io.input = fopen(path, "r");
+	io.io.wrerror = 0;
+	if (!io.input)
+		return error("Could not open %s", path);
+
+	if (output) {
+		io.io.output = fopen(output, "w");
+		if (!io.io.output) {
+			fclose(io.input);
+			return error("Could not write %s", output);
+		}
+	}
+
+	hunk_no = handle_path(sha1, (struct rerere_io *)&io);
+
+	fclose(io.input);
+	if (io.io.wrerror)
+		error("There were errors while writing %s (%s)",
+		      path, strerror(io.io.wrerror));
+	if (io.io.output && fclose(io.io.output))
+		io.io.wrerror = error("Failed to flush %s: %s",
+				      path, strerror(errno));
+
+	if (hunk_no < 0) {
 		if (output)
 			unlink_or_warn(output);
 		return error("Could not parse conflict hunks in %s", path);
 	}
-	if (wrerror)
+	if (io.io.wrerror)
+		return -1;
+	return hunk_no;
+}
+
+struct rerere_io_mem {
+	struct rerere_io io;
+	struct strbuf input;
+};
+
+static int rerere_mem_getline(struct strbuf *sb, struct rerere_io *io_)
+{
+	struct rerere_io_mem *io = (struct rerere_io_mem *)io_;
+	char *ep;
+	size_t len;
+
+	strbuf_release(sb);
+	if (!io->input.len)
+		return -1;
+	ep = strchrnul(io->input.buf, '\n');
+	if (*ep == '\n')
+		ep++;
+	len = ep - io->input.buf;
+	strbuf_add(sb, io->input.buf, len);
+	strbuf_remove(&io->input, 0, len);
+	return 0;
+}
+
+static int handle_cache(const char *path, unsigned char *sha1, const char *output)
+{
+	mmfile_t mmfile[3];
+	mmbuffer_t result = {NULL, 0};
+	struct cache_entry *ce;
+	int pos, len, i, hunk_no;
+	struct rerere_io_mem io;
+
+	/*
+	 * Reproduce the conflicted merge in-core
+	 */
+	len = strlen(path);
+	pos = cache_name_pos(path, len);
+	if (0 <= pos)
 		return -1;
+	pos = -pos - 1;
+
+	for (i = 0; i < 3; i++) {
+		enum object_type type;
+		unsigned long size;
+
+		mmfile[i].size = 0;
+		mmfile[i].ptr = NULL;
+		if (active_nr <= pos)
+			break;
+		ce = active_cache[pos++];
+		if (ce_namelen(ce) != len || memcmp(ce->name, path, len)
+		    || ce_stage(ce) != i + 1)
+			break;
+		mmfile[i].ptr = read_sha1_file(ce->sha1, &type, &size);
+		mmfile[i].size = size;
+	}
+	for (i = 0; i < 3; i++) {
+		if (!mmfile[i].ptr && !mmfile[i].size)
+			mmfile[i].ptr = xstrdup("");
+	}
+	ll_merge(&result, path, &mmfile[0],
+		 &mmfile[1], "ours",
+		 &mmfile[2], "theirs", 0);
+	for (i = 0; i < 3; i++)
+		free(mmfile[i].ptr);
+
+	memset(&io, 0, sizeof(&io));
+	io.io.getline = rerere_mem_getline;
+	if (output)
+		io.io.output = fopen(output, "w");
+	else
+		io.io.output = NULL;
+	strbuf_init(&io.input, 0);
+	strbuf_attach(&io.input, result.ptr, result.size, result.size);
+
+	hunk_no = handle_path(sha1, (struct rerere_io *)&io);
+	strbuf_release(&io.input);
+	if (io.io.output)
+		fclose(io.io.output);
 	return hunk_no;
 }
 
@@ -394,3 +520,52 @@ int rerere(int flags)
 		return 0;
 	return do_plain_rerere(&merge_rr, fd);
 }
+
+static int rerere_forget_one_path(const char *path, struct string_list *rr)
+{
+	const char *filename;
+	char *hex;
+	unsigned char sha1[20];
+	int ret;
+
+	ret = handle_cache(path, sha1, NULL);
+	if (ret < 1)
+		return error("Could not parse conflict hunks in '%s'", path);
+	hex = xstrdup(sha1_to_hex(sha1));
+	filename = rerere_path(hex, "postimage");
+	if (unlink(filename))
+		return (errno == ENOENT
+			? error("no remembered resolution for %s", path)
+			: error("cannot unlink %s: %s", filename, strerror(errno)));
+
+	handle_cache(path, sha1, rerere_path(hex, "preimage"));
+	fprintf(stderr, "Updated preimage for '%s'\n", path);
+
+
+	string_list_insert(path, rr)->util = hex;
+	fprintf(stderr, "Forgot resolution for %s\n", path);
+	return 0;
+}
+
+int rerere_forget(const char **pathspec)
+{
+	int i, fd;
+	struct string_list conflict = { NULL, 0, 0, 1 };
+	struct string_list merge_rr = { NULL, 0, 0, 1 };
+
+	if (read_cache() < 0)
+		return error("Could not read index");
+
+	fd = setup_rerere(&merge_rr, RERERE_NOAUTOUPDATE);
+
+	unmerge_cache(pathspec);
+	find_conflict(&conflict);
+	for (i = 0; i < conflict.nr; i++) {
+		struct string_list_item *it = &conflict.items[i];
+		if (!match_pathspec(pathspec, it->string, strlen(it->string),
+				    0, NULL))
+			continue;
+		rerere_forget_one_path(it->string, &merge_rr);
+	}
+	return write_rr(&merge_rr, fd);
+}
diff --git a/rerere.h b/rerere.h
index 10a94a4ea1..eaa9004dcd 100644
--- a/rerere.h
+++ b/rerere.h
@@ -10,6 +10,7 @@ extern int setup_rerere(struct string_list *, int);
 extern int rerere(int);
 extern const char *rerere_path(const char *hex, const char *file);
 extern int has_rerere_resolution(const char *hex);
+extern int rerere_forget(const char **);
 
 #define OPT_RERERE_AUTOUPDATE(v) OPT_UYN(0, "rerere-autoupdate", (v), \
 	"update the index with reused conflict resolution if possible")
diff --git a/resolve-undo.c b/resolve-undo.c
new file mode 100644
index 0000000000..37d73cd949
--- /dev/null
+++ b/resolve-undo.c
@@ -0,0 +1,176 @@
+#include "cache.h"
+#include "dir.h"
+#include "resolve-undo.h"
+#include "string-list.h"
+
+/* The only error case is to run out of memory in string-list */
+void record_resolve_undo(struct index_state *istate, struct cache_entry *ce)
+{
+	struct string_list_item *lost;
+	struct resolve_undo_info *ui;
+	struct string_list *resolve_undo;
+	int stage = ce_stage(ce);
+
+	if (!stage)
+		return;
+
+	if (!istate->resolve_undo) {
+		resolve_undo = xcalloc(1, sizeof(*resolve_undo));
+		resolve_undo->strdup_strings = 1;
+		istate->resolve_undo = resolve_undo;
+	}
+	resolve_undo = istate->resolve_undo;
+	lost = string_list_insert(ce->name, resolve_undo);
+	if (!lost->util)
+		lost->util = xcalloc(1, sizeof(*ui));
+	ui = lost->util;
+	hashcpy(ui->sha1[stage - 1], ce->sha1);
+	ui->mode[stage - 1] = ce->ce_mode;
+}
+
+static int write_one(struct string_list_item *item, void *cbdata)
+{
+	struct strbuf *sb = cbdata;
+	struct resolve_undo_info *ui = item->util;
+	int i;
+
+	if (!ui)
+		return 0;
+	strbuf_addstr(sb, item->string);
+	strbuf_addch(sb, 0);
+	for (i = 0; i < 3; i++)
+		strbuf_addf(sb, "%o%c", ui->mode[i], 0);
+	for (i = 0; i < 3; i++) {
+		if (!ui->mode[i])
+			continue;
+		strbuf_add(sb, ui->sha1[i], 20);
+	}
+	return 0;
+}
+
+void resolve_undo_write(struct strbuf *sb, struct string_list *resolve_undo)
+{
+	for_each_string_list(write_one, resolve_undo, sb);
+}
+
+struct string_list *resolve_undo_read(void *data, unsigned long size)
+{
+	struct string_list *resolve_undo;
+	size_t len;
+	char *endptr;
+	int i;
+
+	resolve_undo = xcalloc(1, sizeof(*resolve_undo));
+	resolve_undo->strdup_strings = 1;
+
+	while (size) {
+		struct string_list_item *lost;
+		struct resolve_undo_info *ui;
+
+		len = strlen(data) + 1;
+		if (size <= len)
+			goto error;
+		lost = string_list_insert(data, resolve_undo);
+		if (!lost->util)
+			lost->util = xcalloc(1, sizeof(*ui));
+		ui = lost->util;
+		size -= len;
+		data += len;
+
+		for (i = 0; i < 3; i++) {
+			ui->mode[i] = strtoul(data, &endptr, 8);
+			if (!endptr || endptr == data || *endptr)
+				goto error;
+			len = (endptr + 1) - (char*)data;
+			if (size <= len)
+				goto error;
+			size -= len;
+			data += len;
+		}
+
+		for (i = 0; i < 3; i++) {
+			if (!ui->mode[i])
+				continue;
+			if (size < 20)
+				goto error;
+			hashcpy(ui->sha1[i], data);
+			size -= 20;
+			data += 20;
+		}
+	}
+	return resolve_undo;
+
+error:
+	string_list_clear(resolve_undo, 1);
+	error("Index records invalid resolve-undo information");
+	return NULL;
+}
+
+void resolve_undo_clear_index(struct index_state *istate)
+{
+	struct string_list *resolve_undo = istate->resolve_undo;
+	if (!resolve_undo)
+		return;
+	string_list_clear(resolve_undo, 1);
+	free(resolve_undo);
+	istate->resolve_undo = NULL;
+	istate->cache_changed = 1;
+}
+
+int unmerge_index_entry_at(struct index_state *istate, int pos)
+{
+	struct cache_entry *ce;
+	struct string_list_item *item;
+	struct resolve_undo_info *ru;
+	int i, err = 0;
+
+	if (!istate->resolve_undo)
+		return pos;
+
+	ce = istate->cache[pos];
+	if (ce_stage(ce)) {
+		/* already unmerged */
+		while ((pos < istate->cache_nr) &&
+		       ! strcmp(istate->cache[pos]->name, ce->name))
+			pos++;
+		return pos - 1; /* return the last entry processed */
+	}
+	item = string_list_lookup(ce->name, istate->resolve_undo);
+	if (!item)
+		return pos;
+	ru = item->util;
+	if (!ru)
+		return pos;
+	remove_index_entry_at(istate, pos);
+	for (i = 0; i < 3; i++) {
+		struct cache_entry *nce;
+		if (!ru->mode[i])
+			continue;
+		nce = make_cache_entry(ru->mode[i], ru->sha1[i],
+				       ce->name, i + 1, 0);
+		if (add_index_entry(istate, nce, ADD_CACHE_OK_TO_ADD)) {
+			err = 1;
+			error("cannot unmerge '%s'", ce->name);
+		}
+	}
+	if (err)
+		return pos;
+	free(ru);
+	item->util = NULL;
+	return unmerge_index_entry_at(istate, pos);
+}
+
+void unmerge_index(struct index_state *istate, const char **pathspec)
+{
+	int i;
+
+	if (!istate->resolve_undo)
+		return;
+
+	for (i = 0; i < istate->cache_nr; i++) {
+		struct cache_entry *ce = istate->cache[i];
+		if (!match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, NULL))
+			continue;
+		i = unmerge_index_entry_at(istate, i);
+	}
+}
diff --git a/resolve-undo.h b/resolve-undo.h
new file mode 100644
index 0000000000..e4e5c1b1ad
--- /dev/null
+++ b/resolve-undo.h
@@ -0,0 +1,16 @@
+#ifndef RESOLVE_UNDO_H
+#define RESOLVE_UNDO_H
+
+struct resolve_undo_info {
+	unsigned int mode[3];
+	unsigned char sha1[3][20];
+};
+
+extern void record_resolve_undo(struct index_state *, struct cache_entry *);
+extern void resolve_undo_write(struct strbuf *, struct string_list *);
+extern struct string_list *resolve_undo_read(void *, unsigned long);
+extern void resolve_undo_clear_index(struct index_state *);
+extern int unmerge_index_entry_at(struct index_state *, int);
+extern void unmerge_index(struct index_state *, const char **);
+
+#endif
diff --git a/t/t2030-unresolve-info.sh b/t/t2030-unresolve-info.sh
new file mode 100755
index 0000000000..a38bd6df84
--- /dev/null
+++ b/t/t2030-unresolve-info.sh
@@ -0,0 +1,143 @@
+#!/bin/sh
+
+test_description='undoing resolution'
+
+. ./test-lib.sh
+
+check_resolve_undo () {
+	msg=$1
+	shift
+	while case $# in
+	0)	break ;;
+	1|2|3)	die "Bug in check-resolve-undo test" ;;
+	esac
+	do
+		path=$1
+		shift
+		for stage in 1 2 3
+		do
+			sha1=$1
+			shift
+			case "$sha1" in
+			'') continue ;;
+			esac
+			sha1=$(git rev-parse --verify "$sha1")
+			printf "100644 %s %s\t%s\n" $sha1 $stage $path
+		done
+	done >"$msg.expect" &&
+	git ls-files --resolve-undo >"$msg.actual" &&
+	test_cmp "$msg.expect" "$msg.actual"
+}
+
+prime_resolve_undo () {
+	git reset --hard &&
+	git checkout second^0 &&
+	test_tick &&
+	test_must_fail git merge third^0 &&
+	echo merge does not leave anything &&
+	check_resolve_undo empty &&
+	echo different >file &&
+	git add file &&
+	echo resolving records &&
+	check_resolve_undo recorded file initial:file second:file third:file
+}
+
+test_expect_success setup '
+	test_commit initial file first &&
+	git branch side &&
+	git branch another &&
+	test_commit second file second &&
+	git checkout side &&
+	test_commit third file third &&
+	git checkout another &&
+	test_commit fourth file fourth &&
+	git checkout master
+'
+
+test_expect_success 'add records switch clears' '
+	prime_resolve_undo &&
+	test_tick &&
+	git commit -m merged &&
+	echo committing keeps &&
+	check_resolve_undo kept file initial:file second:file third:file &&
+	git checkout second^0 &&
+	echo switching clears &&
+	check_resolve_undo cleared
+'
+
+test_expect_success 'rm records reset clears' '
+	prime_resolve_undo &&
+	test_tick &&
+	git commit -m merged &&
+	echo committing keeps &&
+	check_resolve_undo kept file initial:file second:file third:file &&
+
+	echo merge clears upfront &&
+	test_must_fail git merge fourth^0 &&
+	check_resolve_undo nuked &&
+
+	git rm -f file &&
+	echo resolving records &&
+	check_resolve_undo recorded file initial:file HEAD:file fourth:file &&
+
+	git reset --hard &&
+	echo resetting discards &&
+	check_resolve_undo discarded
+'
+
+test_expect_success 'plumbing clears' '
+	prime_resolve_undo &&
+	test_tick &&
+	git commit -m merged &&
+	echo committing keeps &&
+	check_resolve_undo kept file initial:file second:file third:file &&
+
+	echo plumbing clear &&
+	git update-index --clear-resolve-undo &&
+	check_resolve_undo cleared
+'
+
+test_expect_success 'add records checkout -m undoes' '
+	prime_resolve_undo &&
+	git diff HEAD &&
+	git checkout --conflict=merge file &&
+	echo checkout used the record and removed it &&
+	check_resolve_undo removed &&
+	echo the index and the work tree is unmerged again &&
+	git diff >actual &&
+	grep "^++<<<<<<<" actual
+'
+
+test_expect_success 'unmerge with plumbing' '
+	prime_resolve_undo &&
+	git update-index --unresolve file &&
+	git ls-files -u >actual &&
+	test $(wc -l <actual) = 3
+'
+
+test_expect_success 'rerere and rerere --forget' '
+	mkdir .git/rr-cache &&
+	prime_resolve_undo &&
+	echo record the resolution &&
+	git rerere &&
+	rerere_id=$(cd .git/rr-cache && echo */postimage) &&
+	rerere_id=${rerere_id%/postimage} &&
+	test -f .git/rr-cache/$rerere_id/postimage &&
+	git checkout -m file &&
+	echo resurrect the conflict &&
+	grep "^=======" file &&
+	echo reresolve the conflict &&
+	git rerere &&
+	test "z$(cat file)" = zdifferent &&
+	echo register the resolution again &&
+	git add file &&
+	check_resolve_undo kept file initial:file second:file third:file &&
+	test -z "$(git ls-files -u)" &&
+	git rerere forget file &&
+	! test -f .git/rr-cache/$rerere_id/postimage &&
+	tr "\0" "\n" <.git/MERGE_RR >actual &&
+	echo "$rerere_id	file" >expect &&
+	test_cmp expect actual
+'
+
+test_done