checkout -m: recreate merge when checking out of unmerged index

This teaches git-checkout to recreate a merge out of unmerged
index entries while resolving conflicts.

With this patch, checking out an unmerged path from the index
now have the following possibilities:

 * Without any option, an attempt to checkout an unmerged path
   will atomically fail (i.e. no other cleanly-merged paths are
   checked out either);

 * With "-f", other cleanly-merged paths are checked out, and
   unmerged paths are ignored;

 * With "--ours" or "--theirs, the contents from the specified
   stage is checked out;

 * With "-m" (we should add "--merge" as synonym), the 3-way merge
   is recreated from the staged object names and checked out.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 102 insertions, 2 deletions

diff --git a/builtin-checkout.c b/builtin-checkout.c
index 16bfbb6605..b957193155 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -13,6 +13,9 @@
 #include "diff.h"
 #include "revision.h"
 #include "remote.h"
+#include "blob.h"
+#include "xdiff-interface.h"
+#include "ll-merge.h"
 
 static const char * const checkout_usage[] = {
 	"git checkout [options] <branch>",
@@ -109,6 +112,19 @@ static int check_stage(int stage, struct cache_entry *ce, int pos)
 		     (stage == 2) ? "our" : "their");
 }
 
+static int check_all_stages(struct cache_entry *ce, int pos)
+{
+	if (ce_stage(ce) != 1 ||
+	    active_nr <= pos + 2 ||
+	    strcmp(active_cache[pos+1]->name, ce->name) ||
+	    ce_stage(active_cache[pos+1]) != 2 ||
+	    strcmp(active_cache[pos+2]->name, ce->name) ||
+	    ce_stage(active_cache[pos+2]) != 3)
+		return error("path '%s' does not have all three versions",
+			     ce->name);
+	return 0;
+}
+
 static int checkout_stage(int stage, struct cache_entry *ce, int pos,
 			  struct checkout *state)
 {
@@ -123,6 +139,77 @@ static int checkout_stage(int stage, struct cache_entry *ce, int pos,
 		     (stage == 2) ? "our" : "their");
 }
 
+/* NEEDSWORK: share with merge-recursive */
+static void fill_mm(const unsigned char *sha1, mmfile_t *mm)
+{
+	unsigned long size;
+	enum object_type type;
+
+	if (!hashcmp(sha1, null_sha1)) {
+		mm->ptr = xstrdup("");
+		mm->size = 0;
+		return;
+	}
+
+	mm->ptr = read_sha1_file(sha1, &type, &size);
+	if (!mm->ptr || type != OBJ_BLOB)
+		die("unable to read blob object %s", sha1_to_hex(sha1));
+	mm->size = size;
+}
+
+static int checkout_merged(int pos, struct checkout *state)
+{
+	struct cache_entry *ce = active_cache[pos];
+	const char *path = ce->name;
+	mmfile_t ancestor, ours, theirs;
+	int status;
+	unsigned char sha1[20];
+	mmbuffer_t result_buf;
+
+	if (ce_stage(ce) != 1 ||
+	    active_nr <= pos + 2 ||
+	    strcmp(active_cache[pos+1]->name, path) ||
+	    ce_stage(active_cache[pos+1]) != 2 ||
+	    strcmp(active_cache[pos+2]->name, path) ||
+	    ce_stage(active_cache[pos+2]) != 3)
+		return error("path '%s' does not have all 3 versions", path);
+
+	fill_mm(active_cache[pos]->sha1, &ancestor);
+	fill_mm(active_cache[pos+1]->sha1, &ours);
+	fill_mm(active_cache[pos+2]->sha1, &theirs);
+
+	status = ll_merge(&result_buf, path, &ancestor,
+			  &ours, "ours", &theirs, "theirs", 1);
+	free(ancestor.ptr);
+	free(ours.ptr);
+	free(theirs.ptr);
+	if (status < 0 || !result_buf.ptr) {
+		free(result_buf.ptr);
+		return error("path '%s': cannot merge", path);
+	}
+
+	/*
+	 * NEEDSWORK:
+	 * There is absolutely no reason to write this as a blob object
+	 * and create a phoney cache entry just to leak.  This hack is
+	 * primarily to get to the write_entry() machinery that massages
+	 * the contents to work-tree format and writes out which only
+	 * allows it for a cache entry.  The code in write_entry() needs
+	 * to be refactored to allow us to feed a <buffer, size, mode>
+	 * instead of a cache entry.  Such a refactoring would help
+	 * merge_recursive as well (it also writes the merge result to the
+	 * object database even when it may contain conflicts).
+	 */
+	if (write_sha1_file(result_buf.ptr, result_buf.size,
+			    blob_type, sha1))
+		die("Unable to add merge result for '%s'", path);
+	ce = make_cache_entry(create_ce_mode(active_cache[pos+1]->ce_mode),
+			      sha1,
+			      path, 2, 0);
+	status = checkout_entry(ce, state, NULL);
+	return status;
+}
+
 static int checkout_paths(struct tree *source_tree, const char **pathspec,
 			  struct checkout_opts *opts)
 {
@@ -134,6 +221,7 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	struct commit *head;
 	int errs = 0;
 	int stage = opts->writeout_stage;
+	int merge = opts->merge;
 	int newfd;
 	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
 
@@ -165,6 +253,8 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 				warning("path '%s' is unmerged", ce->name);
 			} else if (stage) {
 				errs |= check_stage(stage, ce, pos);
+			} else if (opts->merge) {
+				errs |= check_all_stages(ce, pos);
 			} else {
 				errs = 1;
 				error("path '%s' is unmerged", ce->name);
@@ -188,6 +278,8 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 			}
 			if (stage)
 				errs |= checkout_stage(stage, ce, pos, &state);
+			else if (merge)
+				errs |= checkout_merged(pos, &state);
 			pos = skip_same_name(ce, pos) - 1;
 		}
 	}
@@ -476,6 +568,11 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 	return ret || opts->writeout_error;
 }
 
+static int git_checkout_config(const char *var, const char *value, void *cb)
+{
+	return git_xmerge_config(var, value, cb);
+}
+
 int cmd_checkout(int argc, const char **argv, const char *prefix)
 {
 	struct checkout_opts opts;
@@ -502,7 +599,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	memset(&opts, 0, sizeof(opts));
 	memset(&new, 0, sizeof(new));
 
-	git_config(git_default_config, NULL);
+	git_config(git_checkout_config, NULL);
 
 	opts.track = git_branch_track;
 
@@ -594,7 +691,7 @@ no_reference:
 			die("invalid path specification");
 
 		/* Checkout paths */
-		if (opts.new_branch || opts.merge) {
+		if (opts.new_branch) {
 			if (argc == 1) {
 				die("git checkout: updating paths is incompatible with switching branches.\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
 			} else {
@@ -602,6 +699,9 @@ no_reference:
 			}
 		}
 
+		if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
+			die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");
+
 		return checkout_paths(source_tree, pathspec, &opts);
 	}