checkout --ours/--theirs: allow checking out one side of a conflicting merge

This lets you to check out 'our' (or 'their') version of an
unmerged path out of the index while resolving conflicts.

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

Documentation/git-checkout.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git checkout' [-q] [-f] [[--track | --no-track] -b <new_branch> [-l]] [-m] [<branch>]
-'git checkout' [-f] [<tree-ish>] [--] <paths>...
+'git checkout' [-f|--ours|--theirs] [<tree-ish>] [--] <paths>...
 
 DESCRIPTION
 -----------
@@ -33,7 +33,9 @@ working tree.
 The index may contain unmerged entries after a failed merge.  By
 default, if you try to check out such an entry from the index, the
 checkout operation will fail and nothing will be checked out.
-Using -f will ignore these unmerged entries.
+Using -f will ignore these unmerged entries.  The contents from a
+specific side of the merge can be checked out of the index by
+using --ours or --theirs.
 
 OPTIONS
 -------
@@ -48,6 +50,11 @@ OPTIONS
 When checking out paths from the index, do not fail upon unmerged
 entries; instead, unmerged entries are ignored.
 
+--ours::
+--theirs::
+	When checking out paths from the index, check out stage #2
+	('ours') or #3 ('theirs') for unmerged paths.
+
 -b::
 	Create a new branch named <new_branch> and start it at
 	<branch>.  The new branch name must pass all checks defined

builtin-checkout.c

@@ -24,6 +24,7 @@ struct checkout_opts {
 	int quiet;
 	int merge;
 	int force;
+	int writeout_stage;
 	int writeout_error;
 
 	const char *new_branch;
@@ -95,6 +96,32 @@ static int skip_same_name(struct cache_entry *ce, int pos)
 	return pos;
 }
 
+static int check_stage(int stage, struct cache_entry *ce, int pos)
+{
+	while (pos < active_nr &&
+	       !strcmp(active_cache[pos]->name, ce->name)) {
+		if (ce_stage(active_cache[pos]) == stage)
+			return 0;
+		pos++;
+	}
+	return error("path '%s' does not have %s version",
+		     ce->name,
+		     (stage == 2) ? "our" : "their");
+}
+
+static int checkout_stage(int stage, struct cache_entry *ce, int pos,
+			  struct checkout *state)
+{
+	while (pos < active_nr &&
+	       !strcmp(active_cache[pos]->name, ce->name)) {
+		if (ce_stage(active_cache[pos]) == stage)
+			return checkout_entry(active_cache[pos], state, NULL);
+		pos++;
+	}
+	return error("path '%s' does not have %s version",
+		     ce->name,
+		     (stage == 2) ? "our" : "their");
+}
 
 static int checkout_paths(struct tree *source_tree, const char **pathspec,
 			  struct checkout_opts *opts)
@@ -106,7 +133,7 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	int flag;
 	struct commit *head;
 	int errs = 0;
-
+	int stage = opts->writeout_stage;
 	int newfd;
 	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
 
@@ -136,6 +163,8 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 				continue;
 			if (opts->force) {
 				warning("path '%s' is unmerged", ce->name);
+			} else if (stage) {
+				errs |= check_stage(stage, ce, pos);
 			} else {
 				errs = 1;
 				error("path '%s' is unmerged", ce->name);
@@ -157,6 +186,8 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 				errs |= checkout_entry(ce, &state, NULL);
 				continue;
 			}
+			if (stage)
+				errs |= checkout_stage(stage, ce, pos, &state);
 			pos = skip_same_name(ce, pos) - 1;
 		}
 	}
@@ -458,6 +489,10 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
 		OPT_SET_INT('t', "track",  &opts.track, "track",
 			BRANCH_TRACK_EXPLICIT),
+		OPT_SET_INT('2', "ours", &opts.writeout_stage, "stage",
+			    2),
+		OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
+			    3),
 		OPT_BOOLEAN('f', NULL, &opts.force, "force"),
 		OPT_BOOLEAN('m', NULL, &opts.merge, "merge"),
 		OPT_END(),
@@ -573,6 +608,8 @@ no_reference:
 	if (new.name && !new.commit) {
 		die("Cannot switch branch to a non-commit.");
 	}
+	if (opts.writeout_stage)
+		die("--ours/--theirs is incompatible with switching branches.");
 
 	return switch_branches(&opts, &new);
 }

t/t7201-co.sh

@@ -382,4 +382,29 @@ test_expect_success 'checkout with an unmerged path can be ignored' '
 	test_cmp sample file
 '
 
+test_expect_success 'checkout unmerged stage' '
+	rm -f .git/index &&
+	O=$(echo original | git hash-object -w --stdin) &&
+	A=$(echo ourside | git hash-object -w --stdin) &&
+	B=$(echo theirside | git hash-object -w --stdin) &&
+	(
+		echo "100644 $A 0	fild" &&
+		echo "100644 $O 1	file" &&
+		echo "100644 $A 2	file" &&
+		echo "100644 $B 3	file" &&
+		echo "100644 $A 0	filf"
+	) | git update-index --index-info &&
+	echo "none of the above" >sample &&
+	echo ourside >expect &&
+	cat sample >fild &&
+	cat sample >file &&
+	cat sample >filf &&
+	git checkout --ours . &&
+	test_cmp expect fild &&
+	test_cmp expect filf &&
+	test_cmp expect file &&
+	git checkout --theirs file &&
+	test ztheirside = "z$(cat file)"
+'
+
 test_done