revert: write REVERT_HEAD pseudoref during conflicted revert

When conflicts are encountered while reverting a commit, it can be
handy to have the name of that commit easily available.  For example,
to produce a copy of the patch to refer to while resolving conflicts:

	$ git revert 2eceb2a8
	error: could not revert 2eceb2a8... awesome, buggy feature
	$ git show -R REVERT_HEAD >the-patch
	$ edit $(git diff --name-only)

Set a REVERT_HEAD pseudoref when "git revert" does not make a commit,
for cases like this.  This also makes it possible for scripts to
distinguish between a revert that encountered conflicts and other
sources of an unmerged index.

After successfully committing, resetting with "git reset", or moving
to another commit with "git checkout" or "git reset", the pseudoref is
no longer useful, so remove it.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -241,6 +241,7 @@ void create_branch(const char *head,
 void remove_branch_state(void)
 {
 	unlink(git_path("CHERRY_PICK_HEAD"));
+	unlink(git_path("REVERT_HEAD"));
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_RR"));
 	unlink(git_path("MERGE_MSG"));

builtin/commit.c

@@ -1514,6 +1514,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	}
 
 	unlink(git_path("CHERRY_PICK_HEAD"));
+	unlink(git_path("REVERT_HEAD"));
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_MSG"));
 	unlink(git_path("MERGE_MODE"));

builtin/revert.c

@@ -289,7 +289,7 @@ static char *get_encoding(const char *message)
 	return NULL;
 }
 
-static void write_cherry_pick_head(struct commit *commit)
+static void write_cherry_pick_head(struct commit *commit, const char *pseudoref)
 {
 	const char *filename;
 	int fd;
@@ -297,7 +297,7 @@ static void write_cherry_pick_head(struct commit *commit)
 
 	strbuf_addf(&buf, "%s\n", sha1_to_hex(commit->object.sha1));
 
-	filename = git_path("CHERRY_PICK_HEAD");
+	filename = git_path(pseudoref);
 	fd = open(filename, O_WRONLY | O_CREAT, 0666);
 	if (fd < 0)
 		die_errno(_("Could not open '%s' for writing"), filename);
@@ -597,7 +597,9 @@ static int do_pick_commit(struct commit *commit, struct replay_opts *opts)
 	 * write it at all.
 	 */
 	if (opts->action == CHERRY_PICK && !opts->no_commit && (res == 0 || res == 1))
-		write_cherry_pick_head(commit);
+		write_cherry_pick_head(commit, "CHERRY_PICK_HEAD");
+	if (opts->action == REVERT && ((opts->no_commit && res == 0) || res == 1))
+		write_cherry_pick_head(commit, "REVERT_HEAD");
 
 	if (res) {
 		error(opts->action == REVERT

t/t3507-cherry-pick-conflict.sh

@@ -253,6 +253,60 @@ test_expect_success 'revert also handles conflicts sanely' '
 	test_cmp expected actual
 '
 
+test_expect_success 'failed revert sets REVERT_HEAD' '
+	pristine_detach initial &&
+	test_must_fail git revert picked &&
+	test_cmp_rev picked REVERT_HEAD
+'
+
+test_expect_success 'successful revert does not set REVERT_HEAD' '
+	pristine_detach base &&
+	git revert base &&
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD &&
+	test_must_fail git rev-parse --verify REVERT_HEAD
+'
+
+test_expect_success 'revert --no-commit sets REVERT_HEAD' '
+	pristine_detach base &&
+	git revert --no-commit base &&
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD &&
+	test_cmp_rev base REVERT_HEAD
+'
+
+test_expect_success 'revert w/dirty tree does not set REVERT_HEAD' '
+	pristine_detach base &&
+	echo foo > foo &&
+	test_must_fail git revert base &&
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD &&
+	test_must_fail git rev-parse --verify REVERT_HEAD
+'
+
+test_expect_success 'GIT_CHERRY_PICK_HELP does not suppress REVERT_HEAD' '
+	pristine_detach initial &&
+	(
+		GIT_CHERRY_PICK_HELP="and then do something else" &&
+		GIT_REVERT_HELP="and then do something else, again" &&
+		export GIT_CHERRY_PICK_HELP GIT_REVERT_HELP &&
+		test_must_fail git revert picked
+	) &&
+	test_must_fail git rev-parse --verify CHERRY_PICK_HEAD &&
+	test_cmp_rev picked REVERT_HEAD
+'
+
+test_expect_success 'git reset clears REVERT_HEAD' '
+	pristine_detach initial &&
+	test_must_fail git revert picked &&
+	git reset &&
+	test_must_fail git rev-parse --verify REVERT_HEAD
+'
+
+test_expect_success 'failed commit does not clear REVERT_HEAD' '
+	pristine_detach initial &&
+	test_must_fail git revert picked &&
+	test_must_fail git commit &&
+	test_cmp_rev picked REVERT_HEAD
+'
+
 test_expect_success 'revert conflict, diff3 -m style' '
 	pristine_detach initial &&
 	git config merge.conflictstyle diff3 &&