Using git commit --amend on a commit with an empty message

Github gists can be cloned as normal git repositories, but the commits made through the web interface appear with an empty commit message. Running git commit --amend against them exposes a slightly odd behaviour of git, which I can also demonstrate as follows:

  $ git init foo && cd foo
  $ touch one && git add one
  $ git commit -m '' --allow-empty-message
  [master (root-commit) 535cb36] 
   0 files changed
   create mode 100644 one

When I try to correct this commit message in an editor, it refuses to proceed, objecting to the existing empty commit message:

  $ git commit --amend
  fatal: commit has empty message

Shouldn't this drop me into the editor and fail only if the resulting message on exit is empty? (For comparison, git commit --amend -m 'oops' will work fine; it's apparently only the edit case which doesn't.)

In fact, we even fail to start the editor if --allow-empty-message is explicitly provided:

  $ git commit --amend --allow-empty-message
  fatal: commit has empty message

Assuming this isn't intentional for some reason I don't understand, I think this is the correct tiny fix? make test succeeds fine both before and after.

-- >8 --
Subject: [PATCH] Allow edit of empty message with commit --amend

If git commit --amend is used on a commit with an empty message, it fails unless -m is given, whether or not --allow-empty-message is specified.

Instead, allow it to proceed to the editor with an empty commit message. Unless --allow-empty-message is in force, it will still abort later if an empty message is saved from the editor. (That check was already present and necessary to prevent a non-empty commit message being edited to an empty one.)

---
 builtin/commit.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index f43eaaf..6515da2 100644
@@ -640,7 +640,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		hook_arg1 = "message";
 	} else if (use_message) {
 		buffer = strstr(use_message_buffer, "\n\n");
-		if (!buffer || buffer[2] == '\0')
+		if (!use_editor && (!buffer || buffer[2] == '\0'))
 			die(_("commit has empty message"));
 		strbuf_add(&sb, buffer + 2, strlen(buffer + 2));
 		hook_arg1 = "commit";

1.7.10

Yeah, it is a "bug" that exists only because nobody sane uses empty message commits, let alone tries to amend such commits, hence went unnoticed for a long time.

The patch looks sane; if we want to keep this as a feature or a bugfix, we may want to pretect it with a new test, though.

Thanks.

Quite. I only noticed it because this is the default behaviour of Github gists and I wanted to replace the empty commit messages with more meaningful ones.

Yes, it's hardly something people will test often. Okay, I'll send a version two with a suitable test.

Best wishes,

Chris.

If git commit --amend is used on a commit with an empty message, it fails unless -m is given, whether or not --allow-empty-message is specified.

Instead, allow it to proceed to the editor with an empty commit message. Unless --allow-empty-message is in force, it will still abort later if an empty message is saved from the editor. (This check was already necessary to prevent a non-empty commit message being edited to an empty one.)

Add a test for --amend --edit of an empty commit message which fails without this fix, as it's a rare case that won't get frequently tested otherwise.

---
 builtin/commit.c  |    2 +-
 t/t7501-commit.sh |   15 +++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

index f43eaaf..6515da2 100644
@@ -640,7 +640,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		hook_arg1 = "message";
 	} else if (use_message) {
 		buffer = strstr(use_message_buffer, "\n\n");
-		if (!buffer || buffer[2] == '\0')
+		if (!use_editor && (!buffer || buffer[2] == '\0'))
 			die(_("commit has empty message"));
 		strbuf_add(&sb, buffer + 2, strlen(buffer + 2));
 		hook_arg1 = "commit";

index b20ca0e..5ad636b 100755
@@ -138,6 +138,21 @@ test_expect_success '--amend --edit' '
 	test_cmp expect msg
 '
 
+test_expect_success '--amend --edit of empty message' '
+	cat >replace <<-\EOF &&
+	#!/bin/sh
+	echo "amended" >"$1"
+	EOF
+	chmod 755 replace &&
+	echo amended >expect &&
+	git commit --allow-empty --allow-empty-message -m "" &&
+	echo more bongo >file &&
+	git add file &&
+	EDITOR=./replace git commit --edit --amend &&
+	git diff-tree -s --format=%s HEAD >msg &&
+	test_cmp expect msg
+'
+
 test_expect_success '-m --edit' '
 	echo amended >expect &&
 	git commit --allow-empty -m buffer &&

1.7.10

Thanks.