[PATCH] merge-message: change meaning of "empty merge message"

[Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>]

In the context of "git merge" the meaning of an "empty message"
is one that contains no line of text. This is not in line with
"git commit" where an "empty message" is one that contains only
whitespaces and/or signed-off-by lines. This could cause surprises
to users who are accustomed to the meaning of an "empty message"
of "git commit".

Prevent such surprises by changing the meaning of an empty 'merge
message' to be in line with that of an empty 'commit message'.

Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
---
 builtin/merge.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/builtin/merge.c b/builtin/merge.c
index 703827f00..db4bf1c40 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -748,6 +748,39 @@ static void abort_commit(struct commit_list *remoteheads, const char *err_msg)
 	exit(1);
 }
 
+/*
+ * Find out if the message in the strbuf contains only whitespace and
+ * Signed-off-by lines.
+ *
+ * This function is the "rest_is_space" function of "commit" with the unwanted
+ * parameter removed.
+ */
+static int message_is_empty(struct strbuf *sb)
+{
+	int i, eol;
+	const char *nl;
+
+	/* Check if the rest is just whitespace and Signed-off-by's. */
+	for (i = 0; i < sb->len; i++) {
+		nl = memchr(sb->buf + i, '\n', sb->len - i);
+		if (nl)
+			eol = nl - sb->buf;
+		else
+			eol = sb->len;
+
+		if (strlen(sign_off_header) <= eol - i &&
+		    starts_with(sb->buf + i, sign_off_header)) {
+			i = eol;
+			continue;
+		}
+		while (i < eol)
+			if (!isspace(sb->buf[i++]))
+				return 0;
+	}
+
+	return 1;
+}
+
 static const char merge_editor_comment[] =
 N_("Please enter a commit message to explain why this merge is necessary,\n"
    "especially if it merges an updated upstream into a topic branch.\n"
@@ -772,7 +805,7 @@ static void prepare_to_commit(struct commit_list *remoteheads)
 	}
 	read_merge_msg(&msg);
 	strbuf_stripspace(&msg, 0 < option_edit);
-	if (!msg.len)
+	if (!msg.len || message_is_empty(&msg))
 		abort_commit(remoteheads, _("Empty commit message."));
 	strbuf_release(&merge_msg);
 	strbuf_addbuf(&merge_msg, &msg);
-- 
2.11.0