parse_date(): '@' prefix forces git-timestamp

The only place that the issue this series addresses was observed
where we read "cat-file commit" output and put it in GIT_AUTHOR_DATE
in order to replay a commit with an ancient timestamp.

With the previous patch alone, "git commit --date='20100917 +0900'"
can be misinterpreted to mean an ancient timestamp, not September in
year 2010.  Guard this codepath by requring an extra '@' in front of
the raw git timestamp on the parsing side. This of course needs to
be compensated by updating get_author_ident_from_commit and the code
for "git commit --amend" to prepend '@' to the string read from the
existing commit in the GIT_AUTHOR_DATE environment variable.

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

builtin/commit.c

@@ -534,6 +534,7 @@ static void determine_author_info(struct strbuf *author_ident)
 
 	if (author_message) {
 		const char *a, *lb, *rb, *eol;
+		size_t len;
 
 		a = strstr(author_message_buffer, "\nauthor ");
 		if (!a)
@@ -554,6 +555,11 @@ static void determine_author_info(struct strbuf *author_ident)
 					 (a + strlen("\nauthor "))));
 		email = xmemdupz(lb + strlen("<"), rb - (lb + strlen("<")));
 		date = xmemdupz(rb + strlen("> "), eol - (rb + strlen("> ")));
+		len = eol - (rb + strlen("> "));
+		date = xmalloc(len + 2);
+		*date = '@';
+		memcpy(date + 1, rb + strlen("> "), len);
+		date[len + 1] = '\0';
 	}
 
 	if (force_author) {

date.c

@@ -637,7 +637,8 @@ int parse_date_basic(const char *date, unsigned long *timestamp, int *offset)
 	*offset = -1;
 	tm_gmt = 0;
 
-	if (!match_object_header_date(date, timestamp, offset))
+	if (*date == '@' &&
+	    !match_object_header_date(date + 1, timestamp, offset))
 		return 0; /* success */
 	for (;;) {
 		int match = 0;

git-sh-setup.sh

@@ -200,7 +200,7 @@ get_author_ident_from_commit () {
 		s/.*/GIT_AUTHOR_EMAIL='\''&'\''/p
 
 		g
-		s/^author [^<]* <[^>]*> \(.*\)$/\1/
+		s/^author [^<]* <[^>]*> \(.*\)$/@\1/
 		s/.*/GIT_AUTHOR_DATE='\''&'\''/p
 
 		q

t/t3400-rebase.sh

@@ -218,4 +218,27 @@ test_expect_success 'rebase -m can copy notes' '
 	test "a note" = "$(git notes show HEAD)"
 '
 
+test_expect_success 'rebase commit with an ancient timestamp' '
+	git reset --hard &&
+
+	>old.one && git add old.one && test_tick &&
+	git commit --date="@12345 +0400" -m "Old one" &&
+	>old.two && git add old.two && test_tick &&
+	git commit --date="@23456 +0500" -m "Old two" &&
+	>old.three && git add old.three && test_tick &&
+	git commit --date="@34567 +0600" -m "Old three" &&
+
+	git cat-file commit HEAD^^ >actual &&
+	grep "author .* 12345 +0400$" actual &&
+	git cat-file commit HEAD^ >actual &&
+	grep "author .* 23456 +0500$" actual &&
+	git cat-file commit HEAD >actual &&
+	grep "author .* 34567 +0600$" actual &&
+
+	git rebase --onto HEAD^^ HEAD^ &&
+
+	git cat-file commit HEAD >actual &&
+	grep "author .* 34567 +0600$" actual
+'
+
 test_done