strbuf: make strbuf_addftime more robust

The return value of strftime is poorly designed; when it
returns 0, the caller cannot tell if the buffer was not
large enough, or if the output was actually 0 bytes. In the
original implementation of strbuf_addftime, we simply punted
and guessed that our 128-byte hint would be large enough.

We can do better, though, if we're willing to treat strftime
like less of a black box. We can munge the incoming format
to make sure that it never produces 0-length output, and
then "fix" the resulting output.  That lets us reliably grow
the buffer based on strftime's return value.

Clever-idea-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
index c7f368c..7c9bec7 100755
--- a/t/t6300-for-each-ref.sh
+++ b/t/t6300-for-each-ref.sh
@@ -235,6 +235,16 @@ test_expect_success 'Check format of strftime date fields' '
 	test_cmp expected actual
 '
 
+test_expect_success 'exercise strftime with odd fields' '
+	echo >expected &&
+	git for-each-ref --format="%(authordate:format:)" refs/heads >actual &&
+	test_cmp expected actual &&
+	long="long format -- $_z40$_z40$_z40$_z40$_z40$_z40$_z40" &&
+	echo $long >expected &&
+	git for-each-ref --format="%(authordate:format:$long)" refs/heads >actual &&
+	test_cmp expected actual
+'
+
 cat >expected <<\EOF
 refs/heads/master
 refs/remotes/origin/master