local_tzoffset: detect errors from tm_to_time_t

When we want to know the local timezone offset at a given
timestamp, we compute it by asking for localtime() at the
given time, and comparing the offset to GMT at that time.
However, there's some juggling between time_t and "struct
tm" which happens, which involves calling our own
tm_to_time_t().

If that function returns an error (e.g., because it only
handles dates up to the year 2099), it returns "-1", which
we treat as a time_t, and is clearly bogus, leading to
bizarre timestamps (that seem to always adjust the time back
to (time_t)(uint32_t)-1, in the year 2106).

It's not a good idea for local_tzoffset() to simply die
here; it would make it hard to run "git log" on a repository
with funny timestamps. Instead, let's just treat such cases
as "zero offset".

Reported-by: Norbert Kiesel <nkiesel@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/date.c b/date.c
index 7c9f76998a..4c7aa9ba85 100644
--- a/date.c
+++ b/date.c
@@ -74,6 +74,8 @@ static int local_tzoffset(unsigned long time)
 	localtime_r(&t, &tm);
 	t_local = tm_to_time_t(&tm);
 
+	if (t_local == -1)
+		return 0; /* error; just use +0000 */
 	if (t_local < t) {
 		eastwest = -1;
 		offset = t - t_local;