Work around portability issue with newer versions of mktime().

Enterprise / PostgreSQL - Tom Lane [sss.pgh.pa.us] - 13 June 2021 18:32 UTC

Recent glibc versions have made mktime() fail if tm_isdst is inconsistent with the prevailing timezone; in particular it fails for tm_isdst = 1 when the zone is UTC. (This seems wildly inconsistent with the POSIX-mandated treatment of "incorrect" values for the other fields of struct tm, so if you ask me it's a bug, but I bet they'll say it's intentional.) This has been observed to cause cosmetic problems when pg_restore'ing an archive created in a different timezone.

To fix, do mktime() using the field values from the archive, and if that fails try again with tm_isdst = -1. This will give a result that's off by the UTC-offset difference from the original zone, but that was true before, too. It's not terribly critical since we don't do anything with the result except possibly print it. (Someday we should flush this entire bit of logic and record a standard-format timestamp in the archive instead. That's not okay for a back-patched bug fix, though.)

Also, guard our only other use of mktime() by having initdb's build_time_t() set tm_isdst = -1 not 0. This case could only have an issue in zones that are DST year-round; but I think some do exist, or could in future.

Per report from Wells Oliver. Back-patch to all supported
versions, since any of them might need to run with a newer glibc.

Discussion: https://postgr.es/m/CAOC+FBWDhDHO7G-i1_n_hjRzCnUeFO+H-Czi1y10mFhRWpBrew@mail.gmail.com

f807e3410f Work around portability issue with newer versions of mktime().
src/bin/initdb/findtimezone.c | 1 +
src/bin/pg_dump/pg_backup_archiver.c | 31 ++++++++++++++++++++++++++-----
2 files changed, 27 insertions(+), 5 deletions(-)

Upstream: git.postgresql.org


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