Horton

Re: UCT (Re: pgsql: Update time zone data files to tzdata release 2019a.)

Andrew Gierth <andrew@tao11.riddles.org.uk>

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From: Andrew Gierth <andrew@tao11.riddles.org.uk>
To: Christoph Berg <myon@debian.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2019-06-04T10:20:14Z
Lists: pgsql-hackers 
>>>>> "Christoph" == Christoph Berg <myon@debian.org> writes:

 >> Etc/UCT is now a backward-compatibility link to Etc/UTC, instead of
 >> being a separate zone that generates the abbreviation "UCT", which
 >> nowadays is typically a typo. Postgres will still accept "UCT" as an
 >> input zone name, but it won't output it.

 Christoph> There is something wrong here. On Debian Buster/unstable,
 Christoph> using system tzdata (2019a-1), if /etc/timezone is
 Christoph> "Etc/UTC":

 Christoph> 11.3's initdb adds timezone = 'UCT' to postgresql.conf
 Christoph> 12beta1's initdb add timezone = 'Etc/UCT' to postgresql.conf

 Christoph> Is that expected behavior?

It's clearly not what users expect and it's clearly the wrong thing to
do, though it's the expected behavior of the current code:

 * On most systems, we rely on trying to match the observable behavior of
 * the C library's localtime() function.  The database zone that matches
 * furthest into the past is the one to use.  Often there will be several
 * zones with identical rankings (since the IANA database assigns multiple
 * names to many zones).  We break ties arbitrarily by preferring shorter,
 * then alphabetically earlier zone names.

I believe I pointed out a long, long time ago that this tie-breaking
strategy was insane, and that the rule should be to prefer canonical
names and use something else only in the case of a strictly better
match.

If TZ is set or if /etc/localtime is a symlink rather than a hardlink or
copy of the zone file, then PG can get the zone name directly rather
than having to do the comparisons, so the above comment doesn't apply;
that gives you a workaround.

-- 
Andrew (irc:RhodiumToad)

Commits

  1. Tweak our special-case logic for the IANA "Factory" timezone.

  2. Avoid choosing "localtime" or "posixrules" as TimeZone during initdb.

  3. Prefer timezone name "UTC" over alternative spellings.

  4. Attempt to identify system timezone by reading /etc/localtime symlink.

  5. Update time zone data files to tzdata release 2019a.