Horton

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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Christoph Berg <myon@debian.org>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2019-06-04T16:43:38Z
Lists: pgsql-hackers 
Hi,

On 2019-06-04 08:53:30 -0700, Andres Freund wrote:
> If I set the system-wide default, using dpkg-reconfigure -plow tzdata,
> to UTC I *do* get Etc/UTC.
> 
> root@alap4:/home/andres/src/postgresql# cat /etc/timezone
> Etc/UTC
> root@alap4:/home/andres/src/postgresql# ls -l /etc/timezone
> -rw-r--r-- 1 root root 8 Jun  4 15:44 /etc/timezone
> 
> selecting default timezone ... Etc/UTC
> 
> This is independent of being built with system or non-system tzdata.
>
> Enabling debugging shows:

Sorry, I was not awake enough while reading the thread (and UCT looks so
similar to UTC).

I do indeed see the behaviour of choosing UCT in 11, but not in
12. Independent of system/non-system tzdata. With system tzdata, I get
the following debug output (after filtering lots of lines wiht out |grep
-v 'scores 0'|grep -v 'uses leap seconds')

TZ "Zulu" gets max score 5200
TZ "UCT" gets max score 5200
TZ "Universal" gets max score 5200
TZ "UTC" gets max score 5200
TZ "Etc/Zulu" gets max score 5200
TZ "Etc/UCT" gets max score 5200
TZ "Etc/Universal" gets max score 5200
TZ "Etc/UTC" gets max score 5200
TZ "localtime" gets max score 5200
TZ "posix/Zulu" gets max score 5200
TZ "posix/UCT" gets max score 5200
TZ "posix/Universal" gets max score 5200
TZ "posix/UTC" gets max score 5200
TZ "posix/Etc/Zulu" gets max score 5200
TZ "posix/Etc/UCT" gets max score 5200
TZ "posix/Etc/Universal" gets max score 5200
TZ "posix/Etc/UTC" gets max score 5200
ok

whereas master only does:

selecting default timezone ... symbolic link "/etc/localtime" contains "/usr/share/zoneinfo/Etc/UTC"
TZ "Etc/UTC" gets max score 5200
Etc/UTC

The reason for the behaviour difference between v12 and 11 is that 12
does:

	/*
	 * Try to avoid the brute-force search by seeing if we can recognize the
	 * system's timezone setting directly.
	 *
	 * Currently we just check /etc/localtime; there are other conventions for
	 * this, but that seems to be the only one used on enough platforms to be
	 * worth troubling over.
	 */
	if (check_system_link_file("/etc/localtime", &tt, resultbuf))
		return resultbuf;

which prevents having to iterate through all of these files, and ending
up with a lot of equivalently scored timezones.

Greetings,

Andres Freund

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.