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

Christoph Berg <myon(at)debian(dot)org> writes:
> In the meantime I realized that I was only testing /etc/timezone
> (which is a plain file with just the zone name), while not touching
> /etc/localtime at all. In this environment, it's a symlink:
> lrwxrwxrwx 1 root root 27 Mär 28 14:49 /etc/localtime -> /usr/share/zoneinfo/Etc/UTC
> ... but the name still gets canonicalized to Etc/UCT or UCT.

Now that I'm home again, I tried to replicate this behavior. I don't
have Debian Buster installed, but I do have an up-to-date Stretch
install, and I can't get it to do this. What I see is that

1. HEAD will follow the spelling appearing in /etc/localtime, if that's
a symlink. It will not pay any attention to /etc/timezone --- but as
far as I can tell, glibc doesn't either. (For instance, if I remove
/etc/localtime, then date(1) starts reporting UTC, independently of
what /etc/timezone might say.)

2. Pre-v12, or if we can't get a valid zone name out of /etc/localtime,
the identify_system_timezone() search settles on "UCT" as being the
shortest and alphabetically first of the various equivalent names for
the zone.

The only way I can get it to pick "Etc/UCT" is if that's what I put
into /etc/localtime. (In which case I maintain that that's not a bug,
or at least not our bug.)

So I'm still mystified by Christoph's report, and am forced to suspect
pilot error -- specifically, /etc/localtime not containing what he said.

Anyway, moving on to the question of what should we do about this,
I don't really have anything better to offer than back-patching 23bd3cec6.
I'm fairly hesitant to do that given the small amount of testing it's
gotten ... but given that it's been in the tree since September, maybe
we can feel like we'd have noticed any really bad problems. I don't have
any use for Andrew's suggestion of looking into zone1970.tab: in the
first place I'm unconvinced that the tzdb guys intend that file to offer
canonical zone names, and in the second place I doubt we can rely on the
file to be present (it's not installed by zic itself), and in the third
place it definitely won't fix this particular issue because it has no
entries for UTC/UCT/GMT etc, only for geographical locations.

Thoughts?

regards, tom lane

PS: As a side note, I do notice an interesting difference between the
timezone database files as they appear on Debian versus what I see on
RHEL or in a PG-generated timezone tree. Debian seems to use symlinks
for multiple equivalent zones:

$ ls -l /usr/share/zoneinfo/U??
-rw-r--r-- 1 root root 127 Mar 27 16:34 /usr/share/zoneinfo/UCT
lrwxrwxrwx 1 root root 3 Mar 27 16:34 /usr/share/zoneinfo/UTC -> UCT
$ ls -l /usr/share/zoneinfo/Etc/U??
lrwxrwxrwx 1 root root 6 Mar 27 16:34 /usr/share/zoneinfo/Etc/UCT -> ../UCT
lrwxrwxrwx 1 root root 6 Mar 27 16:34 /usr/share/zoneinfo/Etc/UTC -> ../UCT

but elsewhere these are hard links:

$ ls -l /usr/share/zoneinfo/U??
-rw-r--r--. 8 root root 118 Mar 26 11:37 /usr/share/zoneinfo/UCT
-rw-r--r--. 8 root root 118 Mar 26 11:37 /usr/share/zoneinfo/UTC
$ ls -l /usr/share/zoneinfo/Etc/U??
-rw-r--r--. 8 root root 118 Mar 26 11:37 /usr/share/zoneinfo/Etc/UCT
-rw-r--r--. 8 root root 118 Mar 26 11:37 /usr/share/zoneinfo/Etc/UTC

However, identify_system_timezone() doesn't treat symlinks differently
from regular files, so this doesn't explain anything about the problem
at hand, AFAICS.