postgres_fdw fails because GMT != UTC

On Thu, 2024-04-04 at 02:19 -0400, Tom Lane wrote:

ERROR:  invalid value for parameter "TimeZone": "UTC"

I am not quite clear on how broken an installation needs to be to
reject "UTC" as a time zone setting, except that the breakage cannot
be subtle.  However, I notice that our code in pgtz.c and other
places treats "GMT" as a hard-wired special case ... but not "UTC".
I wonder if we ought to modify those places to force "UTC" down the
same hard-wired paths.  If we acted like that, this would have worked
no matter how misconfigured the installation was.

An alternative answer could be to change postgres_fdw to send "GMT"
not "UTC".  That's ugly from a standards-compliance viewpoint, but
it would fix this problem even with a non-updated remote server,
and I think postgres_fdw is generally intended to work with even
very old remote servers.

Or we could do both.

I think the first is desirable for reasons of general sanity, and the
second for best compatibility with old versions.

So I vote for "both".

Yours,
Laurenz Albe

On 4 Apr 2024, at 08:19, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Over at [1] we have a complaint of postgres_fdw failing with
a remote-server error

ERROR: invalid value for parameter "TimeZone": "UTC"

I am not quite clear on how broken an installation needs to be to
reject "UTC" as a time zone setting, except that the breakage cannot
be subtle. However, I notice that our code in pgtz.c and other
places treats "GMT" as a hard-wired special case ... but not "UTC".
I wonder if we ought to modify those places to force "UTC" down the
same hard-wired paths. If we acted like that, this would have worked
no matter how misconfigured the installation was.

+1. It makes little sense to support GMT like that but not UTC.

An alternative answer could be to change postgres_fdw to send "GMT"
not "UTC". That's ugly from a standards-compliance viewpoint, but
it would fix this problem even with a non-updated remote server,
and I think postgres_fdw is generally intended to work with even
very old remote servers.

There is always a risk in accomodating broken installations that it might hide
other subtle bugs, but off the cuff that risk seems quite low in this case.

--
Daniel Gustafsson

On Thu, Apr 4, 2024 at 3:49 PM Laurenz Albe <laurenz.albe@cybertec.at> wrote:

On Thu, 2024-04-04 at 02:19 -0400, Tom Lane wrote:

ERROR: invalid value for parameter "TimeZone": "UTC"

I am not quite clear on how broken an installation needs to be to
reject "UTC" as a time zone setting, except that the breakage cannot
be subtle. However, I notice that our code in pgtz.c and other
places treats "GMT" as a hard-wired special case ... but not "UTC".
I wonder if we ought to modify those places to force "UTC" down the
same hard-wired paths. If we acted like that, this would have worked
no matter how misconfigured the installation was.

An alternative answer could be to change postgres_fdw to send "GMT"
not "UTC". That's ugly from a standards-compliance viewpoint, but
it would fix this problem even with a non-updated remote server,
and I think postgres_fdw is generally intended to work with even
very old remote servers.

Or we could do both.

I think the first is desirable for reasons of general sanity, and the
second for best compatibility with old versions.

So I vote for "both".

+1 for both (assuming that the latter does not make the postgres_fdw
code complicated).

Best regards,
Etsuro Fujita

Etsuro Fujita <etsuro.fujita@gmail.com> writes:

On Thu, Apr 4, 2024 at 3:49 PM Laurenz Albe <laurenz.albe@cybertec.at> wrote:

On Thu, 2024-04-04 at 02:19 -0400, Tom Lane wrote:

I am not quite clear on how broken an installation needs to be to
reject "UTC" as a time zone setting, except that the breakage cannot
be subtle. However, I notice that our code in pgtz.c and other
places treats "GMT" as a hard-wired special case ... but not "UTC".
I wonder if we ought to modify those places to force "UTC" down the
same hard-wired paths. If we acted like that, this would have worked
no matter how misconfigured the installation was.

An alternative answer could be to change postgres_fdw to send "GMT"
not "UTC". That's ugly from a standards-compliance viewpoint, but
it would fix this problem even with a non-updated remote server,
and I think postgres_fdw is generally intended to work with even
very old remote servers.

Or we could do both.

+1 for both (assuming that the latter does not make the postgres_fdw
code complicated).

I looked briefly at changing the server like this, and decided that
it would be a little invasive, if only because there would be
documentation and such to update. Example question: should we change
the boot-time default value of the timezone GUC from "GMT" to "UTC"?
Probably, but I doubt we want to back-patch that, nor does it seem
like something to be messing with post-feature-freeze. So I'm
in favor of working on that when the tree opens for v18, but not
right now.

However, we can change postgres_fdw at basically no cost AFAICS.
That's the more important part anyway I think. If your own server
burps because it's got a bad timezone database, you are probably in
a position to do something about that, while you may have no control
over a remote server. (As indeed the original complainant didn't.)

So I propose to apply and back-patch the attached, and leave
it at that for now.

regards, tom lane