From: Tom Lane <tgl@sss.pgh.pa.us>
To: Noah Misch <noah@leadboat.com>
cc: Andres Freund <andres@anarazel.de>, pgsql-hackers@postgresql.org
Subject: Re: BackgroundPsql swallowing errors on windows
In-reply-to: <20250216223951.a3.nmisch@google.com>
References: <wmovm6xcbwh7twdtymxuboaoarbvwj2haasd3sikzlb3dkgz76@n45rzycluzft> <20250216173943.b6.nmisch@google.com> <h6t2voqv6vm4xk4oh5q6og676h7ehv2opbvr2id7m2xot3adh5@t7su2dmuo3bs> <20250216184740.55.nmisch@google.com> <qo3x3ycl5fokh4tumm3lo6rn6ijvl7mmes7k24fm3kuv4xaupt@px4vl5ocnvth> <20250216223951.a3.nmisch@google.com>
Comments: In-reply-to Noah Misch <noah@leadboat.com>
	message dated "Sun, 16 Feb 2025 14:39:51 -0800"
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Date: Sun, 16 Feb 2025 18:18:44 -0500
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Noah Misch <noah@leadboat.com> writes:
> From the slow proxy's perspective, it can't rule out the program under t=
est
> having done those two write() calls.  The proxy doesn't have enough
> information to reconstruct the original four write() calls.  What preven=
ts
> that anomaly?

Yeah, I think it's hopeless to expect that we can disambiguate the
order of writes to two different pipes.  For the problem at hand,
though, it seems like we don't really need to do that.  Rather, the
question is "when we detect that the program-under-test has exited,
can we be sure we have collected all of its output?".  I think that
IPC::Run may be screwing up here, because I have seen non-Windows
CI failures that look like it didn't read all the stderr output.
For example, this pgbench test failure on macOS from [1]:

# Running: pgbench -n -t 1 -Dfoo=3Dbla -Dnull=3Dnull -Dtrue=3Dtrue -Done=3D=
1 -Dzero=3D0.0 -Dbadtrue=3DtrueXXX -Dmaxint=3D9223372036854775807 -Dminint=
=3D-9223372036854775808 -M prepared -f /Users/admin/pgsql/build/testrun/pg=
bench/001_pgbench_with_server/data/t_001_pgbench_with_server_main_data/001=
_pgbench_error_shell_bad_command
[17:27:47.408](0.061s) ok 273 - pgbench script error: shell bad command st=
atus (got 2 vs expected 2)
[17:27:47.409](0.000s) ok 274 - pgbench script error: shell bad command st=
dout /(?^:processed: 0/1)/
[17:27:47.409](0.000s) not ok 275 - pgbench script error: shell bad comman=
d stderr /(?^:\(shell\) .* meta-command failed)/
[17:27:47.409](0.000s) #   Failed test 'pgbench script error: shell bad co=
mmand stderr /(?^:\(shell\) .* meta-command failed)/'
#   at /Users/admin/pgsql/src/bin/pgbench/t/001_pgbench_with_server.pl lin=
e 1466.
#                   ''
#     doesn't match '(?^:\(shell\) .* meta-command failed)'

The program's exited with a failure code as expected, and we saw (some
of?) the expected stdout output, but stderr output is reported to be
empty.

			regards, tom lane

[1] https://cirrus-ci.com/task/6221238034497536