meson: Make test output much more useful on failure (both in CI and locally)

Hello,

This is really useful!

- my $result = run_log($cmd);
- ok($result, $test_name);
+ # Doesn't rely on detecting end of file on the file descriptors,
+ # which can fail, causing the process to hang, notably on Msys
+ # when used with 'pg_ctl start'

The function comments still refer to run_log, for both command_ok and
command_fails

Hi,

On 2026-01-26 11:13:57 +0100, Jelte Fennema-Nio wrote:

I've gotten totally fed up with the amount of clicking I need to do
every time a test fails in CI to find out what exactly failed. The
output that meson gives in its main output is completely useless
currently.

This doesn't really seem meson specific, right? It just seems about the output
of our own test tooling?

Meson can't really output something that we hide from it...

I agree that what we currently do is pretty unhelpful.

Subject: [PATCH v1 1/5] meson: Include pg_regress diffs in meson output

This isn't about meson, it's about adding a new argument to pg_regress.

I think we'd need to limit the size of the output here somewhat. If e.g. the
server crashes, you end up with all subsequent tests failing and printing out
a couple hundred kB of pointless diff output.

@@ -1521,22 +1523,40 @@ results_differ(const char *testname, const char *resultsfile, const char *defaul
* append to the diffs summary file.
*/

-	/* Write diff header */
-	difffile = fopen(difffilename, "a");
+	difffile = fopen(difffilename, "a+");
if (difffile)
{
+		fseek(difffile, 0, SEEK_END);
+		startpos = ftell(difffile);
fprintf(difffile,
"diff %s %s %s\n",
pretty_diff_opts, best_expect_file, resultsfile);
+		fflush(difffile);
+
+		/* Run diff */
+		snprintf(cmd, sizeof(cmd),
+				 "diff %s \"%s\" \"%s\" >> \"%s\"",
+				 pretty_diff_opts, best_expect_file, resultsfile, difffilename);
+		run_diff(cmd, difffilename);
+
+		/* Emit diff as TAP diagnostics if requested */
+		if (print_diffs)
+		{
+			char		line[1024];
+
+			fseek(difffile, startpos, SEEK_SET);
+			while (fgets(line, sizeof(line), difffile))
+			{
+				size_t		len = strlen(line);
+
+				if (len > 0 && line[len - 1] == '\n')
+					line[len - 1] = '\0';
+				diag("%s", line);
+			}
+		}
fclose(difffile);
}

I'm a bit confused. Why are we rerunning diffs and appending to the diff file?

From c1db07a8bba54e6b903766585f1b521ef526de12 Mon Sep 17 00:00:00 2001
From: Jelte Fennema-Nio <postgres@jeltef.nl>
Date: Mon, 26 Jan 2026 09:09:11 +0100
Subject: [PATCH v1 2/5] perl tap: Show failed command output

This adds the output of failed commands to the TAP output. Before a
failed libpq_pipeline test would look like this:

Failed test 'libpq_pipeline cancel'
at /home/jelte/work/postgres-3/src/test/modules/libpq_pipeline/t/001_libpq_pipeline.pl line 55.

Now you can actually see the reason of the failure:

Failed test 'libpq_pipeline cancel'
at /home/jelte/work/postgres-3/src/test/modules/libpq_pipeline/t/001_libpq_pipeline.pl line 55.
----- command failed -----
libpq_pipeline -r 700 cancel port=14309 host=/tmp/htMib451qD dbname='postgres' max_protocol_version=latest
--------- stderr ---------
test cancellations...
libpq_pipeline:315: unexpected number of rows received: 1
--------------------------
---
src/test/perl/PostgreSQL/Test/Utils.pm | 36 +++++++++++++++++++++++---
1 file changed, 32 insertions(+), 4 deletions(-)

diff --git a/src/test/perl/PostgreSQL/Test/Utils.pm b/src/test/perl/PostgreSQL/Test/Utils.pm
index ff843eecc6e..bd1e981c6f0 100644
--- a/src/test/perl/PostgreSQL/Test/Utils.pm
+++ b/src/test/perl/PostgreSQL/Test/Utils.pm
@@ -955,8 +955,22 @@ sub command_ok
{
local $Test::Builder::Level = $Test::Builder::Level + 1;
my ($cmd, $test_name) = @_;
-	my $result = run_log($cmd);
-	ok($result, $test_name);
+	# Doesn't rely on detecting end of file on the file descriptors,
+	# which can fail, causing the process to hang, notably on Msys
+	# when used with 'pg_ctl start'
+	my $stdoutfile = File::Temp->new();
+	my $stderrfile = File::Temp->new();

I'm afraid that the creation of this many additional tempfiles would slow down
the tests on some platforms...

+	my $result = IPC::Run::run $cmd, '>' => $stdoutfile, '2>' => $stderrfile;
+	ok($result, $test_name) or do
+	{
+		my $stdout = slurp_file($stdoutfile);
+		my $stderr = slurp_file($stderrfile);
+		diag("----- command failed -----");
+		diag(join(" ", @$cmd));
+		diag("--------- stdout ---------"), diag($stdout) if $stdout;
+		diag("--------- stderr ---------"), diag($stderr) if $stderr;
+		diag("--------------------------");
+	};

Outputting the entire file seems to again have the danger of completely
swamping the output.

return;
}

@@ -972,8 +986,22 @@ sub command_fails
{
local $Test::Builder::Level = $Test::Builder::Level + 1;
my ($cmd, $test_name) = @_;
-	my $result = run_log($cmd);
-	ok(!$result, $test_name);
+	# Doesn't rely on detecting end of file on the file descriptors,
+	# which can fail, causing the process to hang, notably on Msys
+	# when used with 'pg_ctl start'

Huh? Any more details?

+	my $stdoutfile = File::Temp->new();
+	my $stderrfile = File::Temp->new();
+	my $result = IPC::Run::run $cmd, '>' => $stdoutfile, '2>' => $stderrfile;
+	ok(!$result, $test_name) or do
+	{
+		my $stdout = slurp_file($stdoutfile);
+		my $stderr = slurp_file($stderrfile);
+		diag("-- command succeeded unexpectedly --");
+		diag(join(" ", @$cmd));
+		diag("-------------- stdout --------------"), diag($stdout) if $stdout;
+		diag("-------------- stderr --------------"), diag($stderr) if $stderr;
+		diag("------------------------------------");
+	};

I don't think it makes sense to duplicate the logic for this multiple times.

From c88f1fbf6462da56a1f0739dd83a6284d6bdd389 Mon Sep 17 00:00:00 2001
From: Jelte Fennema-Nio <postgres@jeltef.nl>
Date: Mon, 26 Jan 2026 09:32:15 +0100
Subject: [PATCH v1 3/5] perl tap: Show die reason in TAP output
...
diff --git a/src/test/perl/PostgreSQL/Test/Utils.pm b/src/test/perl/PostgreSQL/Test/Utils.pm
index bd1e981c6f0..df328d0f571 100644
--- a/src/test/perl/PostgreSQL/Test/Utils.pm
+++ b/src/test/perl/PostgreSQL/Test/Utils.pm
@@ -206,6 +206,14 @@ INIT
# test may still fail, but it's more likely to report useful facts.
$SIG{PIPE} = 'IGNORE';

+	# Emit die messages as TAP diagnostics so they appear in test output.
+	$SIG{__DIE__} = sub {
+		return if $^S;    # Ignore dies inside eval
+		my $msg = shift;
+		chomp $msg;
+		diag("die: $msg");
+	};
+

This looked familiar - turns out I had complained about this in the past and
then forgotten about it :(

/messages/by-id/20220222181924.eehi7o4pmneeb4hm@alap3.anarazel.de

At least at the time I found it necessary to separately test for $^S, to avoid
triggering the logic for syntax errors. Not sure why I concluded that though.

In a reply Andrew suggested to use done_testing(), which avoids the pointless
"Tests were run but no plan was declared and done_testing() was not seen."
which does seem like an improvement.

From a6b324678293d65f3a417b8f66889b3f3f6a9c29 Mon Sep 17 00:00:00 2001
From: Jelte Fennema-Nio <postgres@jeltef.nl>
Date: Mon, 26 Jan 2026 10:04:44 +0100
Subject: [PATCH v1 4/5] perl tap: Include caller in die messages of psql

This way by looking at the error output you can actually figure out
where it occured, instead of just knowing that is in the very often
called psql function.

I think this is too localized a fix. Right now every die in a .pm file has
this issue. The easiest fix would probably be to just replace all uses of die
in .pm files with croak (which we already, inconsistently, use).

stderr:
# die: error running SQL: 'psql:<stdin>:1: ERROR: syntax error at or near "I"
# LINE 1: I am the worst
# ^'
# while running 'psql --no-psqlrc --no-align --tuples-only --quiet --dbname port=20878 host=/tmp/K5V48mzFx9 dbname='postgres' --file - --variable ON_ERROR_STOP=1' with sql 'I am the worst' at /home/andres/src/postgresql/src/bin/scripts/t/020_createdb.pl line 19.

Note that the failure now is reported as coming from the actual test file.

We could also use Test::More->builder->croak(), but that's a heck of a lot
longer, and would require us to be more liberal about sprinkling

local $Test::Builder::Level = $Test::Builder::Level + 1;
around. I don't see an improvement in the output either.

In contrast Carp.pm just looks for a caller outside of the current package. If
we are worried about errors across Cluster.pm/Utils.pm, we could just fix that
in one place by using @CARP_NOT.

Maybe there's a way to do that more centrally?

Greetings,

Andres Freund

On Mon, 26 Jan 2026 at 17:42, Andres Freund <andres@anarazel.de> wrote:

This doesn't really seem meson specific, right? It just seems about the output
of our own test tooling?

Yes, it's general improvements. But the problem is worse in meson than
for because that really only shows the TAP output. The Makefile based
builds will show a bit more information currently.

I think we'd need to limit the size of the output here somewhat. If e.g. the
server crashes, you end up with all subsequent tests failing and printing out
a couple hundred kB of pointless diff output.

Fair enough.

I'm a bit confused. Why are we rerunning diffs and appending to the diff file?

Now I'm confused. We're running the same amount of diffs as before.
Did you miss the removal of the /* Run diff */ below the block that
you quoted?

I'm afraid that the creation of this many additional tempfiles would slow down
the tests on some platforms...

< snip>

@@ -972,8 +986,22 @@ sub command_fails
{
local $Test::Builder::Level = $Test::Builder::Level + 1;
my ($cmd, $test_name) = @_;
-     my $result = run_log($cmd);
-     ok(!$result, $test_name);
+     # Doesn't rely on detecting end of file on the file descriptors,
+     # which can fail, causing the process to hang, notably on Msys
+     # when used with 'pg_ctl start'

Huh? Any more details?

I copy pasted that comment and the pipe to tempfiles from
command_like_safe. I did this after I initially tried to pipe to
variables, but that made the test that we run in SanityCheck get stuck
forever.

I don't get why this is needed either.

+     my $stdoutfile = File::Temp->new();
+     my $stderrfile = File::Temp->new();
+     my $result = IPC::Run::run $cmd, '>' => $stdoutfile, '2>' => $stderrfile;
+     ok(!$result, $test_name) or do
+     {
+             my $stdout = slurp_file($stdoutfile);
+             my $stderr = slurp_file($stderrfile);
+             diag("-- command succeeded unexpectedly --");
+             diag(join(" ", @$cmd));
+             diag("-------------- stdout --------------"), diag($stdout) if $stdout;
+             diag("-------------- stderr --------------"), diag($stderr) if $stderr;
+             diag("------------------------------------");
+     };

I don't think it makes sense to duplicate the logic for this multiple times.

It seemed a bit much to make it generic over the input, as it would
need to remove dashes accordingly. Especially since it only has two
copies.

This looked familiar - turns out I had complained about this in the past and
then forgotten about it :(

/messages/by-id/20220222181924.eehi7o4pmneeb4hm@alap3.anarazel.de

At least at the time I found it necessary to separately test for $^S, to avoid
triggering the logic for syntax errors. Not sure why I concluded that though.

In a reply Andrew suggested to use done_testing(), which avoids the pointless
"Tests were run but no plan was declared and done_testing() was not seen."
which does seem like an improvement.

I'll take a look at doing that.

I think this is too localized a fix. Right now every die in a .pm file has
this issue. The easiest fix would probably be to just replace all uses of die
in .pm files with croak (which we already, inconsistently, use).

I didn't even know that function existed (due to my pretty much
non-existent knowlegde of perl).

On Mon Jan 26, 2026 at 5:42 PM CET, Andres Freund wrote:

I agree that what we currently do is pretty unhelpful.

Attached is v2, which I think fixes all the issues that you mentioned.

Hi,

Thank you for working on this! I think this is a nice improvement.

On Tue, 27 Jan 2026 at 17:48, Jelte Fennema-Nio <postgres@jeltef.nl> wrote:

On Mon Jan 26, 2026 at 5:42 PM CET, Andres Freund wrote:

I agree that what we currently do is pretty unhelpful.

Attached is v2, which I think fixes all the issues that you mentioned.

v2-0001:

This doesn't really read the first 80 lines of the combined diff.
Lines with more than 1023 characters will be counted and printed as
multiple lines. I don't think this is a problem but clearing this
would be helpful. Other than that, it looks good to me.

v2-0002:

I am just curious if test failure can cause file descriptors not being
properly closed and then the IPC::Run:run() call will hang
indefinitely like the pg_ctl case. Other than that, it looks good to
me.

v2-0003 and v2-0004 look good to me.

--
Regards,
Nazir Bilal Yavuz
Microsoft

On Tue, 3 Feb 2026 at 11:20, Nazir Bilal Yavuz <byavuz81@gmail.com> wrote:

v2-0001:

This doesn't really read the first 80 lines of the combined diff.
Lines with more than 1023 characters will be counted and printed as
multiple lines. I don't think this is a problem but clearing this
would be helpful. Other than that, it looks good to me.

I was annoyed by this, so I actually fixed it (manually tested by
changing the buffer to 10 bytes). This meant I needed to introduce
DIAG_DETAIL and DIAG_END, like we already had NOTE_DETAIL and NOTE_END.

v2-0002:

I am just curious if test failure can cause file descriptors not being
properly closed and then the IPC::Run:run() call will hang
indefinitely like the pg_ctl case. Other than that, it looks good to
me.

No, this issue only happens because for the 'pg_ctl restart' process,
its the child postmaster process outlives the pg_ctl process, WHILE
still having the stdout/stderr file descriptor open. As long as all the
subprocesses exit, the file descriptor will be closed by the OS
automatically. So, this problem really only occurs in these daemon
spawning situations. Which for us in practice is only for pg_ctl
start/restart.

On 2026-02-06 Fr 3:44 AM, Jelte Fennema-Nio wrote:

On Tue, 3 Feb 2026 at 11:20, Nazir Bilal Yavuz <byavuz81@gmail.com>
wrote:

v2-0001:

This doesn't really read the first 80 lines of the combined diff.
Lines with more than 1023 characters will be counted and printed as
multiple lines. I don't think this is a problem but clearing this
would be helpful. Other than that, it looks good to me.

I was annoyed by this, so I actually fixed it (manually tested by
changing the buffer to 10 bytes). This meant I needed to introduce
DIAG_DETAIL and DIAG_END, like we already had NOTE_DETAIL and NOTE_END.

v2-0002:

I am just curious if test failure can cause file descriptors not being
properly closed and then the IPC::Run:run() call will hang
indefinitely like the pg_ctl case. Other than that, it looks good to
me.

No, this issue only happens because for the 'pg_ctl restart' process,
its the child postmaster process outlives the pg_ctl process, WHILE
still having the stdout/stderr file descriptor open. As long as all the
subprocesses exit, the file descriptor will be closed by the OS
automatically. So, this problem really only occurs in these daemon
spawning situations. Which for us in practice is only for pg_ctl
start/restart.

I've had a brief look at these. They generally look OK - I didn't see
any obvious issues.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com

- my $result = run_log($cmd);
- ok($result, $test_name);
+ my ($stdout, $stderr);
+ my $result = IPC::Run::run $cmd, '>' => \$stdout, '2>' => \$stderr;
+ ok($result, $test_name) or do
+ {
+ diag("---------- command failed ----------");
+ _diag_command_output($cmd, $stdout, $stderr);
+ };

Previously this printed a line about running the command even on
success, and the new version removes that. Was this intentional? It's
not mentioned in the commit message and that seems like a useful
feature to me.

For example:

Before:
grep "# Run" build/testrun/initdb/001_initdb/log/regress_log_001_initdb | wc -l
35

After:
grep "# Run" build/testrun/initdb/001_initdb/log/regress_log_001_initdb | wc -l
11

On Wed Mar 4, 2026 at 11:56 PM CET, Zsolt Parragi wrote:

Previously this printed a line about running the command even on
success, and the new version removes that. Was this intentional? It's
not mentioned in the commit message and that seems like a useful
feature to me.

Fixed this now. I also fixed two off-by-one errors in counting the
number of outputted lines (found by local AI review).

And I also changed the logic to write to the diff file to close and
re-open, because I realized that on Windows the new logic would create
these errors when trying to write to the diffs file:

[09:00:49.251] stderr:
[09:00:49.251] The process cannot access the file because it is being used by another process.
[09:00:49.251] The process cannot access the file because it is being used by another process.
[09:00:49.251] The process cannot access the file because it is being used by another process.
[09:00:49.251] The process cannot access the file because it is being used by another process.
[09:00:49.251] # 4 of 239 tests failed.
[09:00:49.251] # The differences that caused some tests to fail can be viewed in the file "C:/cirrus/build/testrun/regress/regress/regression.diffs".
[09:00:49.251] # A copy of the test summary that you see above is saved in the file "C:/cirrus/build/testrun/regress/regress/regression.out".
[09:00:49.251]

Finally I added a new commit which starts to use command_ok in
002_pg_upgrade.pl and 027_stream_regress.pl to get the new nicer output.

Finally I added a new commit which starts to use command_ok in
002_pg_upgrade.pl and 027_stream_regress.pl to get the new nicer output.

I tried this out, and am very pleased with the results.

I added an obvious error to a regression output file and, now I see:

# -------------- stderr --------------
# # diff -U3
/home/corey/src/postgres/src/test/regress/expected/stats_import.out
/home/corey/src/postgres/build/testrun/recovery/027_stream_regress/data/results/stats_import.out
# # --- /home/corey/src/postgres/src/test/regress/expected/stats_import.out
2026-03-19 15:20:35.413217825 -0400
# # +++
/home/corey/src/postgres/build/testrun/recovery/027_stream_regress/data/results/stats_import.out
2026-03-19 15:20:57.565988863 -0400
# # @@ -25,8 +25,6 @@
# # 31
# # (1 row)
# #
# # --- OBVIOUS ERROR IS OBVIOUS
# # -
# # -- Create a view that is used purely for the type based on
pg_statistic.
# # CREATE VIEW stats_import.pg_statistic_flat_t AS
# # SELECT
# # 1 of 243 tests failed.
# # The differences that caused some tests to fail can be viewed in the
file
"/home/corey/src/postgres/build/testrun/recovery/027_stream_regress/data/regression.diffs".
# # A copy of the test summary that you see above is saved in the file
"/home/corey/src/postgres/build/testrun/recovery/027_stream_regress/data/regression.out".
# ------------------------------------
# Looks like you failed 1 test of 11.

(test program exited with status code 1)
―――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――

That's great, because ofttimes I want to find the diff -U3 line, replace
the command with "meld" (a graphical diff tool) and then examine the
differences from there.

v4-0001:

+ * useful information is actually in the first few lines

nitpick: comment sentence needs a closing '.'

v4-0002, v4-0003, v4-0004: look good to me

v4-0005: the commit message is probably too bulky to be used as-is, but the
committer was going to change that anyway and right now too much
information is way better than not enough.

I can't wait to use this.

On Thu Mar 19, 2026 at 8:44 PM CET, Corey Huinker wrote:

Finally I added a new commit which starts to use command_ok in
002_pg_upgrade.pl and 027_stream_regress.pl to get the new nicer output.

I tried this out, and am very pleased with the results.

Thanks for the review.

nitpick: comment sentence needs a closing '.'

Fixed

v4-0005: the commit message is probably too bulky to be used as-is, but the
committer was going to change that anyway and right now too much
information is way better than not enough.

Shortened it using your example run.

I can't wait to use this.

Me too :) I've also added the Author/Reviewed-By/Discussion footers to
the commits to make the committers job easier.

Shortened it using your example run.

Glad I was able to help.

I can't wait to use this.

Me too :) I've also added the Author/Reviewed-By/Discussion footers to
the commits to make the committers job easier.

I've held off on doing that in my proposed commits so as not to be
presumptuous, but I can see where having it available would be a
convenience for the committer. This will be a good test of that.

Applies clean to master, passes tests. Ship it.

On 2026-03-20 Fr 2:13 PM, Corey Huinker wrote:

Shortened it using your example run.

Glad I was able to help.

I can't wait to use this.

Me too :) I've also added the Author/Reviewed-By/Discussion footers to
the commits to make the committers job easier.

I've held off on doing that in my proposed commits so as not to be
presumptuous, but I can see where having it available would be a
convenience for the committer. This will be a good test of that.

Applies clean to master, passes tests. Ship it.

Committed with minor tidy up. The main change was to add a @CARP_NOT
setting in Utils.pm, so that croak() would look back past Cluster.pm to
the TAP script caller.

cheers

andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com

On Thu, 2 Apr 2026 at 14:26, Andrew Dunstan <andrew@dunslane.net> wrote:

Committed with minor tidy up. The main change was to add a @CARP_NOT setting in Utils.pm, so that croak() would look back past Cluster.pm to the TAP script caller.

Thanks!

On 02.04.26 14:25, Andrew Dunstan wrote:

On 2026-03-20 Fr 2:13 PM, Corey Huinker wrote:

Shortened it using your example run.

Glad I was able to help.

I can't wait to use this.

Me too :) I've also added the Author/Reviewed-By/Discussion footers to
the commits to make the committers job easier.

I've held off on doing that in my proposed commits so as not to be
presumptuous, but I can see where having it available would be a
convenience for the committer. This will be a good test of that.

Applies clean to master, passes tests. Ship it.

Committed with minor tidy up. The main change was to add a @CARP_NOT
setting in Utils.pm, so that croak() would look back past Cluster.pm to
the TAP script caller.

I would like to register a vote against this new behavior:

pg_regress: Include diffs in TAP output

When pg_regress fails it is often tedious to find the actual diffs,
especially in CI where you must navigate a file browser. Emit the
first
80 lines of the combined regression.diffs as TAP diagnostics so the
failure reason is visible directly in the test output.

I find this annoying.

What happens, and this is admittedly my particular experience, is that
the diff lines are wider than the terminal width, and so 80 lines in the
file might end up being 200 lines on screen, and then the summary of the
test failure disappears from the screen and the diff output is garbled
and useless, and so the whole output is now less useful than before.

I could see this maybe being useful if the entire diff file is, say,
less than 50 lines. But I don't see how seeing a truncated diff by
default can be useful.

The commit message makes reference to "especially on CI". Maybe this
new behavior should be triggered by being on CI, or the output not being
a terminal, or something like that.

On Tue, 7 Apr 2026 at 15:15, Peter Eisentraut <peter@eisentraut.org> wrote:

I could see this maybe being useful if the entire diff file is, say,
less than 50 lines. But I don't see how seeing a truncated diff by
default can be useful.

The idea behind that is that if it's a big diff it's often because the
server crashed. And the top of the diff will then tell you which query
crashed the server. Similarly if some important command like CREATE
TABLE failed, the diff after that is also useless.

Are you saying that 80 diff lines are too many? And to limit it to 40
or 50 instead by default. Could you try changing the number locally
and if that would address the annoyance? Or would it only be solved by
having no diff output at all? And would being able to configure the
amount of lines using a env variable help?

The commit message makes reference to "especially on CI". Maybe this
new behavior should be triggered by being on CI, or the output not being
a terminal, or something like that.

I definitely want the diff in my terminal too, when testing locally.
There are far too many regression.diffs file in my build directory.

On 07.04.26 16:25, Jelte Fennema-Nio wrote:

On Tue, 7 Apr 2026 at 15:15, Peter Eisentraut <peter@eisentraut.org> wrote:

I could see this maybe being useful if the entire diff file is, say,
less than 50 lines. But I don't see how seeing a truncated diff by
default can be useful.

The idea behind that is that if it's a big diff it's often because the
server crashed. And the top of the diff will then tell you which query
crashed the server.

That is not my experience. Often times, when a test out of a group of
20 crashes, then the crashing test will hang for a while while the core
file is being written out, while the other 19 tests fail immediately
because of the server restart. And so the top of the diff will just
show all the other tests aborting, not the actual crashing test.

Are you saying that 80 diff lines are too many? And to limit it to 40
or 50 instead by default. Could you try changing the number locally
and if that would address the annoyance? Or would it only be solved by
having no diff output at all? And would being able to configure the
amount of lines using a env variable help?

80 is too many. I have a reasonably big monitor, and my echo $LINES
says 60. On the laptop screen it's 40. And you need to leave some room
so that the summary of the test run can be shown, so like 10 lines. So
I think 25 or 30 is about the maximum that would be reasonable. (Or you
actually take getenv("LINES") into account.)

But again, one of the problems is that the diff lines could be longer
than the terminal width, and then these computations no longer work. If
you look at the diffs with "less" or similar, then you can manage that.
If you could print the start of the diff while truncating it to the
terminal line length, then that could work.

Independent of that, there appears to be a technical problem with the
implementation. Consider a recent failed build on the buildfarm:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=dodo&amp;dt=2026-04-07%2012%3A45%3A07

The buildfarm output actually handles this quite nicely: The bottom of
the output contains the test result summary, and if you want to know
more detail, you click on the links right below.

But note that the additional diff output appears somewhere in the middle
of the test run. There appears to be a problem synchronizing multiple
output streams.

(But also note that this new feature is possibly redundant with what the
buildfarm output already provides, so it might be worth configuring this
differently for the buildfarm client.)