Subject: Re: Add Pipelining support in psql

From: "a.kozhemyakin" <[email-masked]>
Date: Wed, 16 Apr 2025 21:31:59 +0700

Hello,

After commit 2cce0fe on master

When executing query:
psql postgres <<EOF
CREATE TABLE psql_pipeline();
\startpipeline
COPY psql_pipeline FROM STDIN;
SELECT 'val1';
\syncpipeline
\getresults
EOF

ERROR:  unexpected message type 0x50 during COPY from stdin
CONTEXT:  COPY psql_pipeline, line 1
Pipeline aborted, command did not run
psql: common.c:1510: discardAbortedPipelineResults: Assertion `res == 
((void *)0) || result_status == PGRES_PIPELINE_ABORTED' failed.
Aborted (core dumped)

The psql crashes with the stack trace:
(gdb) bt
#0  __pthread_kill_implementation (no_tid=0, signo=6, 
threadid=<optimized out>) at ./nptl/pthread_kill.c:44
#1  __pthread_kill_internal (signo=6, threadid=<optimized out>) at 
./nptl/pthread_kill.c:78
#2  __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) 
at ./nptl/pthread_kill.c:89
#3  0x0000760edd24527e in __GI_raise (sig=sig@entry=6) at 
../sysdeps/posix/raise.c:26
#4  0x0000760edd2288ff in __GI_abort () at ./stdlib/abort.c:79
#5  0x0000760edd22881b in __assert_fail_base (fmt=0x760edd3d01e8 
"%s%s%s:%u: %s%sAssertion `%s' failed.\n%n",
     assertion=assertion@entry=0x5ba46ab79850 "res == ((void *)0) || 
result_status == PGRES_PIPELINE_ABORTED", file=file@entry=0x5ba46ab6fcad 
"common.c",
     line=line@entry=1510, function=function@entry=0x5ba46ab9c780 
<__PRETTY_FUNCTION__.3> "discardAbortedPipelineResults") at 
./assert/assert.c:96
#6  0x0000760edd23b517 in __assert_fail 
(assertion=assertion@entry=0x5ba46ab79850 "res == ((void *)0) || 
result_status == PGRES_PIPELINE_ABORTED",
     file=file@entry=0x5ba46ab6fcad "common.c", line=line@entry=1510,
     function=function@entry=0x5ba46ab9c780 <__PRETTY_FUNCTION__.3> 
"discardAbortedPipelineResults") at ./assert/assert.c:105
#7  0x00005ba46ab2bd40 in discardAbortedPipelineResults () at common.c:1510
#8  ExecQueryAndProcessResults (query=query@entry=0x5ba4a2ec1e10 "SELECT 
'val1';", elapsed_msec=elapsed_msec@entry=0x7ffeb07262a8,
     svpt_gone_p=svpt_gone_p@entry=0x7ffeb07262a7, 
is_watch=is_watch@entry=false, min_rows=min_rows@entry=0, 
opt=opt@entry=0x0, printQueryFout=0x0)
     at common.c:1811
#9  0x00005ba46ab2983f in SendQuery (query=0x5ba4a2ec1e10 "SELECT 
'val1';") at common.c:1212
#10 0x00005ba46ab3f66a in MainLoop (source=source@entry=0x760edd4038e0 
<_IO_2_1_stdin_>) at mainloop.c:515
#11 0x00005ba46ab23f2a in process_file (filename=0x0, 
use_relative_path=use_relative_path@entry=false) at command.c:4870
#12 0x00005ba46ab1e9d9 in main (argc=<optimized out>, 
argv=0x7ffeb07269d8) at startup.c:420

06.03.2025 11:20, Michael Paquier пишет:
> On Wed, Mar 05, 2025 at 03:25:12PM +0100, Daniel Verite wrote:
>> 	Anthonin Bonnefoy wrote:
>>> I do see the idea to make it easier to convert existing scripts into
>>> using pipelining. The main focus of the initial implementation was
>>> more on protocol regression tests with psql, so that's not necessarily
>>> something I had in mind.
>> Understood. Yet pipelining can accelerate considerably certain scripts
>> when client-server latency is an issue. We should expect end users to
>> benefit from it too.
> That was not a test case we had in mind originally here, but if it is
> possible to keep the implementation simple while supporting your
> demand, well, let's do it.  If it's not that straight-forward, let's
> use the new meta-command, forbidding \g and \gx based on your
> arguments from upthread.
> --
> Michael

---

From: Michael Paquier <[email-masked]>
Date: Wed, 16 Apr 2025 09:18:01 -0700

On Wed, Apr 16, 2025 at 09:31:59PM +0700, a.kozhemyakin wrote:
> psql: common.c:1510: discardAbortedPipelineResults: Assertion `res == ((void
> *)0) || result_status == PGRES_PIPELINE_ABORTED' failed.
> Aborted (core dumped)

Reproduced here, thanks for the report.  I'll look at that at the
beginning of next week, adding an open item for now.
--
Michael

---

From: Anthonin Bonnefoy <[email-masked]>
Date: Tue, 22 Apr 2025 14:37:19 +0200

On Tue, Apr 22, 2025 at 2:06 AM Michael Paquier <[email-masked]> wrote:
> I am wondering if we could not be smarter with the handling of
> the counters, but I really doubt that there is much more we can do
> under a PGRES_FATAL_ERROR.

One thing that bothers me is that the reported error is silently
discarded within discardAbortedPipelineResults.

psql -f bug_assertion.sql
psql:bug_assertion.sql:7: ERROR:  unexpected message type 0x50 during
COPY from stdin
CONTEXT:  COPY psql_pipeline, line 1
psql:bug_assertion.sql:7: Pipeline aborted, command did not run

This should ideally report the "FATAL:  terminating connection because
protocol synchronization was lost" sent by the backend process.

Also, we still have a triggered assertion failure with the following:
CREATE TABLE psql_pipeline(a text);
\startpipeline
COPY psql_pipeline FROM STDIN;
SELECT 'val1';
\syncpipeline
\endpipeline
...
Assertion failed: (pset.piped_syncs == 0), function
ExecQueryAndProcessResults, file common.c, line 2153.

A possible alternative could be to abort discardAbortedPipelineResults
when we encounter a res != NULL + FATAL error and let the outer loop
handle it. As you said, the pipeline flow is borked so there's not
much to salvage. The outer loop would read and print all error
messages until the closed connection is detected. Then,
CheckConnection will reset the connection which will reset the
pipeline state.

While testing this change, I was initially looking for the "FATAL:
terminating connection because protocol synchronization was lost"
message in the tests. However, this was failing on Windows[1] as the
FATAL message wasn't reported on stderr. I'm not sure why yet.

[1]: https://cirrus-ci.com/task/5051031505076224?logs=check_world#L240-L246