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Re: Refactoring postmaster's code to cleanup after child exit

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Tomas Vondra <tomas@vondra.me>
Cc: Thomas Munro <thomas.munro@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, Andres Freund <andres@anarazel.de>
Date: 2024-12-09T20:55:45Z
Lists: pgsql-hackers 
On 09/12/2024 14:47, Tomas Vondra wrote:
> On 12/9/24 13:30, Heikki Linnakangas wrote:
>> On 09/12/2024 01:12, Tomas Vondra wrote:
>>> On 11/14/24 15:13, Heikki Linnakangas wrote:
>>>> On 09/10/2024 23:40, Heikki Linnakangas wrote:
>>>>> I pushed the first three patches, with the new test and one of the
>>>>> small
>>>>> refactoring patches. Thanks for all the comments so far! Here is a new
>>>>> version of the remaining patches.
>>>>>
>>> Hi, the TAP test 001_connection_limits.pl introduced by 6a1d0d470e84
>>> seems to have problems with valgrind :-( I reliably get this failure:
>>
>> How exactly do you run the test with valgrind? What platform?
>>
> 
> It failed for me on both amd64 (Fedora 41) and rpi5 32/64-bit (Debian).
> 
>> It works for me, with this:
>>
>> (cd build && ninja && rm -rf tmp_install && meson test --suite setup &&
>> valgrind --leak-check=no --gen-suppressions=all --suppressions=/home/
>> heikki/git-sandbox/postgresql/src/tools/valgrind.supp --time-stamp=yes
>> --error-markers=VALGRINDERROR-BEGIN,VALGRINDERROR-END --log-file=$HOME/
>> pg-valgrind/%p.log --trace-children=yes meson test --suite postmaster )
>>
> 
> I have a patch that tweaks pg_ctl/pg_regress to execute valgrind, so I
> just do
> 
> ./configure --enable-debug --prefix=/home/user/builds/master
> --enable-depend --enable-cassert --enable-tap-tests CPPFLAGS="-O0 -ggdb3
> -DUSE_VALGRIND"
> 
> and then the usual "make check" or whatever.
> 
> The patch has a hardcoded path to the .supp file, and places the
> valgrind log into /tmp. It has worked for me fine up until that commit,
> and it still seems to be working in every other test directory.

Ok, I was able to reproduce this with that setup.

Unsurprisingly, it's a timing issue. It can be reproduced without 
valgrind by adding this delay:

diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 289059435a9..1eb6bad72ca 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -583,6 +583,7 @@ errfinish(const char *filename, int lineno, const 
char *funcname)
  		 * FATAL termination.  The postmaster may or may not consider this
  		 * worthy of panic, depending on which subprocess returns it.
  		 */
+		sleep(1);
  		proc_exit(1);
  	}

The test opens a connection that is expected to fail with the "remaining 
connection slots are reserved for roles with the SUPERUSER attribute" 
error. Right after that, it opens a new connection as superuser, and 
expects it to succeed. But if the previous backend hasn't exited yet, 
the new connection fails with "too many clients already".

Not sure how to fix this. A small sleep in the test would work, but in 
principle there's no delay that's guaranteed to be enough. A more robust 
solution would be to run a "select count(*) from pg_stat_activity" and 
wait until the number of connections are what's expected. I'll try that 
and see how complicated that gets..

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix test name and username used in failed connection attempts

  2. tests: Don't fail due to high default timeout in postmaster/003_start_stop

  3. tests: Fix race condition in postmaster/002_connection_limits

  4. Pass MyPMChildSlot as an explicit argument to child process

  5. Assign a child slot to every postmaster child process

  6. Kill dead-end children when there's nothing else left

  7. Replace postmaster.c's own backend type codes with BackendType

  8. Use an shmem_exit callback to remove backend from PMChildFlags on exit

  9. Add test for dead-end backends

  10. Add test for connection limits

  11. Fix garbled process name on backend crash

  12. Consolidate postmaster code to launch background processes

  13. Fix comment on processes being kept over a restart

  14. Refactor code to handle death of a backend or bgworker in postmaster

  15. Make BackgroundWorkerList doubly-linked

  16. Minor refactoring of assign_backendlist_entry()

  17. Fix outdated comment; all running bgworkers are in BackendList

  18. Allow building with MSVC and Strawberry perl

  19. Fix rare failure in LDAP tests.

  20. Allow notifications to bgworkers without database connections.