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Try to handle torn reads of pg_control in frontend.
- 5725e4ebe7a9 16.1 cited

Incorrect checksum in control file with pg_rewind test

Maksim.Melnikov <m.melnikov@postgrespro.ru> — 2025-09-04T15:18:30Z

Hi, hackers!

I've got test failure for pg_rewind tests and it seems we have
read/write races
for pg_control file. The test error is incorrect checksum in control file.
Build was compiled with -DEXEC_BACKEND flag.

# +++ tap check in src/bin/pg_rewind +++
Bailout called. Further testing stopped: pg_ctl start failed
t/001_basic.pl ...............
Dubious, test returned 255 (wstat 65280, 0xff00)
All 20 subtests passed

2025-05-07 15:00:39.353 MSK [2002308] LOG: starting backup recovery
with redo LSN 0/2000028, checkpoint LSN 0/2000070, on timeline ID 1
2025-05-07 15:00:39.354 MSK [2002307] FATAL: incorrect checksum in
control file
2025-05-07 15:00:39.354 MSK [2002308] LOG: redo starts at 0/2000028
2025-05-07 15:00:39.354 MSK [2002308] LOG: completed backup recovery
with redo LSN 0/2000028 and end LSN 0/2000138
2025-05-07 15:00:39.354 MSK [2002301] LOG: background writer process
(PID 2002307) exited with exit code 1
2025-05-07 15:00:39.354 MSK [2002301] LOG: terminating any other active
server processes
2025-05-07 15:00:39.355 MSK [2002301] LOG: shutting down because
restart_after_crash is off
2025-05-07 15:00:39.356 MSK [2002301] LOG: database system is shut down
# No postmaster PID for node "primary_remote"
[15:00:39.438](0.238s) Bail out! pg_ctl start failed

Failure occurred during restart the primary node to check that rewind
went correctly.
Error is very rare and difficult to reproduce.

It seems we have race between process that replays WAL on start and
update control
file and other sub-processes that read control file and were started
with exec.
As the result sub-processes can read partially updated file with
incorrect crc.
The reason is that LocalProcessControlFile don't acquire ControlFileLock
and it
can't do it.

I found thread
https://www.postgresql.org/message-id/flat/20221123014224.xisi44byq3cf5psi%40awork3.anarazel.de,
where the similiar issue was discussed for frontend programs. The
decision was
to retry control file read in case of crc failures. Details can be found
in commit
5725e4ebe7a936f724f21e7ee1e84e54a70bfd83. My suggestion is to use this
approach
here. Patch is attached.

Best regards,
Maksim Melnikov

Re: Incorrect checksum in control file with pg_rewind test

Maksim.Melnikov <m.melnikov@postgrespro.ru> — 2025-11-07T15:19:39Z

Hi,
just to clarify, it isn't pg_rewind related issue and can fire 
spontaneously.
I don't have any strong scenario how to reproduce it, tests sometimes 
fired on our local CI, but as you can see on thread [1],
where the same issue for frontends was discussed, it is very hard to 
reproduce and there wasn't scenario how to do it too.

Some dirty hacks to reproduce it was described here [2], and I've tried 
it on master branch:
First of all I applied patch 
0001-XXX-Dirty-hack-to-clobber-control-file-for-testing.patch from [2], 
then compile app with
-DEXEC_BACKEND and exec command in psql
do $$ begin loop perform pg_update_control_file(); end loop; end; $$;
Also I've run pgbench command
for run in {1..5000}; do pgbench -c50 -t100 -j6 -S postgres ; done
And eventually got error

2025-11-07 17:58:33.139 MSK [2472504] FATAL:  incorrect checksum in 
control file
2025-11-07 17:58:33.141 MSK [2472501] LOG:  could not receive data from 
client: Connection reset by peer
2025-11-07 17:58:33.143 MSK [2472505] LOG:  could not send data to 
client: Broken pipe
2025-11-07 17:58:33.143 MSK [2472505] FATAL:  connection to client lost

Best regards,
Maksim Melnikov

[1] 
https://www.postgresql.org/message-id/flat/20221123014224.xisi44byq3cf5psi%40awork3.anarazel.de
[2] 
https://www.postgresql.org/message-id/CA%2BhUKGK-BEe38aKNqHJDQ86LUW-CMwF5F9bo1JtJVg71FoDv_w%40mail.gmail.com
[3] 
https://www.postgresql.org/message-id/f59335a4-83ff-438a-a30e-7cf2200276b6%40postgrespro.ru

Re: Incorrect checksum in control file with pg_rewind test

Ivan Kovmir <ivan.kovmir@cybertec.at> — 2025-11-14T13:15:10Z

I can reproduce the bug on the master branch with the following steps:

1. Apply 0001-XXX-Dirty-hack-to-clobber-control-file-for-testing.patch [1]
2. Compile PostgreSQL with `-DEXEC_BACKEND` C compiler flag option.
3. Run `initdb`
4. Run `postgres`
5. Run `pgbench -i`
6. Run `psql -c 'do $$ begin loop perform pg_update_control_file(); end
loop; end; $$;'`
7. Run `for run in {1..5000}; do pgbench -c50 -t100 -j6 -S postgres; done`
in parallel with the previous command.
8. Wait a while.

[1]
https://www.postgresql.org/message-id/CA%2BhUKGK-BEe38aKNqHJDQ86LUW-CMwF5F9bo1JtJVg71FoDv_w%40mail.gmail.com

Re: Incorrect checksum in control file with pg_rewind test

Maksim.Melnikov <m.melnikov@postgrespro.ru> — 2026-05-12T08:30:55Z

On 4/21/26 15:12, Alexander Korotkov wrote:
> Thank you for spotting this issue and proposing a patch.  The fork
> builds don't have this problem, because fork replicated contents of
> LocalControlFile to the new process.  And the postmaster has
> consistent snapshot of control file as there is no concurrent process
> which could write it and that moment.  But EXEC_BACKEND, even with
> your patch, may end up different processes with different contents of
> LocalControlFile.  I don't see it could cause a material bug right
> now, but I see this as undesirable divergence between fork and
> EXEC_BACKEND behaviors.  I propose an alternative approach copy the
> contents of control file to the new process via BackendParameters.
> This approach solves two problems at once: no torn reads, and no
> divergence between fork and EXEC_BACKEND.

Hi, Alexander!

Thanks for response! Your patch LGTM.

Best regards,
Maksim Melnikov