Re: "postmaster became multithreaded" is reachable

On Thu, Jan 23, 2025 at 01:22:13PM -0500, Tom Lane wrote:
> ExitPostmaster() says:
>
> /*
> * There is no known cause for a postmaster to become multithreaded after
> * startup. Recheck to account for the possibility of unknown causes.
> * This message uses LOG level, because an unclean shutdown at this point
> * would usually not look much different from a clean shutdown.
> */
> if (pthread_is_threaded_np() != 0)
> ereport(LOG,
> (errcode(ERRCODE_INTERNAL_ERROR),
> errmsg_internal("postmaster became multithreaded"),
> errdetail("Please report this to <%s>.", PACKAGE_BUGREPORT)));
>
> We now have a report[1] of a way to trigger this error message.
> It seems to boil down to "cause an error before PostmasterMain
> has reached its test of pthread_is_threaded_np()", for example
>
> $ # build with --enable-nls, using MacPorts' gettext package
> $ unset LANG
> $ postgres -D /foo
> postgres: could not access directory "/foo": No such file or directory
> Run initdb or pg_basebackup to initialize a PostgreSQL data directory.
> 2025-01-23 18:16:03.843 GMT [41524] LOG: postmaster became multithreaded
> 2025-01-23 18:16:03.843 GMT [41524] DETAIL: Please report this to <pgsql-bugs(at)lists(dot)postgresql(dot)org>.

That makes sense. Thanks for confirming it.

> I suggest therefore that we remove the "Please report a bug"
> bit and instead use the much more helpful hint appearing in
> PostmasterMain:
>
> errhint("Set the LC_ALL environment variable to a valid locale.")));

That suffices. It took a lot of years for the "Please report" message to
yield a report, so I'm not much concerned about losing reports from this
change.

> An alternative idea (that maybe should also be done in PostmasterMain)
> is to report the HINT only #ifdef __darwin__ and on other platforms
> use the "Please report a bug" text.

I gather pthread_is_threaded_np() is still macOS-only, in which case the
!darwin branch would be future-proofing only.

Another alternative would be to track whether we reached the startup-time
check, emitting a message at exit only if we did reach the startup-time check.
If a postmaster passes the startup-time check and fails the exit-time check,
changing the environment won't help. Even so, I'm content with your first
proposal.

> [1] https://www.postgresql.org/message-id/18783-d1873b95a59b9103%40postgresql.org