"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@lists.postgresql.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.

Noah Misch <noah@leadboat.com> writes:

On Thu, Jan 23, 2025 at 01:22:13PM -0500, Tom Lane wrote:

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.

Ah, I had not realized that. Perhaps the comments about this code
should mention that?

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.

Yeah, seems like overcomplication. Let's go with the simple way.

regards, tom lane

On Thu, Jan 23, 2025 at 01:51:32PM -0500, Tom Lane wrote:

Noah Misch <noah@leadboat.com> writes:

On Thu, Jan 23, 2025 at 01:22:13PM -0500, Tom Lane wrote:

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.

Ah, I had not realized that. Perhaps the comments about this code
should mention that?

Sure, your committed comment works for me.

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.

Yeah, seems like overcomplication. Let's go with the simple way.

postgr.es/c/4f15759bdcddd23e874526a6b2c0ff86e0beb042 looks fine. Thanks.