[Tom Lane <tgl@sss.pgh.pa.us>]

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>.

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.")));

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.

			regards, tom lane

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