Don't ask for bug reports about pthread_is_threaded_np() != 0.

We thought that this condition was unreachable in ExitPostmaster,
but actually it's possible if you have both a misconfigured locale
setting and some other mistake that causes PostmasterMain to bail
out before reaching its own check of pthread_is_threaded_np().

Given the lack of other reports, let's not ask for bug reports if
this occurs; instead just give the same hint as in PostmasterMain.

Bug: #18783
Reported-by: anani191181515@gmail.com
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/18783-d1873b95a59b9103@postgresql.org
Discussion: https://postgr.es/m/206317.1737656533@sss.pgh.pa.us
Backpatch-through: 13

src/backend/postmaster/postmaster.c

@@ -1382,6 +1382,8 @@ PostmasterMain(int argc, char *argv[])
 	 * calls fork() without an immediate exec(), both of which have undefined
 	 * behavior in a multithreaded program.  A multithreaded postmaster is the
 	 * normal case on Windows, which offers neither fork() nor sigprocmask().
+	 * Currently, macOS is the only platform having pthread_is_threaded_np(),
+	 * so we need not worry whether this HINT is appropriate elsewhere.
 	 */
 	if (pthread_is_threaded_np() != 0)
 		ereport(FATAL,
@@ -5158,15 +5160,16 @@ ExitPostmaster(int status)
 
 	/*
 	 * There is no known cause for a postmaster to become multithreaded after
-	 * startup.  Recheck to account for the possibility of unknown causes.
+	 * startup.  However, we might reach here via an error exit before
+	 * reaching the test in PostmasterMain, so provide the same hint as there.
 	 * 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)));
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("postmaster became multithreaded"),
+				 errhint("Set the LC_ALL environment variable to a valid locale.")));
 #endif
 
 	/* should cleanup shared memory and kill all backends */