On Darwin, detect and report a multithreaded postmaster.

Darwin --enable-nls builds use a substitute setlocale() that may start a
thread.  Buildfarm member orangutan experienced BackendList corruption
on account of different postmaster threads executing signal handlers
simultaneously.  Furthermore, a multithreaded postmaster risks undefined
behavior from sigprocmask() and fork().  Emit LOG messages about the
problem and its workaround.  Back-patch to 9.0 (all supported versions).

src/backend/postmaster/postmaster.c

@@ -92,6 +92,10 @@
 #include <dns_sd.h>
 #endif
 
+#ifdef HAVE_PTHREAD_IS_THREADED_NP
+#include <pthread.h>
+#endif
+
 #include "access/transam.h"
 #include "access/xlog.h"
 #include "bootstrap/bootstrap.h"
@@ -1105,6 +1109,24 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	RemovePgTempFiles();
 
+#ifdef HAVE_PTHREAD_IS_THREADED_NP
+
+	/*
+	 * On Darwin, libintl replaces setlocale() with a version that calls
+	 * CFLocaleCopyCurrent() when its second argument is "" and every relevant
+	 * environment variable is unset or empty.  CFLocaleCopyCurrent() makes
+	 * the process multithreaded.  The postmaster calls sigprocmask() and
+	 * 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().
+	 */
+	if (pthread_is_threaded_np() != 0)
+		ereport(LOG,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("postmaster became multithreaded during startup"),
+		 errhint("Set the LC_ALL environment variable to a valid locale.")));
+#endif
+
 	/*
 	 * Remember postmaster startup time
 	 */
@@ -1535,6 +1557,15 @@ ServerLoop(void)
 			TouchSocketLockFile();
 			last_touch_time = now;
 		}
+
+#ifdef HAVE_PTHREAD_IS_THREADED_NP
+
+		/*
+		 * With assertions enabled, check regularly for appearance of
+		 * additional threads.  All builds check at start and exit.
+		 */
+		Assert(pthread_is_threaded_np() == 0);
+#endif
 	}
 }
 
@@ -4209,6 +4240,18 @@ SubPostmasterMain(int argc, char *argv[])
 static void
 ExitPostmaster(int status)
 {
+#ifdef HAVE_PTHREAD_IS_THREADED_NP
+
+	/*
+	 * There is no known cause for a postmaster to become multithreaded after
+	 * startup.  Recheck to account for the possibility of unknown causes.
+	 */
+	if (pthread_is_threaded_np() != 0)
+		ereport(LOG,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("postmaster became multithreaded")));
+#endif
+
 	/* should cleanup shared memory and kill all backends */
 
 	/*