diff --git a/configure b/configure
index 2a8c4fd1f2..fce49088a8 100755
--- a/configure
+++ b/configure
@@ -11366,7 +11366,7 @@ fi
 LIBS_including_readline="$LIBS"
 LIBS=`echo "$LIBS" | sed -e 's/-ledit//g' -e 's/-lreadline//g'`
 
-for ac_func in cbrt dlopen fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll pstat readlink setproctitle setsid shm_open sigprocmask symlink sync_file_range towlower utime utimes wcstombs wcstombs_l
+for ac_func in cbrt dlopen fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll pstat pthread_is_threaded_np readlink setproctitle setsid shm_open sigprocmask symlink sync_file_range towlower utime utimes wcstombs wcstombs_l
 do :
   as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
 ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"
diff --git a/configure.in b/configure.in
index 8ca03ab4ae..6aa69fb19f 100644
--- a/configure.in
+++ b/configure.in
@@ -1265,7 +1265,7 @@ PGAC_FUNC_GETTIMEOFDAY_1ARG
 LIBS_including_readline="$LIBS"
 LIBS=`echo "$LIBS" | sed -e 's/-ledit//g' -e 's/-lreadline//g'`
 
-AC_CHECK_FUNCS([cbrt dlopen fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll pstat readlink setproctitle setsid shm_open sigprocmask symlink sync_file_range towlower utime utimes wcstombs wcstombs_l])
+AC_CHECK_FUNCS([cbrt dlopen fdatasync getifaddrs getpeerucred getrlimit mbstowcs_l memmove poll pstat pthread_is_threaded_np readlink setproctitle setsid shm_open sigprocmask symlink sync_file_range towlower utime utimes wcstombs wcstombs_l])
 
 AC_REPLACE_FUNCS(fseeko)
 case $host_os in
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index c3cff7ccc4..f33c4fed81 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -87,6 +87,10 @@
 #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"
@@ -1200,6 +1204,24 @@ 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
 	 */
@@ -1657,6 +1679,15 @@ ServerLoop(void)
 			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
+
 		/*
 		 * If we already sent SIGQUIT to children and they are slow to shut
 		 * down, it's time to send them SIGKILL.  This doesn't happen
@@ -4745,6 +4776,18 @@ 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 */
 
 	/*
diff --git a/src/common/exec.c b/src/common/exec.c
index b31d621e37..e5575d5f50 100644
--- a/src/common/exec.c
+++ b/src/common/exec.c
@@ -556,8 +556,20 @@ set_pglocale_pgservice(const char *argv0, const char *app)
 
 	/* don't set LC_ALL in the backend */
 	if (strcmp(app, PG_TEXTDOMAIN("postgres")) != 0)
+	{
 		setlocale(LC_ALL, "");
 
+		/*
+		 * One could make a case for reproducing here PostmasterMain()'s test
+		 * for whether the process is multithreaded.  Unlike the postmaster,
+		 * no frontend program calls sigprocmask() or otherwise provides for
+		 * mutual exclusion between signal handlers.  While frontends using
+		 * fork(), if multithreaded, are formally exposed to undefined
+		 * behavior, we have not witnessed a concrete bug.  Therefore,
+		 * complaining about multithreading here may be mere pedantry.
+		 */
+	}
+
 	if (find_my_exec(argv0, my_exec_path) < 0)
 		return;
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 465281c60e..83f04e9382 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -394,6 +394,9 @@
 /* Define to 1 if the PS_STRINGS thing exists. */
 #undef HAVE_PS_STRINGS
 
+/* Define to 1 if you have the `pthread_is_threaded_np' function. */
+#undef HAVE_PTHREAD_IS_THREADED_NP
+
 /* Define to 1 if you have the <pwd.h> header file. */
 #undef HAVE_PWD_H