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).

3 files changed, 58 insertions, 0 deletions

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 3dbf82b339d..d294f9e0af2 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -91,6 +91,10 @@
 #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"
@@ -1233,6 +1237,24 @@ 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
 	 */
@@ -1675,6 +1697,15 @@ ServerLoop(void)
 			TouchSocketLockFiles();
 			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
 	}
 }
 
@@ -4632,6 +4663,18 @@ 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/include/pg_config.h.in b/src/include/pg_config.h.in
index ce48dfb691c..b6a6a48807e 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -387,6 +387,9 @@
 /* Define if you have POSIX threads libraries and header files. */
 #undef HAVE_PTHREAD
 
+/* 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
 
diff --git a/src/port/exec.c b/src/port/exec.c
index 8ce7c618ffc..077ac58b2b2 100644
--- a/src/port/exec.c
+++ b/src/port/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;