Use FD_CLOEXEC on ListenSockets

It's good hygiene if e.g. an extension launches a subprogram when
being loaded. We went through some effort to close them in the child
process in EXEC_BACKEND mode, but it's better to not hand them down to
the child process in the first place. We still need to close them
after fork when !EXEC_BACKEND, but it's a little simpler.

In the passing, LOG a message if closing the client connection or
listen socket fails. Shouldn't happen, but if it does, would be nice
to know.

Reviewed-by: Tristan Partin, Andres Freund, Thomas Munro
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi

src/backend/libpq/pqcomm.c

@@ -458,6 +458,9 @@ StreamServerPort(int family, const char *hostName, unsigned short portNumber,
 		}
 
 #ifndef WIN32
+		/* Don't give the listen socket to any subprograms we execute. */
+		if (fcntl(fd, F_SETFD, FD_CLOEXEC) < 0)
+			elog(FATAL, "fcntl(F_SETFD) failed on socket: %m");
 
 		/*
 		 * Without the SO_REUSEADDR flag, a new postmaster can't be started
@@ -831,7 +834,8 @@ StreamConnection(pgsocket server_fd, Port *port)
 void
 StreamClose(pgsocket sock)
 {
-	closesocket(sock);
+	if (closesocket(sock) != 0)
+		elog(LOG, "could not close client or listen socket: %m");
 }
 
 /*

src/backend/postmaster/postmaster.c

@@ -496,7 +496,6 @@ typedef struct
 	Port		port;
 	InheritableSocket portsocket;
 	char		DataDir[MAXPGPATH];
-	pgsocket	ListenSocket[MAXLISTEN];
 	int32		MyCancelKey;
 	int			MyPMChildSlot;
 #ifndef WIN32
@@ -2545,8 +2544,6 @@ ConnFree(Port *port)
 void
 ClosePostmasterPorts(bool am_syslogger)
 {
-	int			i;
-
 	/* Release resources held by the postmaster's WaitEventSet. */
 	if (pm_wait_set)
 	{
@@ -2573,8 +2570,12 @@ ClosePostmasterPorts(bool am_syslogger)
 	/*
 	 * Close the postmaster's listen sockets.  These aren't tracked by fd.c,
 	 * so we don't call ReleaseExternalFD() here.
+	 *
+	 * The listen sockets are marked as FD_CLOEXEC, so this isn't needed in
+	 * EXEC_BACKEND mode.
 	 */
-	for (i = 0; i < MAXLISTEN; i++)
+#ifndef EXEC_BACKEND
+	for (int i = 0; i < MAXLISTEN; i++)
 	{
 		if (ListenSocket[i] != PGINVALID_SOCKET)
 		{
@@ -2582,6 +2583,7 @@ ClosePostmasterPorts(bool am_syslogger)
 			ListenSocket[i] = PGINVALID_SOCKET;
 		}
 	}
+#endif
 
 	/*
 	 * If using syslogger, close the read side of the pipe.  We don't bother
@@ -6038,8 +6040,6 @@ save_backend_variables(BackendParameters *param, Port *port,
 
 	strlcpy(param->DataDir, DataDir, MAXPGPATH);
 
-	memcpy(&param->ListenSocket, &ListenSocket, sizeof(ListenSocket));
-
 	param->MyCancelKey = MyCancelKey;
 	param->MyPMChildSlot = MyPMChildSlot;
 
@@ -6271,8 +6271,6 @@ restore_backend_variables(BackendParameters *param, Port *port)
 
 	SetDataDir(param->DataDir);
 
-	memcpy(&ListenSocket, &param->ListenSocket, sizeof(ListenSocket));
-
 	MyCancelKey = param->MyCancelKey;
 	MyPMChildSlot = param->MyPMChildSlot;