Make latch.c more paranoid about child-process cases.

Although the postmaster doesn't currently create a self-pipe or any
latches, there's discussion of it doing so in future.  It's also
conceivable that a shared_preload_libraries extension would try to
create such a thing in the postmaster process today.  In that case
the self-pipe FDs would be inherited by forked child processes.
latch.c was entirely unprepared for such a case and could suffer an
assertion failure, or worse try to use the inherited pipe if somebody
called WaitLatch without having called InitializeLatchSupport in that
process.  Make it keep track of whether InitializeLatchSupport has been
called in the *current* process, and do the right thing if state has
been inherited from a parent.

Apply FD_CLOEXEC to file descriptors created in latch.c (the self-pipe,
as well as epoll event sets).  This ensures that child processes spawned
in backends, the archiver, etc cannot accidentally or intentionally mess
with these FDs.  It also ensures that we end up with the right state
for the self-pipe in EXEC_BACKEND processes, which otherwise wouldn't
know to close the postmaster's self-pipe FDs.

Back-patch to 9.6, mainly to keep latch.c looking similar in all branches
it exists in.

Discussion: https://postgr.es/m/8322.1493240739@sss.pgh.pa.us

src/backend/storage/ipc/latch.c

@@ -118,6 +118,9 @@ static volatile sig_atomic_t waiting = false;
 static int	selfpipe_readfd = -1;
 static int	selfpipe_writefd = -1;
 
+/* Process owning the self-pipe --- needed for checking purposes */
+static int	selfpipe_owner_pid = 0;
+
 /* Private function prototypes */
 static void sendSelfPipeByte(void);
 static void drainSelfPipe(void);
@@ -146,7 +149,41 @@ InitializeLatchSupport(void)
 #ifndef WIN32
 	int			pipefd[2];
 
-	Assert(selfpipe_readfd == -1);
+	if (IsUnderPostmaster)
+	{
+		/*
+		 * We might have inherited connections to a self-pipe created by the
+		 * postmaster.  It's critical that child processes create their own
+		 * self-pipes, of course, and we really want them to close the
+		 * inherited FDs for safety's sake.
+		 */
+		if (selfpipe_owner_pid != 0)
+		{
+			/* Assert we go through here but once in a child process */
+			Assert(selfpipe_owner_pid != MyProcPid);
+			/* Release postmaster's pipe FDs; ignore any error */
+			(void) close(selfpipe_readfd);
+			(void) close(selfpipe_writefd);
+			/* Clean up, just for safety's sake; we'll set these below */
+			selfpipe_readfd = selfpipe_writefd = -1;
+			selfpipe_owner_pid = 0;
+		}
+		else
+		{
+			/*
+			 * Postmaster didn't create a self-pipe ... or else we're in an
+			 * EXEC_BACKEND build, in which case it doesn't matter since the
+			 * postmaster's pipe FDs were closed by the action of FD_CLOEXEC.
+			 */
+			Assert(selfpipe_readfd == -1);
+		}
+	}
+	else
+	{
+		/* In postmaster or standalone backend, assert we do this but once */
+		Assert(selfpipe_readfd == -1);
+		Assert(selfpipe_owner_pid == 0);
+	}
 
 	/*
 	 * Set up the self-pipe that allows a signal handler to wake up the
@@ -154,23 +191,30 @@ InitializeLatchSupport(void)
 	 * that SetLatch won't block if the event has already been set many times
 	 * filling the kernel buffer. Make the read-end non-blocking too, so that
 	 * we can easily clear the pipe by reading until EAGAIN or EWOULDBLOCK.
+	 * Also, make both FDs close-on-exec, since we surely do not want any
+	 * child processes messing with them.
 	 */
 	if (pipe(pipefd) < 0)
 		elog(FATAL, "pipe() failed: %m");
 	if (fcntl(pipefd[0], F_SETFL, O_NONBLOCK) == -1)
-		elog(FATAL, "fcntl() failed on read-end of self-pipe: %m");
+		elog(FATAL, "fcntl(F_SETFL) failed on read-end of self-pipe: %m");
 	if (fcntl(pipefd[1], F_SETFL, O_NONBLOCK) == -1)
-		elog(FATAL, "fcntl() failed on write-end of self-pipe: %m");
+		elog(FATAL, "fcntl(F_SETFL) failed on write-end of self-pipe: %m");
+	if (fcntl(pipefd[0], F_SETFD, FD_CLOEXEC) == -1)
+		elog(FATAL, "fcntl(F_SETFD) failed on read-end of self-pipe: %m");
+	if (fcntl(pipefd[1], F_SETFD, FD_CLOEXEC) == -1)
+		elog(FATAL, "fcntl(F_SETFD) failed on write-end of self-pipe: %m");
 
 	selfpipe_readfd = pipefd[0];
 	selfpipe_writefd = pipefd[1];
+	selfpipe_owner_pid = MyProcPid;
 #else
 	/* currently, nothing to do here for Windows */
 #endif
 }
 
 /*
- * Initialize a backend-local latch.
+ * Initialize a process-local latch.
  */
 void
 InitLatch(volatile Latch *latch)
@@ -181,7 +225,7 @@ InitLatch(volatile Latch *latch)
 
 #ifndef WIN32
 	/* Assert InitializeLatchSupport has been called in this process */
-	Assert(selfpipe_readfd >= 0);
+	Assert(selfpipe_readfd >= 0 && selfpipe_owner_pid == MyProcPid);
 #else
 	latch->event = CreateEvent(NULL, TRUE, FALSE, NULL);
 	if (latch->event == NULL)
@@ -199,6 +243,10 @@ InitLatch(volatile Latch *latch)
  * containing the latch with ShmemInitStruct. (The Unix implementation
  * doesn't actually require that, but the Windows one does.) Because of
  * this restriction, we have no concurrency issues to worry about here.
+ *
+ * Note that other handles created in this module are never marked as
+ * inheritable.  Thus we do not need to worry about cleaning up child
+ * process references to postmaster-private latches or WaitEventSets.
  */
 void
 InitSharedLatch(volatile Latch *latch)
@@ -244,7 +292,7 @@ OwnLatch(volatile Latch *latch)
 
 #ifndef WIN32
 	/* Assert InitializeLatchSupport has been called in this process */
-	Assert(selfpipe_readfd >= 0);
+	Assert(selfpipe_readfd >= 0 && selfpipe_owner_pid == MyProcPid);
 #endif
 
 	if (latch->owner_pid != 0)
@@ -277,7 +325,7 @@ DisownLatch(volatile Latch *latch)
  * is incurred when WL_TIMEOUT is given, so avoid using a timeout if possible.
  *
  * The latch must be owned by the current process, ie. it must be a
- * backend-local latch initialized with InitLatch, or a shared latch
+ * process-local latch initialized with InitLatch, or a shared latch
  * associated with the current process by calling OwnLatch.
  *
  * Returns bit mask indicating which condition(s) caused the wake-up. Note
@@ -517,9 +565,9 @@ CreateWaitEventSet(MemoryContext context, int nevents)
 	set->nevents_space = nevents;
 
 #if defined(WAIT_USE_EPOLL)
-	set->epoll_fd = epoll_create(nevents);
+	set->epoll_fd = epoll_create1(EPOLL_CLOEXEC);
 	if (set->epoll_fd < 0)
-		elog(ERROR, "epoll_create failed: %m");
+		elog(ERROR, "epoll_create1 failed: %m");
 #elif defined(WAIT_USE_WIN32)
 
 	/*
@@ -540,6 +588,12 @@ CreateWaitEventSet(MemoryContext context, int nevents)
 
 /*
  * Free a previously created WaitEventSet.
+ *
+ * Note: preferably, this shouldn't have to free any resources that could be
+ * inherited across an exec().  If it did, we'd likely leak those resources in
+ * many scenarios.  For the epoll case, we ensure that by setting FD_CLOEXEC
+ * when the FD is created.  For the Windows case, we assume that the handles
+ * involved are non-inheritable.
  */
 void
 FreeWaitEventSet(WaitEventSet *set)
@@ -586,7 +640,7 @@ FreeWaitEventSet(WaitEventSet *set)
  * used to modify previously added wait events using ModifyWaitEvent().
  *
  * In the WL_LATCH_SET case the latch must be owned by the current process,
- * i.e. it must be a backend-local latch initialized with InitLatch, or a
+ * i.e. it must be a process-local latch initialized with InitLatch, or a
  * shared latch associated with the current process by calling OwnLatch.
  *
  * In the WL_SOCKET_READABLE/WRITEABLE case, EOF and error conditions are