Don't leak a signalfd when using latches in the postmaster.

At the time of commit 6a2a70a02 we didn't use latch infrastructure in
the postmaster.  We're planning to start doing that, so we'd better make
sure that the signalfd inherited from a postmaster is not duplicated and
then leaked in the child.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2BZ-HpOj1JsO9eWUP%2Bar7npSVinsC_npxSy%2BjdOMsx%3DGg%40mail.gmail.com

src/backend/storage/ipc/latch.c

@@ -283,6 +283,22 @@ InitializeLatchSupport(void)
 #ifdef WAIT_USE_SIGNALFD
 	sigset_t	signalfd_mask;
 
+	if (IsUnderPostmaster)
+	{
+		/*
+		 * It would probably be safe to re-use the inherited signalfd since
+		 * signalfds only see the current process's pending signals, but it
+		 * seems less surprising to close it and create our own.
+		 */
+		if (signal_fd != -1)
+		{
+			/* Release postmaster's signal FD; ignore any error */
+			(void) close(signal_fd);
+			signal_fd = -1;
+			ReleaseExternalFD();
+		}
+	}
+
 	/* Block SIGURG, because we'll receive it through a signalfd. */
 	sigaddset(&UnBlockSig, SIGURG);