Fix some more bugs in signal handlers and process shutdown logic.

WalSndKill was doing things exactly backwards: it should first clear
MyWalSnd (to stop signal handlers from touching MyWalSnd->latch),
then disown the latch, and only then mark the WalSnd struct unused by
clearing its pid field.

Also, WalRcvSigUsr1Handler and worker_spi_sighup failed to preserve
errno, which is surely a requirement for any signal handler.

Per discussion of recent buildfarm failures.  Back-patch as far
as the relevant code exists.

contrib/worker_spi/worker_spi.c

@@ -42,6 +42,7 @@
 #include "tcop/utility.h"
 
 PG_MODULE_MAGIC;
+
 PG_FUNCTION_INFO_V1(worker_spi_launch);
 
 void		_PG_init(void);
@@ -82,15 +83,19 @@ worker_spi_sigterm(SIGNAL_ARGS)
 
 /*
  * Signal handler for SIGHUP
- *		Set a flag to let the main loop to reread the config file, and set
+ *		Set a flag to tell the main loop to reread the config file, and set
  *		our latch to wake it up.
  */
 static void
 worker_spi_sighup(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	got_sighup = true;
 	if (MyProc)
 		SetLatch(&MyProc->procLatch);
+
+	errno = save_errno;
 }
 
 /*

src/backend/replication/walreceiver.c

@@ -740,7 +740,11 @@ WalRcvSigHupHandler(SIGNAL_ARGS)
 static void
 WalRcvSigUsr1Handler(SIGNAL_ARGS)
 {
+	int			save_errno = errno;
+
 	latch_sigusr1_handler();
+
+	errno = save_errno;
 }
 
 /* SIGTERM: set flag for main loop, or shutdown immediately if safe */

src/backend/replication/walsender.c

@@ -1401,17 +1401,23 @@ InitWalSenderSlot(void)
 static void
 WalSndKill(int code, Datum arg)
 {
-	Assert(MyWalSnd != NULL);
+	WalSnd	   *walsnd = MyWalSnd;
+
+	Assert(walsnd != NULL);
+
+	/*
+	 * Clear MyWalSnd first; then disown the latch.  This is so that signal
+	 * handlers won't try to touch the latch after it's no longer ours.
+	 */
+	MyWalSnd = NULL;
+
+	DisownLatch(&walsnd->latch);
 
 	/*
 	 * Mark WalSnd struct no longer in use. Assume that no lock is required
 	 * for this.
 	 */
-	MyWalSnd->pid = 0;
-	DisownLatch(&MyWalSnd->latch);
-
-	/* WalSnd struct isn't mine anymore */
-	MyWalSnd = NULL;
+	walsnd->pid = 0;
 }
 
 /*