Allow WaitLatch() to be used without a latch.

Due to flaws in commit 3347c982, using WaitLatch() without
WL_LATCH_SET could cause an assertion failure or crash.  Repair.

While here, also add a check that the latch we're switching to belongs
to this backend, when changing from one latch to another.

Discussion: https://postgr.es/m/CA%2BhUKGK1607VmtrDUHQXrsooU%3Dap4g4R2yaoByWOOA3m8xevUQ%40mail.gmail.com

src/backend/storage/ipc/latch.c

@@ -924,7 +924,22 @@ ModifyWaitEvent(WaitEventSet *set, int pos, uint32 events, Latch *latch)
 
 	if (events == WL_LATCH_SET)
 	{
+		if (latch && latch->owner_pid != MyProcPid)
+			elog(ERROR, "cannot wait on a latch owned by another process");
 		set->latch = latch;
+		/*
+		 * On Unix, we don't need to modify the kernel object because the
+		 * underlying pipe is the same for all latches so we can return
+		 * immediately.  On Windows, we need to update our array of handles,
+		 * but we leave the old one in place and tolerate spurious wakeups if
+		 * the latch is disabled.
+		 */
+#if defined(WAIT_USE_WIN32)
+		if (!latch)
+			return;
+#else
+		return;
+#endif
 	}
 
 #if defined(WAIT_USE_EPOLL)
@@ -1386,7 +1401,7 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 			/* There's data in the self-pipe, clear it. */
 			drainSelfPipe();
 
-			if (set->latch->is_set)
+			if (set->latch && set->latch->is_set)
 			{
 				occurred_events->fd = PGINVALID_SOCKET;
 				occurred_events->events = WL_LATCH_SET;
@@ -1536,7 +1551,7 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 			/* There's data in the self-pipe, clear it. */
 			drainSelfPipe();
 
-			if (set->latch->is_set)
+			if (set->latch && set->latch->is_set)
 			{
 				occurred_events->fd = PGINVALID_SOCKET;
 				occurred_events->events = WL_LATCH_SET;
@@ -1645,7 +1660,7 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 			/* There's data in the self-pipe, clear it. */
 			drainSelfPipe();
 
-			if (set->latch->is_set)
+			if (set->latch && set->latch->is_set)
 			{
 				occurred_events->fd = PGINVALID_SOCKET;
 				occurred_events->events = WL_LATCH_SET;
@@ -1812,7 +1827,7 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 		if (!ResetEvent(set->latch->event))
 			elog(ERROR, "ResetEvent failed: error code %lu", GetLastError());
 
-		if (set->latch->is_set)
+		if (set->latch && set->latch->is_set)
 		{
 			occurred_events->fd = PGINVALID_SOCKET;
 			occurred_events->events = WL_LATCH_SET;