Fix volatile-safety issue in asyncQueueReadAllNotifications().

The "pos" variable is modified within PG_TRY and then referenced
within PG_CATCH, so for strict POSIX conformance it must be marked
volatile.  Superficially the code looked safe because pos's address
was taken, which was sufficient to force it into memory ... but it's
not sufficient to ensure that the compiler applies updates exactly
where the program text says to.  The volatility marking has to extend
into a couple of subroutines too, but I think that's probably a good
thing because the risk of out-of-order updates is mostly in those
subroutines not asyncQueueReadAllNotifications() itself.  In principle
the compiler could have re-ordered operations such that an error could
be thrown while "pos" had an incorrect value.

It's unclear how real the risk is here, but for safety back-patch
to all active branches.

Author: tglsfdc

src/backend/commands/async.c

@@ -369,13 +369,13 @@ static void Exec_UnlistenAllCommit(void);
 static bool IsListeningOn(const char *channel);
 static void asyncQueueUnregister(void);
 static bool asyncQueueIsFull(void);
-static bool asyncQueueAdvance(QueuePosition *position, int entryLength);
+static bool asyncQueueAdvance(volatile QueuePosition *position, int entryLength);
 static void asyncQueueNotificationToEntry(Notification *n, AsyncQueueEntry *qe);
 static ListCell *asyncQueueAddEntries(ListCell *nextNotify);
 static void asyncQueueFillWarning(void);
 static bool SignalBackends(void);
 static void asyncQueueReadAllNotifications(void);
-static bool asyncQueueProcessPageEntries(QueuePosition *current,
+static bool asyncQueueProcessPageEntries(volatile QueuePosition *current,
 							 QueuePosition stop,
 							 char *page_buffer);
 static void asyncQueueAdvanceTail(void);
@@ -1202,7 +1202,7 @@ asyncQueueIsFull(void)
  * returns true, else false.
  */
 static bool
-asyncQueueAdvance(QueuePosition *position, int entryLength)
+asyncQueueAdvance(volatile QueuePosition *position, int entryLength)
 {
 	int			pageno = QUEUE_POS_PAGE(*position);
 	int			offset = QUEUE_POS_OFFSET(*position);
@@ -1792,7 +1792,7 @@ DisableNotifyInterrupt(void)
 static void
 asyncQueueReadAllNotifications(void)
 {
-	QueuePosition pos;
+	volatile QueuePosition pos;
 	QueuePosition oldpos;
 	QueuePosition head;
 	bool		advanceTail;
@@ -1952,7 +1952,7 @@ asyncQueueReadAllNotifications(void)
  * The QueuePosition *current is advanced past all processed messages.
  */
 static bool
-asyncQueueProcessPageEntries(QueuePosition *current,
+asyncQueueProcessPageEntries(volatile QueuePosition *current,
 							 QueuePosition stop,
 							 char *page_buffer)
 {