Avoid acquiring spinlock when checking if recovery has finished, for speed.

RecoveryIsInProgress() can be called very frequently. During normal
operation, it just checks a backend-local variable and returns quickly,
but during hot standby, it checks a spinlock-protected shared variable.
Those spinlock acquisitions can become a point of contention on a busy
hot standby system.

Replace the spinlock acquisition with a memory barrier.

Per discussion with Andres Freund, Ants Aasma and Merlin Moncure.

Author: hlinnaka

src/backend/access/transam/xlog.c

@@ -7367,13 +7367,13 @@ RecoveryInProgress(void)
 		return false;
 	else
 	{
-		/* use volatile pointer to prevent code rearrangement */
+		/*
+		 * use volatile pointer to make sure we make a fresh read of the
+		 * shared variable.
+		 */
 		volatile XLogCtlData *xlogctl = XLogCtl;
 
-		/* spinlock is essential on machines with weak memory ordering! */
-		SpinLockAcquire(&xlogctl->info_lck);
 		LocalRecoveryInProgress = xlogctl->SharedRecoveryInProgress;
-		SpinLockRelease(&xlogctl->info_lck);
 
 		/*
 		 * Initialize TimeLineID and RedoRecPtr when we discover that recovery
@@ -7382,7 +7382,20 @@ RecoveryInProgress(void)
 		 * this, see also LocalSetXLogInsertAllowed.)
 		 */
 		if (!LocalRecoveryInProgress)
+		{
+			/*
+			 * If we just exited recovery, make sure we read TimeLineID and
+			 * RedoRecPtr after SharedRecoveryInProgress (for machines with
+			 * weak memory ordering).
+			 */
+			pg_memory_barrier();
 			InitXLOGAccess();
+		}
+		/*
+		 * Note: We don't need a memory barrier when we're still in recovery.
+		 * We might exit recovery immediately after return, so the caller
+		 * can't rely on 'true' meaning that we're still in recovery anyway.
+		 */
 
 		return LocalRecoveryInProgress;
 	}