Reduce more the number of calls to GetMaxBackends()

Some of the code paths changed by aa64f23 can reduce the number of times
GetMaxBackends() is called.  The performance gain is marginal, but most
of the code changed by this commit already did that.  Hence, let's be
clean and apply the same rule everywhere, for consistency.

Some of the code paths, like in deadlock.c, involve only assertions.
These are left unchanged.

Reviewed-by: Nathan Bossart, Robert Haas
Discussion: https://postgr.es/m/YgMpGZhPOjNfS7er@paquier.xyz

src/backend/commands/async.c

@@ -1633,6 +1633,7 @@ SignalBackends(void)
 	int32	   *pids;
 	BackendId  *ids;
 	int			count;
+	int			max_backends = GetMaxBackends();
 
 	/*
 	 * Identify backends that we need to signal.  We don't want to send
@@ -1642,8 +1643,8 @@ SignalBackends(void)
 	 * XXX in principle these pallocs could fail, which would be bad. Maybe
 	 * preallocate the arrays?  They're not that large, though.
 	 */
-	pids = (int32 *) palloc(GetMaxBackends() * sizeof(int32));
-	ids = (BackendId *) palloc(GetMaxBackends() * sizeof(BackendId));
+	pids = (int32 *) palloc(max_backends * sizeof(int32));
+	ids = (BackendId *) palloc(max_backends * sizeof(BackendId));
 	count = 0;
 
 	LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);

src/backend/storage/lmgr/lock.c

@@ -2924,6 +2924,7 @@ GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode, int *countp)
 	LWLock	   *partitionLock;
 	int			count = 0;
 	int			fast_count = 0;
+	int			max_backends = GetMaxBackends();
 
 	if (lockmethodid <= 0 || lockmethodid >= lengthof(LockMethods))
 		elog(ERROR, "unrecognized lock method: %d", lockmethodid);
@@ -2942,12 +2943,12 @@ GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode, int *countp)
 			vxids = (VirtualTransactionId *)
 				MemoryContextAlloc(TopMemoryContext,
 								   sizeof(VirtualTransactionId) *
-								   (GetMaxBackends() + max_prepared_xacts + 1));
+								   (max_backends + max_prepared_xacts + 1));
 	}
 	else
 		vxids = (VirtualTransactionId *)
 			palloc0(sizeof(VirtualTransactionId) *
-					(GetMaxBackends() + max_prepared_xacts + 1));
+					(max_backends + max_prepared_xacts + 1));
 
 	/* Compute hash code and partition lock, and look up conflicting modes. */
 	hashcode = LockTagHashCode(locktag);
@@ -3104,7 +3105,7 @@ GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode, int *countp)
 
 	LWLockRelease(partitionLock);
 
-	if (count > GetMaxBackends() + max_prepared_xacts)	/* should never happen */
+	if (count > max_backends + max_prepared_xacts)	/* should never happen */
 		elog(PANIC, "too many conflicting locks found");
 
 	vxids[count].backendId = InvalidBackendId;

src/backend/utils/adt/lockfuncs.c

@@ -559,13 +559,14 @@ pg_safe_snapshot_blocking_pids(PG_FUNCTION_ARGS)
 	int		   *blockers;
 	int			num_blockers;
 	Datum	   *blocker_datums;
+	int			max_backends = GetMaxBackends();
 
 	/* A buffer big enough for any possible blocker list without truncation */
-	blockers = (int *) palloc(GetMaxBackends() * sizeof(int));
+	blockers = (int *) palloc(max_backends * sizeof(int));
 
 	/* Collect a snapshot of processes waited for by GetSafeSnapshot */
 	num_blockers =
-		GetSafeSnapshotBlockingPids(blocked_pid, blockers, GetMaxBackends());
+		GetSafeSnapshotBlockingPids(blocked_pid, blockers, max_backends);
 
 	/* Convert int array to Datum array */
 	if (num_blockers > 0)