Remove redundant ReplicationSlotsComputeRequiredLSN calls

The function ReplicationSlotsComputeRequiredLSN is used to calculate the
oldest slots' required LSN. It is called every time when restart_lsn
value of any slot is changed (for example, when a slot is advancing).
The slot's oldest required LSN is used to remote old WAL segments in two
places - when checkpoint or restart point is created (CreateCheckPoint,
CreateRestartPoint functions). Old WAL segments seems to be truncated in
these two functions only.

The idea of the patch is to call ReplicationSlotsComputeRequiredLSN in
CreateCheckPoint or CreateRestartPoint functions only, before call of
RemoveOldXlogFiles function where old WAL segments are removed. There
is no obvious need to recalculate oldest required LSN every time when a
slot's restart_lsn is changed.

The value of the oldest required lsn can affect on slot invalidation.
The function InvalidateObsoleteReplicationSlots with non zero second
parameter (oldestSegno) is called in CreateCheckPoint,
CreateRestartPoint functions only where slot invalidation occurs with
reason RS_INVAL_WAL_REMOVED. Once we update the oldest slots' required
lsn in the beginning of these functions, the proposed patch should not
break the behaviour of slot invalidation function in this case.

Author: Vitaly Davydov

src/backend/access/transam/xlog.c

@@ -7320,6 +7320,7 @@ CreateCheckPoint(int flags)
 	/*
 	 * Get the current minimum LSN to be used later in WAL segments cleanup.
 	 */
+	ReplicationSlotsComputeRequiredLSN();
 	slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN();
 
 	/*
@@ -7519,6 +7520,7 @@ CreateCheckPoint(int flags)
 										   _logSegNo, InvalidOid,
 										   InvalidTransactionId))
 	{
+		ReplicationSlotsComputeRequiredLSN();
 		slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN();
 		CheckPointReplicationSlots(shutdown);
 
@@ -7888,6 +7890,7 @@ CreateRestartPoint(int flags)
 	/*
 	 * Get the current minimum LSN to be used later in WAL segments cleanup.
 	 */
+	ReplicationSlotsComputeRequiredLSN();
 	slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN();
 
 	if (log_checkpoints)
@@ -7983,6 +7986,7 @@ CreateRestartPoint(int flags)
 										   _logSegNo, InvalidOid,
 										   InvalidTransactionId))
 	{
+		ReplicationSlotsComputeRequiredLSN();
 		slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN();
 		CheckPointReplicationSlots(flags & CHECKPOINT_IS_SHUTDOWN);
 

src/backend/replication/logical/logical.c

@@ -1905,7 +1905,6 @@ LogicalConfirmReceivedLocation(XLogRecPtr lsn)
 			SpinLockRelease(&MyReplicationSlot->mutex);
 
 			ReplicationSlotsComputeRequiredXmin(false);
-			ReplicationSlotsComputeRequiredLSN();
 		}
 	}
 	else

src/backend/replication/logical/slotsync.c

@@ -335,7 +335,6 @@ update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
 		SpinLockRelease(&slot->mutex);
 
 		ReplicationSlotsComputeRequiredXmin(false);
-		ReplicationSlotsComputeRequiredLSN();
 	}
 
 	return updated_config || updated_xmin_or_lsn;
@@ -502,9 +501,6 @@ reserve_wal_for_local_slot(XLogRecPtr restart_lsn)
 		slot->data.restart_lsn = restart_lsn;
 		SpinLockRelease(&slot->mutex);
 
-		/* Prevent WAL removal as fast as possible */
-		ReplicationSlotsComputeRequiredLSN();
-
 		XLByteToSeg(slot->data.restart_lsn, segno, wal_segment_size);
 
 		/*

src/backend/replication/slot.c

@@ -1008,7 +1008,6 @@ ReplicationSlotDropPtr(ReplicationSlot *slot)
 	 * limits.
 	 */
 	ReplicationSlotsComputeRequiredXmin(false);
-	ReplicationSlotsComputeRequiredLSN();
 
 	/*
 	 * If removing the directory fails, the worst thing that will happen is
@@ -1494,9 +1493,6 @@ ReplicationSlotReserveWal(void)
 		slot->data.restart_lsn = restart_lsn;
 		SpinLockRelease(&slot->mutex);
 
-		/* prevent WAL removal as fast as possible */
-		ReplicationSlotsComputeRequiredLSN();
-
 		/*
 		 * If all required WAL is still there, great, otherwise retry. The
 		 * slot should prevent further removal of WAL, unless there's a
@@ -2014,7 +2010,6 @@ InvalidateObsoleteReplicationSlots(uint32 possible_causes,
 	if (invalidated)
 	{
 		ReplicationSlotsComputeRequiredXmin(false);
-		ReplicationSlotsComputeRequiredLSN();
 	}
 
 	return invalidated;

src/backend/replication/slotfuncs.c

@@ -583,7 +583,6 @@ pg_replication_slot_advance(PG_FUNCTION_ARGS)
 	 * advancing potentially done.
 	 */
 	ReplicationSlotsComputeRequiredXmin(false);
-	ReplicationSlotsComputeRequiredLSN();
 
 	ReplicationSlotRelease();
 
@@ -819,7 +818,6 @@ copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
 
 		ReplicationSlotMarkDirty();
 		ReplicationSlotsComputeRequiredXmin(false);
-		ReplicationSlotsComputeRequiredLSN();
 		ReplicationSlotSave();
 
 #ifdef USE_ASSERT_CHECKING

src/backend/replication/walsender.c

@@ -2384,7 +2384,6 @@ PhysicalConfirmReceivedLocation(XLogRecPtr lsn)
 	if (changed)
 	{
 		ReplicationSlotMarkDirty();
-		ReplicationSlotsComputeRequiredLSN();
 		PhysicalWakeupLogicalWalSnd();
 	}