Allow pg_create_physical_replication_slot() to reserve WAL.

When creating a physical slot it's often useful to immediately reserve
the current WAL position instead of only doing after the first feedback
message arrives. That e.g. allows slots to guarantee that all the WAL
for a base backup will be available afterwards.

Logical slots already have to reserve WAL during creation, so generalize
that logic into being usable for both physical and logical slots.

Catversion bump because of the new parameter.

Author: Gurjeet Singh
Reviewed-By: Andres Freund
Discussion: CABwTF4Wh_dBCzTU=49pFXR6coR4NW1ynb+vBqT+Po=7fuq5iCw@mail.gmail.com

1 files changed, 1 insertions, 46 deletions

diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 5411e599eb2..5a07e1d9a69 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -250,52 +250,7 @@ CreateInitDecodingContext(char *plugin,
 	StrNCpy(NameStr(slot->data.plugin), plugin, NAMEDATALEN);
 	SpinLockRelease(&slot->mutex);
 
-	/*
-	 * The replication slot mechanism is used to prevent removal of required
-	 * WAL. As there is no interlock between this and checkpoints required WAL
-	 * could be removed before ReplicationSlotsComputeRequiredLSN() has been
-	 * called to prevent that. In the very unlikely case that this happens
-	 * we'll just retry.
-	 */
-	while (true)
-	{
-		XLogSegNo	segno;
-
-		/*
-		 * Let's start with enough information if we can, so log a standby
-		 * snapshot and start decoding at exactly that position.
-		 */
-		if (!RecoveryInProgress())
-		{
-			XLogRecPtr	flushptr;
-
-			/* start at current insert position */
-			slot->data.restart_lsn = GetXLogInsertRecPtr();
-
-			/* make sure we have enough information to start */
-			flushptr = LogStandbySnapshot();
-
-			/* and make sure it's fsynced to disk */
-			XLogFlush(flushptr);
-		}
-		else
-			slot->data.restart_lsn = GetRedoRecPtr();
-
-		/* 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
-		 * concurrent ReplicationSlotsComputeRequiredLSN() after we've written
-		 * the new restart_lsn above, so normally we should never need to loop
-		 * more than twice.
-		 */
-		XLByteToSeg(slot->data.restart_lsn, segno);
-		if (XLogGetLastRemovedSegno() < segno)
-			break;
-	}
-
+	ReplicationSlotReserveWal();
 
 	/* ----
 	 * This is a bit tricky: We need to determine a safe xmin horizon to start