From: Andres Freund <andres@anarazel.de>
Date: Tue, 28 Jan 2014 17:51:33 +0000 (+0100)
Subject: Don't let ReplicationSlotAcquire succeed in the midst of ReplicationSlotCreate.
X-Git-Url: http://waps.l3s.uni-hannover.de/gitweb/?a=commitdiff_plain;h=7d97b442a2a49532f4eccf4098179756eab476be;p=users%2Frhaas%2Fpostgres.git

Don't let ReplicationSlotAcquire succeed in the midst of ReplicationSlotCreate.
---

diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index f3894f1502..cc376b8b0a 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -261,27 +261,32 @@ ReplicationSlotCreate(const char *name, bool db_specific)
 
 	/*
 	 * We need to briefly prevent any other backend from iterating over the
-	 * slots while we flip the in_use flag.
+	 * slots while we flip the in_use flag. We also need to set the active
+	 * flag while holding the ControlLock as otherwise a concurrent
+	 * SlotAcquire() could acquire the slot as well.
 	 */
 	LWLockAcquire(ReplicationSlotControlLock, LW_EXCLUSIVE);
-	slot->in_use = true;
-	LWLockRelease(ReplicationSlotControlLock);
 
-	/*
-	 * Now that the slot has been marked as in_use, it's safe to let somebody
-	 * else try to allocate a slot.
-	 */
-	LWLockRelease(ReplicationSlotAllocationLock);
+	slot->in_use = true;
 
 	/* We can now mark the slot active, and that makes it our slot. */
 	{
 		volatile ReplicationSlot *vslot = slot;
 
 		SpinLockAcquire(&slot->mutex);
+		Assert(!vslot->active);
 		vslot->active = true;
 		SpinLockRelease(&slot->mutex);
 		MyReplicationSlot = slot;
 	}
+
+	LWLockRelease(ReplicationSlotControlLock);
+
+	/*
+	 * Now that the slot has been marked as in_use and in_active, it's safe to
+	 * let somebody else try to allocate a slot.
+	 */
+	LWLockRelease(ReplicationSlotAllocationLock);
 }
 
 /*