Fix race in ReplicationSlotRelease() for ephemeral slots

When releasing an ephemeral replication slot, ReplicationSlotRelease()
drops the slot via ReplicationSlotDropAcquired().

However, after dropping the slot, ReplicationSlotRelease() continued
to use its local "slot" pointer, which still referenced the dropped
slot's former shared-memory entry. It could then update fields such as
effective_xmin in that entry.

Once an ephemeral slot has been dropped (via ReplicationSlotDropAcquired()),
its slot array entry can be reused immediately by another backend
creating a new slot. As a result, those updates could corrupt
the state of an unrelated replication slot.

Fix by skipping those shared-memory updates for phemeral slots and
performing them only for non-ephemeral slots, whose shared-memory
entries remain valid after release.

Backpatch to all supported versions.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Masao Fujii <masao.fujii@gmail.com>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/TY4PR01MB177184FF9EE916F577E1F554194082@TY4PR01MB17718.jpnprd01.prod.outlook.com
Backpatch-through: 14

diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 1f4aad52c4a3138583d2000ed08a08ce50f73916..6e3a561691a5347d42f0bb7c04640550098f67ec 100644 (file)
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -587,35 +587,37 @@ ReplicationSlotRelease(void)
                 */
                ReplicationSlotDropAcquired();
        }
-
-       /*
-        * If slot needed to temporarily restrain both data and catalog xmin to
-        * create the catalog snapshot, remove that temporary constraint.
-        * Snapshots can only be exported while the initial snapshot is still
-        * acquired.
-        */
-       if (!TransactionIdIsValid(slot->data.xmin) &&
-               TransactionIdIsValid(slot->effective_xmin))
-       {
-               SpinLockAcquire(&slot->mutex);
-               slot->effective_xmin = InvalidTransactionId;
-               SpinLockRelease(&slot->mutex);
-               ReplicationSlotsComputeRequiredXmin(false);
-       }
-
-       if (slot->data.persistency == RS_PERSISTENT)
+       else
        {
                /*
-                * Mark persistent slot inactive.  We're not freeing it, just
-                * disconnecting, but wake up others that may be waiting for it.
+                * If slot needed to temporarily restrain both data and catalog xmin
+                * to create the catalog snapshot, remove that temporary constraint.
+                * Snapshots can only be exported while the initial snapshot is still
+                * acquired.
                 */
-               SpinLockAcquire(&slot->mutex);
-               slot->active_pid = 0;
-               SpinLockRelease(&slot->mutex);
-               ConditionVariableBroadcast(&slot->active_cv);
-       }
+               if (!TransactionIdIsValid(slot->data.xmin) &&
+                       TransactionIdIsValid(slot->effective_xmin))
+               {
+                       SpinLockAcquire(&slot->mutex);
+                       slot->effective_xmin = InvalidTransactionId;
+                       SpinLockRelease(&slot->mutex);
+                       ReplicationSlotsComputeRequiredXmin(false);
+               }
 
-       MyReplicationSlot = NULL;
+               if (slot->data.persistency == RS_PERSISTENT)
+               {
+                       /*
+                        * Mark persistent slot inactive.  We're not freeing it, just
+                        * disconnecting, but wake up others that may be waiting for it.
+                        */
+                       SpinLockAcquire(&slot->mutex);
+                       slot->active_pid = 0;
+                       SpinLockRelease(&slot->mutex);
+                       ConditionVariableBroadcast(&slot->active_cv);
+               }
+
+               MyReplicationSlot = NULL;
+       }
 
        /* might not have been set when we've been a plain slot */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);