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 4246d0a51e1d9231fe2a0957a32a40ec71ce2585..e60ccc424ddacfd8b3ad67ed1e8316cb337a84a8 100644 (file)
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -737,44 +737,46 @@ 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))
+       else
        {
-               SpinLockAcquire(&slot->mutex);
-               slot->effective_xmin = InvalidTransactionId;
-               SpinLockRelease(&slot->mutex);
-               ReplicationSlotsComputeRequiredXmin(false);
-       }
-
-       /*
-        * Set the time since the slot has become inactive. We get the current
-        * time beforehand to avoid system call while holding the spinlock.
-        */
-       now = GetCurrentTimestamp();
+               /*
+                * 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)
-       {
                /*
-                * Mark persistent slot inactive.  We're not freeing it, just
-                * disconnecting, but wake up others that may be waiting for it.
+                * Set the time since the slot has become inactive. We get the current
+                * time beforehand to avoid system call while holding the spinlock.
                 */
-               SpinLockAcquire(&slot->mutex);
-               slot->active_pid = 0;
-               ReplicationSlotSetInactiveSince(slot, now, false);
-               SpinLockRelease(&slot->mutex);
-               ConditionVariableBroadcast(&slot->active_cv);
-       }
-       else
-               ReplicationSlotSetInactiveSince(slot, now, true);
+               now = GetCurrentTimestamp();
 
-       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;
+                       ReplicationSlotSetInactiveSince(slot, now, false);
+                       SpinLockRelease(&slot->mutex);
+                       ConditionVariableBroadcast(&slot->active_cv);
+               }
+               else
+                       ReplicationSlotSetInactiveSince(slot, now, true);
+
+               MyReplicationSlot = NULL;
+       }
 
        /* might not have been set when we've been a plain slot */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);