Prevent concurrent SimpleLruTruncate() for any given SLRU.

The SimpleLruTruncate() header comment states the new coding rule.  To
achieve this, add locktype "frozenid" and two LWLocks.  This closes a
rare opportunity for data loss, which manifested as "apparent
wraparound" or "could not access status of transaction" errors.  Data
loss is more likely in pg_multixact, due to released branches' thin
margin between multiStopLimit and multiWrapLimit.  If a user's physical
replication primary logged ":  apparent wraparound" messages, the user
should rebuild standbys of that primary regardless of symptoms.  At less
risk is a cluster having emitted "not accepting commands" errors or
"must be vacuumed" warnings at some point.  One can test a cluster for
this data loss by running VACUUM FREEZE in every database.  Back-patch
to 9.5 (all supported versions).

Discussion: https://postgr.es/m/20190218073103.GA1434723@rfd.leadboat.com

1 files changed, 27 insertions, 10 deletions

diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index 71b7577afc0..4c1286eb988 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -244,19 +244,22 @@ typedef struct QueueBackendStatus
 /*
  * Shared memory state for LISTEN/NOTIFY (excluding its SLRU stuff)
  *
- * The AsyncQueueControl structure is protected by the NotifyQueueLock.
+ * The AsyncQueueControl structure is protected by the NotifyQueueLock and
+ * NotifyQueueTailLock.
  *
- * When holding the lock in SHARED mode, backends may only inspect their own
- * entries as well as the head and tail pointers. Consequently we can allow a
- * backend to update its own record while holding only SHARED lock (since no
- * other backend will inspect it).
+ * When holding NotifyQueueLock in SHARED mode, backends may only inspect
+ * their own entries as well as the head and tail pointers. Consequently we
+ * can allow a backend to update its own record while holding only SHARED lock
+ * (since no other backend will inspect it).
  *
- * When holding the lock in EXCLUSIVE mode, backends can inspect the entries
- * of other backends and also change the head and tail pointers.
+ * When holding NotifyQueueLock in EXCLUSIVE mode, backends can inspect the
+ * entries of other backends and also change the head pointer. When holding
+ * both NotifyQueueLock and NotifyQueueTailLock in EXCLUSIVE mode, backends
+ * can change the tail pointer.
  *
  * NotifySLRULock is used as the control lock for the pg_notify SLRU buffers.
- * In order to avoid deadlocks, whenever we need both locks, we always first
- * get NotifyQueueLock and then NotifySLRULock.
+ * In order to avoid deadlocks, whenever we need multiple locks, we first get
+ * NotifyQueueTailLock, then NotifyQueueLock, and lastly NotifySLRULock.
  *
  * Each backend uses the backend[] array entry with index equal to its
  * BackendId (which can range from 1 to MaxBackends).  We rely on this to make
@@ -2177,6 +2180,10 @@ asyncQueueAdvanceTail(void)
 	int			newtailpage;
 	int			boundary;
 
+	/* Restrict task to one backend per cluster; see SimpleLruTruncate(). */
+	LWLockAcquire(NotifyQueueTailLock, LW_EXCLUSIVE);
+
+	/* Compute the new tail. */
 	LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
 	min = QUEUE_HEAD;
 	for (BackendId i = QUEUE_FIRST_LISTENER; i > 0; i = QUEUE_NEXT_LISTENER(i))
@@ -2185,7 +2192,6 @@ asyncQueueAdvanceTail(void)
 		min = QUEUE_POS_MIN(min, QUEUE_BACKEND_POS(i));
 	}
 	oldtailpage = QUEUE_POS_PAGE(QUEUE_TAIL);
-	QUEUE_TAIL = min;
 	LWLockRelease(NotifyQueueLock);
 
 	/*
@@ -2205,6 +2211,17 @@ asyncQueueAdvanceTail(void)
 		 */
 		SimpleLruTruncate(NotifyCtl, newtailpage);
 	}
+
+	/*
+	 * Advertise the new tail.  This changes asyncQueueIsFull()'s verdict for
+	 * the segment immediately prior to the new tail, allowing fresh data into
+	 * that segment.
+	 */
+	LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
+	QUEUE_TAIL = min;
+	LWLockRelease(NotifyQueueLock);
+
+	LWLockRelease(NotifyQueueTailLock);
 }
 
 /*