async_notify_freeze_xids.txt

diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index 4bd37d5beb5..ba06234dc8e 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -2168,6 +2168,120 @@ asyncQueueAdvanceTail(void)
 	LWLockRelease(NotifyQueueTailLock);
 }
 
+/*
+ * AsyncNotifyFreezeXids
+ *
+ * Prepare the async notification queue for CLOG truncation by freezing
+ * transaction IDs that are about to become inaccessible.
+ *
+ * This function is called by VACUUM before advancing datfrozenxid. It scans
+ * the notification queue and replaces XIDs that would become inaccessible
+ * after CLOG truncation with special markers:
+ * - Committed transactions are set to FrozenTransactionId
+ * - Aborted/crashed transactions are set to InvalidTransactionId
+ *
+ * Only XIDs < newFrozenXid are processed, as those are the ones whose CLOG
+ * pages will be truncated. If XID < newFrozenXid, it cannot still be running
+ * (or it would have held back newFrozenXid through ProcArray).
+ * Therefore, if TransactionIdDidCommit returns false, we know the transaction
+ * either aborted explicitly or crashed, and we can safely mark it invalid.
+ */
+void
+AsyncNotifyFreezeXids(TransactionId newFrozenXid)
+{
+	QueuePosition pos;
+	QueuePosition head;
+	int64		curpage = -1;
+	int			slotno = -1;
+	char	   *page_buffer = NULL;
+	bool		page_dirty = false;
+
+	/*
+	 * Acquire locks in the correct order to avoid deadlocks. As per the
+	 * locking protocol: NotifyQueueTailLock, then NotifyQueueLock, then SLRU
+	 * bank locks.
+	 *
+	 * We only need SHARED mode since we're just reading the head/tail
+	 * positions, not modifying them.
+	 */
+	LWLockAcquire(NotifyQueueTailLock, LW_SHARED);
+	LWLockAcquire(NotifyQueueLock, LW_SHARED);
+
+	pos = QUEUE_TAIL;
+	head = QUEUE_HEAD;
+
+	/* Release NotifyQueueLock early, we only needed to read the positions */
+	LWLockRelease(NotifyQueueLock);
+
+	/*
+	 * Scan the queue from tail to head, freezing XIDs as needed. We hold
+	 * NotifyQueueTailLock throughout to ensure the tail doesn't move while
+	 * we're working.
+	 */
+	while (!QUEUE_POS_EQUAL(pos, head))
+	{
+		AsyncQueueEntry *qe;
+		TransactionId xid;
+		int64		pageno = QUEUE_POS_PAGE(pos);
+		int			offset = QUEUE_POS_OFFSET(pos);
+
+		/* If we need a different page, release old lock and get new one */
+		if (pageno != curpage)
+		{
+			LWLock	   *lock;
+
+			/* Release previous page if any */
+			if (slotno >= 0)
+			{
+				if (page_dirty)
+				{
+					NotifyCtl->shared->page_dirty[slotno] = true;
+					page_dirty = false;
+				}
+				LWLockRelease(SimpleLruGetBankLock(NotifyCtl, curpage));
+			}
+
+			lock = SimpleLruGetBankLock(NotifyCtl, pageno);
+			LWLockAcquire(lock, LW_EXCLUSIVE);
+			slotno = SimpleLruReadPage(NotifyCtl, pageno, true,
+									   InvalidTransactionId);
+			page_buffer = NotifyCtl->shared->page_buffer[slotno];
+			curpage = pageno;
+		}
+
+		qe = (AsyncQueueEntry *) (page_buffer + offset);
+		xid = qe->xid;
+
+		if (TransactionIdIsNormal(xid) &&
+			TransactionIdPrecedes(xid, newFrozenXid))
+		{
+			if (TransactionIdDidCommit(xid))
+			{
+				qe->xid = FrozenTransactionId;
+				page_dirty = true;
+			}
+			else
+			{
+				qe->xid = InvalidTransactionId;
+				page_dirty = true;
+			}
+		}
+
+		/* Advance to next entry */
+		asyncQueueAdvance(&pos, qe->length);
+	}
+
+	/* Release final page lock if we acquired one */
+	if (slotno >= 0)
+	{
+		if (page_dirty)
+			NotifyCtl->shared->page_dirty[slotno] = true;
+		LWLockRelease(SimpleLruGetBankLock(NotifyCtl, curpage));
+	}
+
+	LWLockRelease(NotifyQueueTailLock);
+}
+
 /*
  * ProcessIncomingNotify
  *
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index ed03e3bd50d..2e8dbf03bd5 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -37,6 +37,7 @@
 #include "catalog/namespace.h"
 #include "catalog/pg_database.h"
 #include "catalog/pg_inherits.h"
+#include "commands/async.h"
 #include "commands/cluster.h"
 #include "commands/defrem.h"
 #include "commands/progress.h"
@@ -1758,6 +1759,13 @@ vac_update_datfrozenxid(void)
 
 	dbform = (Form_pg_database) GETSTRUCT(tuple);
 
+	/*
+	 * Before advancing datfrozenxid, freeze any old transaction IDs in the
+	 * async notification queue to prevent "could not access status of
+	 * transaction" errors when LISTEN is called after CLOG truncation.
+	 */
+	AsyncNotifyFreezeXids(newFrozenXid);
+
 	/*
 	 * As in vac_update_relstats(), we ordinarily don't want to let
 	 * datfrozenxid go backward; but if it's "in the future" then it must be
diff --git a/src/include/commands/async.h b/src/include/commands/async.h
index f75c3df9556..aaec7314c10 100644
--- a/src/include/commands/async.h
+++ b/src/include/commands/async.h
@@ -46,4 +46,7 @@ extern void HandleNotifyInterrupt(void);
 /* process interrupts */
 extern void ProcessNotifyInterrupt(bool flush);
 
+/* freeze old transaction IDs in notify queue (called by VACUUM) */
+extern void AsyncNotifyFreezeXids(TransactionId newFrozenXid);
+
 #endif							/* ASYNC_H */