v63-0002-Resume-retaining-the-information-for-conflict-de.patch

From c46ddfc5c135b1994e33a4953ec5c754e04ecfdd Mon Sep 17 00:00:00 2001
From: Zhijie Hou <houzj.fnst@fujitsu.com>
Date: Wed, 20 Aug 2025 14:04:34 +0800
Subject: [PATCH v63 2/2] Resume retaining the information for conflict
 detection

The patch allows the launcher to re-initialized invalidated slot, if at
least one apply worker has confirmed that the retention duration is now within
the max_conflict_retention_duration.
---
 doc/src/sgml/ref/create_subscription.sgml  |   4 +
 src/backend/replication/logical/launcher.c |  81 +++++++--
 src/backend/replication/logical/worker.c   | 201 +++++++++++++++++++--
 3 files changed, 256 insertions(+), 30 deletions(-)

diff --git a/doc/src/sgml/ref/create_subscription.sgml b/doc/src/sgml/ref/create_subscription.sgml
index 52822b2fbb5..f032dc2df59 100644
--- a/doc/src/sgml/ref/create_subscription.sgml
+++ b/doc/src/sgml/ref/create_subscription.sgml
@@ -541,6 +541,10 @@ CREATE SUBSCRIPTION <replaceable class="parameter">subscription_name</replaceabl
           <literal>max_conflict_retention_duration</literal> set within the
           corresponding subscription. To re-enable retention manually, you can
           disable <literal>retain_dead_tuples</literal> and re-enable it.
+          Alternatively, the retention will be automatically resumed once at
+          least one apply worker confirms that the retention duration is within
+          the specified limit, or if a new apply worker with retain_dead_tuples
+          enabled is started.
          </para>
          <para>
           Note that overall retention will not stop if other subscriptions
diff --git a/src/backend/replication/logical/launcher.c b/src/backend/replication/logical/launcher.c
index 20e910e5156..25401a1d3c8 100644
--- a/src/backend/replication/logical/launcher.c
+++ b/src/backend/replication/logical/launcher.c
@@ -102,9 +102,12 @@ static int	logicalrep_pa_worker_count(Oid subid);
 static void logicalrep_launcher_attach_dshmem(void);
 static void ApplyLauncherSetWorkerStartTime(Oid subid, TimestampTz start_time);
 static TimestampTz ApplyLauncherGetWorkerStartTime(Oid subid);
-static void compute_min_nonremovable_xid(LogicalRepWorker *worker, TransactionId *xmin);
+static void compute_min_nonremovable_xid(LogicalRepWorker *worker,
+										 bool can_advance_xmin,
+										 TransactionId *xmin);
 static bool acquire_conflict_slot_if_exists(void);
 static void update_conflict_slot_xmin(TransactionId new_xmin);
+static void init_conflict_slot_xmin(void);
 
 
 /*
@@ -1251,6 +1254,14 @@ ApplyLauncherMain(Datum main_arg)
 				 * subscription was enabled.
 				 */
 				CreateConflictDetectionSlot();
+
+				/*
+				 * Initialize slot.xmin as a subscription resumes retention of
+				 * information useful for conflict detection.
+				 */
+				if (sub->retentionactive &&
+					!TransactionIdIsValid(MyReplicationSlot->data.xmin))
+					init_conflict_slot_xmin();
 			}
 
 			if (!sub->enabled)
@@ -1267,9 +1278,8 @@ ApplyLauncherMain(Datum main_arg)
 				 * required for conflict detection among all running apply
 				 * workers that enables retain_dead_tuples.
 				 */
-				if (sub->retaindeadtuples && sub->retentionactive &&
-					can_advance_xmin)
-					compute_min_nonremovable_xid(w, &xmin);
+				if (sub->retaindeadtuples && sub->retentionactive)
+					compute_min_nonremovable_xid(w, can_advance_xmin, &xmin);
 
 				/* worker is running already */
 				continue;
@@ -1380,7 +1390,8 @@ ApplyLauncherMain(Datum main_arg)
  * in *xmin.
  */
 static void
-compute_min_nonremovable_xid(LogicalRepWorker *worker, TransactionId *xmin)
+compute_min_nonremovable_xid(LogicalRepWorker *worker, bool can_advance_xmin,
+							 TransactionId *xmin)
 {
 	TransactionId nonremovable_xid;
 
@@ -1396,7 +1407,27 @@ compute_min_nonremovable_xid(LogicalRepWorker *worker, TransactionId *xmin)
 	nonremovable_xid = worker->oldest_nonremovable_xid;
 	SpinLockRelease(&worker->mutex);
 
-	Assert(TransactionIdIsValid(nonremovable_xid));
+	/*
+	 * Assign slot.xmin to the apply worker's oldest_nonremovable_xid if the
+	 * latter is invalid. This ensures the apply worker continues to maintain
+	 * the oldest_nonremovable_xid (see resume_conflict_info_retention).
+	 */
+	if (!TransactionIdIsValid(nonremovable_xid))
+	{
+		nonremovable_xid = MyReplicationSlot->data.xmin;
+
+		SpinLockAcquire(&worker->mutex);
+		worker->oldest_nonremovable_xid = nonremovable_xid;
+		SpinLockRelease(&worker->mutex);
+
+		/* Notify the apply worker to start the next cycle of management */
+		LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
+		logicalrep_worker_wakeup_ptr(worker);
+		LWLockRelease(LogicalRepWorkerLock);
+	}
+
+	if (!can_advance_xmin)
+		return;
 
 	if (!TransactionIdIsValid(*xmin) ||
 		TransactionIdPrecedes(nonremovable_xid, *xmin))
@@ -1457,23 +1488,15 @@ update_conflict_slot_xmin(TransactionId new_xmin)
 }
 
 /*
- * Create and acquire the replication slot used to retain information for
- * conflict detection, if not yet.
+ * Initialize the xmin for the conflict detection slot.
  */
-void
-CreateConflictDetectionSlot(void)
+static void
+init_conflict_slot_xmin(void)
 {
 	TransactionId xmin_horizon;
 
-	/* Exit early, if the replication slot is already created and acquired */
-	if (MyReplicationSlot)
-		return;
-
-	ereport(LOG,
-			errmsg("creating replication conflict detection slot"));
-
-	ReplicationSlotCreate(CONFLICT_DETECTION_SLOT, false, RS_PERSISTENT, false,
-						  false, false);
+	Assert(MyReplicationSlot &&
+		   !TransactionIdIsValid(MyReplicationSlot->data.xmin));
 
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
@@ -1493,6 +1516,26 @@ CreateConflictDetectionSlot(void)
 	ReplicationSlotSave();
 }
 
+/*
+ * Create and acquire the replication slot used to retain information for
+ * conflict detection, if not yet.
+ */
+void
+CreateConflictDetectionSlot(void)
+{
+	/* Exit early, if the replication slot is already created and acquired */
+	if (MyReplicationSlot)
+		return;
+
+	ereport(LOG,
+			errmsg("creating replication conflict detection slot"));
+
+	ReplicationSlotCreate(CONFLICT_DETECTION_SLOT, false, RS_PERSISTENT, false,
+						  false, false);
+
+	init_conflict_slot_xmin();
+}
+
 /*
  * Is current process the logical replication launcher?
  */
diff --git a/src/backend/replication/logical/worker.c b/src/backend/replication/logical/worker.c
index 1b2d6a3e001..0ef3806961e 100644
--- a/src/backend/replication/logical/worker.c
+++ b/src/backend/replication/logical/worker.c
@@ -177,11 +177,21 @@
  *   Update pg_subscription.subretentionactive to false within a new
  *   transaction, and set oldest_nonremovable_xid to InvalidTransactionId.
  *
+ * - RDT_RESUME_CONFLICT_INFO_RETENTION:
+ *   Update pg_subscription.subretentionactive to true within a new transaction,
+ *   and wait for the launcher to initialize the oldest_nonremovable_xid.
+ *
  * The overall state progression is: GET_CANDIDATE_XID ->
  * REQUEST_PUBLISHER_STATUS -> WAIT_FOR_PUBLISHER_STATUS -> (loop to
  * REQUEST_PUBLISHER_STATUS till concurrent remote transactions end) ->
  * WAIT_FOR_LOCAL_FLUSH -> (RDT_STOP_CONFLICT_INFO_RETENTION if the wait time
- * exceeds max_conflict_retention_duration) -> loop back to GET_CANDIDATE_XID.
+ * exceeds max_conflict_retention_duration) ->
+ * (RDT_RESUME_CONFLICT_INFO_RETENTION if the wait time has decreased to within
+ * acceptable limits) -> loop back to GET_CANDIDATE_XID.
+ *
+ * Note that the state could transition to RDT_RESUME_CONFLICT_INFO_RETENTION at
+ * any phase if the retention has been stopped, but
+ * max_conflict_retention_duration is now set to 0.
  *
  * Retaining the dead tuples for this period is sufficient for ensuring
  * eventual consistency using last-update-wins strategy, as dead tuples are
@@ -380,6 +390,7 @@ typedef enum
 	RDT_WAIT_FOR_PUBLISHER_STATUS,
 	RDT_WAIT_FOR_LOCAL_FLUSH,
 	RDT_STOP_CONFLICT_INFO_RETENTION,
+	RDT_RESUME_CONFLICT_INFO_RETENTION,
 } RetainDeadTuplesPhase;
 
 /*
@@ -424,6 +435,10 @@ typedef struct RetainDeadTuplesData
 	long		table_sync_wait_time;	/* time spent waiting for table sync
 										 * to finish */
 
+	bool		wait_for_initial_xid;	/* wait for the launcher to initialize
+										 * the apply worker's
+										 * oldest_nonremovable_xid */
+
 	/*
 	 * The following fields are used to determine the timing for the next
 	 * round of transaction ID advancement.
@@ -567,6 +582,9 @@ static void wait_for_local_flush(RetainDeadTuplesData *rdt_data);
 static void stop_conflict_info_retention(RetainDeadTuplesData *rdt_data);
 static void reset_retention_data_fields(RetainDeadTuplesData *rdt_data);
 static bool should_stop_conflict_info_retention(RetainDeadTuplesData *rdt_data);
+static bool should_resume_retention_immediately(RetainDeadTuplesData *rdt_data,
+												bool status_received);
+static void resume_conflict_info_retention(RetainDeadTuplesData *rdt_data);
 static void adjust_xid_advance_interval(RetainDeadTuplesData *rdt_data,
 										bool new_xid_found);
 
@@ -4361,6 +4379,13 @@ maybe_advance_nonremovable_xid(RetainDeadTuplesData *rdt_data,
 	if (!can_advance_nonremovable_xid(rdt_data))
 		return;
 
+	/*
+	 * Resume retention immediately if required. (See
+	 * should_resume_retention_immediately() for details).
+	 */
+	if (should_resume_retention_immediately(rdt_data, status_received))
+		rdt_data->phase = RDT_RESUME_CONFLICT_INFO_RETENTION;
+
 	process_rdt_phase_transition(rdt_data, status_received);
 }
 
@@ -4383,10 +4408,6 @@ can_advance_nonremovable_xid(RetainDeadTuplesData *rdt_data)
 	if (!MySubscription->retaindeadtuples)
 		return false;
 
-	/* No need to advance if we have already stopped retaining */
-	if (!TransactionIdIsValid(MyLogicalRepWorker->oldest_nonremovable_xid))
-		return false;
-
 	return true;
 }
 
@@ -4415,6 +4436,9 @@ process_rdt_phase_transition(RetainDeadTuplesData *rdt_data,
 		case RDT_STOP_CONFLICT_INFO_RETENTION:
 			stop_conflict_info_retention(rdt_data);
 			break;
+		case RDT_RESUME_CONFLICT_INFO_RETENTION:
+			resume_conflict_info_retention(rdt_data);
+			break;
 	}
 }
 
@@ -4671,6 +4695,18 @@ wait_for_local_flush(RetainDeadTuplesData *rdt_data)
 	if (last_flushpos < rdt_data->remote_lsn)
 		return;
 
+	/*
+	 * If conflict info retention was previously stopped due to a timeout, and
+	 * the time required to advance the non-removable transaction ID has now
+	 * decreased to within acceptable limits, resume the rentention.
+	 */
+	if (!TransactionIdIsValid(MyLogicalRepWorker->oldest_nonremovable_xid))
+	{
+		rdt_data->phase = RDT_RESUME_CONFLICT_INFO_RETENTION;
+		process_rdt_phase_transition(rdt_data, false);
+		return;
+	}
+
 	/*
 	 * Reaching here means the remote WAL position has been received, and all
 	 * transactions up to that position on the publisher have been applied and
@@ -4737,6 +4773,112 @@ stop_conflict_info_retention(RetainDeadTuplesData *rdt_data)
 	ApplyLauncherWakeup();
 
 	reset_retention_data_fields(rdt_data);
+
+	/* process the next phase */
+	process_rdt_phase_transition(rdt_data, false);
+}
+
+/*
+ * Workhorse for the RDT_RESUME_CONFLICT_INFO_RETENTION phase.
+ */
+static void
+resume_conflict_info_retention(RetainDeadTuplesData *rdt_data)
+{
+	if (rdt_data->wait_for_initial_xid)
+	{
+		TransactionId nonremovable_xid;
+
+		SpinLockAcquire(&MyLogicalRepWorker->mutex);
+		nonremovable_xid = MyLogicalRepWorker->oldest_nonremovable_xid;
+		SpinLockRelease(&MyLogicalRepWorker->mutex);
+
+		/*
+		 * Proceed to the next phase if the catalog has been updated and the
+		 * launcher has initialized slot.xmin and assigned it to
+		 * oldest_nonremovable_xid.
+		 *
+		 * It might seem feasible to directly check the conflict detection
+		 * slot.xmin instead of relying on the launcher to assign the worker's
+		 * oldest_nonremovable_xid; however, that could lead to a race
+		 * condition where slot.xmin is set to InvalidTransactionId
+		 * immediately after the check. In such cases, oldest_nonremovable_xid
+		 * would no longer be protected by a replication slot and could become
+		 * unreliable if a wraparound occurs.
+		 */
+		if (TransactionIdIsValid(nonremovable_xid))
+		{
+			rdt_data->wait_for_initial_xid = false;
+
+			reset_retention_data_fields(rdt_data);
+
+			/* process the next phase */
+			process_rdt_phase_transition(rdt_data, false);
+		}
+
+		/*
+		 * Return early since the catalog has been updated and we are waiting
+		 * for oldest_nonremovable_xid to be initialized.
+		 */
+		return;
+	}
+
+	/*
+	 * Proceed to the next phase if retention has not been stopped yet. This
+	 * occurs when transitioning from the RDT_STOP_CONFLICT_INFO_RETENTION
+	 * phase but subretentionactive has not been updated due to the inability
+	 * to start a new transaction (see stop_conflict_info_retention).
+	 */
+	if (TransactionIdIsValid(MyLogicalRepWorker->oldest_nonremovable_xid))
+	{
+		reset_retention_data_fields(rdt_data);
+
+		/* process the next phase */
+		process_rdt_phase_transition(rdt_data, false);
+
+		return;
+	}
+
+	/*
+	 * Do not update the catalog during an active transaction. The transaction
+	 * may be started during change application, leading to a possible
+	 * rollback of catalog updates if the application fails subsequently.
+	 */
+	if (IsTransactionState())
+		return;
+
+	StartTransactionCommand();
+
+	/*
+	 * Updating pg_subscription might involve TOAST table access, so ensure we
+	 * have a valid snapshot.
+	 */
+	PushActiveSnapshot(GetTransactionSnapshot());
+
+	/* Set pg_subscription.subretentionactive to true */
+	UpdateDeadTupleRetentionStatus(MySubscription->oid, true);
+
+	PopActiveSnapshot();
+	CommitTransactionCommand();
+
+	ereport(LOG,
+			errmsg("logical replication worker for subscription \"%s\" will resume retaining the information for detecting conflicts",
+				   MySubscription->name),
+			MySubscription->maxconflretention
+			? errdetail("The retention duration for information used in conflict detection is now within the acceptable limit of %u ms.",
+						MySubscription->maxconflretention)
+			: errdetail("The retention duration for information used in conflict detection is now indefinite."));
+
+	/* Notify launcher to update the xmin of the conflict slot */
+	ApplyLauncherWakeup();
+
+	/*
+	 * The next step is to wait for the launcher to initialize the
+	 * oldest_nonremovable_xid.
+	 */
+	rdt_data->wait_for_initial_xid = true;
+
+	/* process the next phase */
+	process_rdt_phase_transition(rdt_data, false);
 }
 
 /*
@@ -4766,9 +4908,8 @@ reset_retention_data_fields(RetainDeadTuplesData *rdt_data)
  * RDT_STOP_CONFLICT_INFO_RETENTION phase and return true. Otherwise, return
  * false.
  *
- * Currently, the retention will not resume automatically unless user manually
- * disables retain_dead_tuples and re-enables it after confirming that the
- * replication slot has been dropped.
+ * The retention will resume automatically if the worker has confirmed that the
+ * retention duration is now within the max_conflict_retention_duration.
  */
 static bool
 should_stop_conflict_info_retention(RetainDeadTuplesData *rdt_data)
@@ -4799,14 +4940,52 @@ should_stop_conflict_info_retention(RetainDeadTuplesData *rdt_data)
 									rdt_data->table_sync_wait_time))
 		return false;
 
-	rdt_data->phase = RDT_STOP_CONFLICT_INFO_RETENTION;
+	/* Stop retention if not yet */
+	if (TransactionIdIsValid(MyLogicalRepWorker->oldest_nonremovable_xid))
+	{
+		rdt_data->phase = RDT_STOP_CONFLICT_INFO_RETENTION;
 
-	/* process the next phase */
-	process_rdt_phase_transition(rdt_data, false);
+		/* process the next phase */
+		process_rdt_phase_transition(rdt_data, false);
+	}
 
 	return true;
 }
 
+/*
+ * Check whether retention should be resumed immediately if it has been
+ * previously stopped, but max_conflict_retention_duration is now set to 0.
+ */
+static bool
+should_resume_retention_immediately(RetainDeadTuplesData *rdt_data, bool status_received)
+{
+	/* Return false if retention is already being resumed */
+	if (rdt_data->phase == RDT_RESUME_CONFLICT_INFO_RETENTION)
+		return false;
+
+	/* Return false if max_conflict_retention_duration is not 0 */
+	if (MySubscription->maxconflretention)
+		return false;
+
+	/*
+	 * Do not resume when waiting for publisher status, as doing so may result
+	 * in the message being processed after the data and phase have been
+	 * reset, potentially causing it to be mistakenly identified as a new
+	 * message. This could lead to the premature advancement of
+	 * oldest_nonremovable_xid.
+	 */
+	if (rdt_data->phase == RDT_WAIT_FOR_PUBLISHER_STATUS &&
+		!status_received)
+		return false;
+
+	/*
+	 * Resume retention if we are in the process of stopping or have already
+	 * stopped retention.
+	 */
+	return rdt_data->phase == RDT_STOP_CONFLICT_INFO_RETENTION ||
+		!TransactionIdIsValid(MyLogicalRepWorker->oldest_nonremovable_xid);
+}
+
 /*
  * Adjust the interval for advancing non-removable transaction IDs.
  *
-- 
2.50.1.windows.1