v30-0002-Maintain-the-replication-slot-in-logical-launche.patch

From b72ef318efd7af105fcac6f6fc2acd86a98a7eb5 Mon Sep 17 00:00:00 2001
From: Hou Zhijie <houzj.fnst@cn.fujitsu.com>
Date: Thu, 26 Sep 2024 12:11:34 +0800
Subject: [PATCH v30 2/7] Maintain the replication slot in logical launcher to
 retain dead tuples

This patch enables the logical replication launcher to create and maintain a
replication slot named pg_conflict_detection.

The launcher periodically collects the oldest_nonremovable_xid from all apply
workers. It then computes the minimum transaction ID and advances the xmin
value of the replication slot if it precedes the computed value.

The interval for updating the slot (nap time) is dynamically adjusted based on
the activity of the apply workers. The launcher waits for a certain period
before performing the next update, with the duration varying depending on
whether the xmin value of the replication slot was updated during the last
cycle.
---
 doc/src/sgml/config.sgml                      |   2 +
 doc/src/sgml/func.sgml                        |  14 +-
 doc/src/sgml/protocol.sgml                    |   2 +
 doc/src/sgml/ref/create_subscription.sgml     |   4 +-
 src/backend/access/transam/xlogrecovery.c     |   2 +-
 src/backend/commands/subscriptioncmds.c       |   2 +-
 src/backend/replication/logical/launcher.c    | 180 +++++++++++++++++-
 .../replication/logical/reorderbuffer.c       |   2 +-
 src/backend/replication/logical/worker.c      |   3 +
 src/backend/replication/slot.c                |  34 +++-
 src/include/replication/logicallauncher.h     |   1 +
 src/include/replication/slot.h                |  11 +-
 12 files changed, 245 insertions(+), 12 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index c1674c22cb2..58ce478c493 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -4945,6 +4945,8 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
           new setting.
           This setting has no effect if <varname>primary_conninfo</varname> is not
           set or the server is not in standby mode.
+          The name cannot be <literal>pg_conflict_detection</literal>, as it is
+          reserved for logical replication conflict detection.
          </para>
         </listitem>
        </varlistentry>
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 574a544d9fa..af4028ce2ba 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -29711,7 +29711,9 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
        </para>
        <para>
         Creates a new physical replication slot named
-        <parameter>slot_name</parameter>. The optional second parameter,
+        <parameter>slot_name</parameter>. The name cannot be
+        <literal>pg_conflict_detection</literal>, as it is reserved for
+        logical replication conflict detection. The optional second parameter,
         when <literal>true</literal>, specifies that the <acronym>LSN</acronym> for this
         replication slot be reserved immediately; otherwise
         the <acronym>LSN</acronym> is reserved on first connection from a streaming
@@ -29755,7 +29757,9 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
        <para>
         Creates a new logical (decoding) replication slot named
         <parameter>slot_name</parameter> using the output plugin
-        <parameter>plugin</parameter>. The optional third
+        <parameter>plugin</parameter>. The name cannot be
+        <literal>pg_conflict_detection</literal>, as it is reserved for
+        logical replication conflict detection. The optional third
         parameter, <parameter>temporary</parameter>, when set to true, specifies that
         the slot should not be permanently stored to disk and is only meant
         for use by the current session. Temporary slots are also
@@ -29785,6 +29789,8 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
        <para>
         Copies an existing physical replication slot named <parameter>src_slot_name</parameter>
         to a physical replication slot named <parameter>dst_slot_name</parameter>.
+        The new slot name cannot be <literal>pg_conflict_detection</literal>,
+        as it is reserved for logical replication conflict detection.
         The copied physical slot starts to reserve WAL from the same <acronym>LSN</acronym> as the
         source slot.
         <parameter>temporary</parameter> is optional. If <parameter>temporary</parameter>
@@ -29807,7 +29813,9 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
         Copies an existing logical replication slot
         named <parameter>src_slot_name</parameter> to a logical replication
         slot named <parameter>dst_slot_name</parameter>, optionally changing
-        the output plugin and persistence.  The copied logical slot starts
+        the output plugin and persistence.  The name cannot be
+        <literal>pg_conflict_detection</literal>, as it is reserved for
+        logical replication conflict detection.  The copied logical slot starts
         from the same <acronym>LSN</acronym> as the source logical slot.  Both
         <parameter>temporary</parameter> and <parameter>plugin</parameter> are
         optional; if they are omitted, the values of the source slot are used.
diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
index aab1fc06de2..edadb8348ed 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -2220,6 +2220,8 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
          <para>
           The name of the slot to create. Must be a valid replication slot
           name (see <xref linkend="streaming-replication-slots-manipulation"/>).
+          The name cannot be <literal>pg_conflict_detection</literal>, as it
+          is reserved for logical replication conflict detection.
          </para>
         </listitem>
        </varlistentry>
diff --git a/doc/src/sgml/ref/create_subscription.sgml b/doc/src/sgml/ref/create_subscription.sgml
index 57dec28a5df..eec85cde880 100644
--- a/doc/src/sgml/ref/create_subscription.sgml
+++ b/doc/src/sgml/ref/create_subscription.sgml
@@ -169,7 +169,9 @@ CREATE SUBSCRIPTION <replaceable class="parameter">subscription_name</replaceabl
         <listitem>
          <para>
           Name of the publisher's replication slot to use.  The default is
-          to use the name of the subscription for the slot name.
+          to use the name of the subscription for the slot name. The name cannot
+          be <literal>pg_conflict_detection</literal>, as it is reserved for
+          logical replication conflict detection.
          </para>
 
          <para>
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index 6ce979f2d8b..2dcda37bc77 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -4760,7 +4760,7 @@ bool
 check_primary_slot_name(char **newval, void **extra, GucSource source)
 {
 	if (*newval && strcmp(*newval, "") != 0 &&
-		!ReplicationSlotValidateName(*newval, WARNING))
+		!ReplicationSlotValidateName(*newval, false, WARNING))
 		return false;
 
 	return true;
diff --git a/src/backend/commands/subscriptioncmds.c b/src/backend/commands/subscriptioncmds.c
index 4aec73bcc6b..46d4e65da97 100644
--- a/src/backend/commands/subscriptioncmds.c
+++ b/src/backend/commands/subscriptioncmds.c
@@ -210,7 +210,7 @@ parse_subscription_options(ParseState *pstate, List *stmt_options,
 			if (strcmp(opts->slot_name, "none") == 0)
 				opts->slot_name = NULL;
 			else
-				ReplicationSlotValidateName(opts->slot_name, ERROR);
+				ReplicationSlotValidateName(opts->slot_name, false, ERROR);
 		}
 		else if (IsSet(supported_opts, SUBOPT_COPY_DATA) &&
 				 strcmp(defel->defname, "copy_data") == 0)
diff --git a/src/backend/replication/logical/launcher.c b/src/backend/replication/logical/launcher.c
index 8e42787a426..8cef4460848 100644
--- a/src/backend/replication/logical/launcher.c
+++ b/src/backend/replication/logical/launcher.c
@@ -32,6 +32,7 @@
 #include "postmaster/interrupt.h"
 #include "replication/logicallauncher.h"
 #include "replication/origin.h"
+#include "replication/slot.h"
 #include "replication/walreceiver.h"
 #include "replication/worker_internal.h"
 #include "storage/ipc.h"
@@ -91,7 +92,6 @@ static dshash_table *last_start_times = NULL;
 static bool on_commit_launcher_wakeup = false;
 
 
-static void ApplyLauncherWakeup(void);
 static void logicalrep_launcher_onexit(int code, Datum arg);
 static void logicalrep_worker_onexit(int code, Datum arg);
 static void logicalrep_worker_detach(void);
@@ -100,6 +100,9 @@ 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 create_conflict_slot_if_not_exists(void);
+static void advance_conflict_slot_xmin(FullTransactionId new_xmin);
+static void drop_conflict_slot_if_exists(void);
 
 
 /*
@@ -1106,7 +1109,10 @@ ApplyLauncherWakeupAtCommit(void)
 		on_commit_launcher_wakeup = true;
 }
 
-static void
+/*
+ * Wakeup the launcher immediately.
+ */
+void
 ApplyLauncherWakeup(void)
 {
 	if (LogicalRepCtx->launcher_pid != 0)
@@ -1119,6 +1125,8 @@ ApplyLauncherWakeup(void)
 void
 ApplyLauncherMain(Datum main_arg)
 {
+	bool		slot_maybe_exist = true;
+
 	ereport(DEBUG1,
 			(errmsg_internal("logical replication launcher started")));
 
@@ -1147,6 +1155,8 @@ ApplyLauncherMain(Datum main_arg)
 		MemoryContext subctx;
 		MemoryContext oldctx;
 		long		wait_time = DEFAULT_NAPTIME_PER_CYCLE;
+		bool		can_advance_xmin = true;
+		FullTransactionId xmin = InvalidFullTransactionId;
 
 		CHECK_FOR_INTERRUPTS();
 
@@ -1166,15 +1176,56 @@ ApplyLauncherMain(Datum main_arg)
 			TimestampTz now;
 			long		elapsed;
 
+			/*
+			 * Create the conflict slot before starting the worker to prevent
+			 * it from unnecessarily maintaining its oldest_nonremovable_xid.
+			 */
+			create_conflict_slot_if_not_exists();
+
 			if (!sub->enabled)
+			{
+				can_advance_xmin = false;
 				continue;
+			}
 
 			LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
 			w = logicalrep_worker_find(sub->oid, InvalidOid, false);
 			LWLockRelease(LogicalRepWorkerLock);
 
 			if (w != NULL)
+			{
+				/*
+				 * Collect non-removable transaction IDs from all apply
+				 * workers to determine the xmin for advancing the replication
+				 * slot used in conflict detection.
+				 */
+				if (can_advance_xmin)
+				{
+					FullTransactionId nonremovable_xid;
+
+					SpinLockAcquire(&w->relmutex);
+					nonremovable_xid = w->oldest_nonremovable_xid;
+					SpinLockRelease(&w->relmutex);
+
+					/*
+					 * Stop advancing xmin if an invalid non-removable
+					 * transaction ID is found, otherwise update xmin.
+					 */
+					if (!FullTransactionIdIsValid(nonremovable_xid))
+						can_advance_xmin = false;
+					else if (!FullTransactionIdIsValid(xmin) ||
+							 FullTransactionIdPrecedes(nonremovable_xid, xmin))
+						xmin = nonremovable_xid;
+				}
+
 				continue;		/* worker is running already */
+			}
+
+			/*
+			 * The worker has not yet started, so there is no valid
+			 * non-removable transaction ID available for advancement.
+			 */
+			can_advance_xmin = false;
 
 			/*
 			 * If the worker is eligible to start now, launch it.  Otherwise,
@@ -1207,6 +1258,27 @@ ApplyLauncherMain(Datum main_arg)
 			}
 		}
 
+		/*
+		 * Maintain the xmin value of the replication slot for conflict
+		 * detection if needed.
+		 */
+		if (sublist)
+		{
+			if (can_advance_xmin)
+				advance_conflict_slot_xmin(xmin);
+
+			slot_maybe_exist = true;
+		}
+
+		/*
+		 * Drop the slot if we're no longer retaining dead tuples.
+		 */
+		else if (slot_maybe_exist)
+		{
+			drop_conflict_slot_if_exists();
+			slot_maybe_exist = false;
+		}
+
 		/* Switch back to original memory context. */
 		MemoryContextSwitchTo(oldctx);
 		/* Clean the temporary memory. */
@@ -1234,6 +1306,110 @@ ApplyLauncherMain(Datum main_arg)
 	/* Not reachable */
 }
 
+/*
+ * Create and acquire the replication slot used to retain dead tuples for
+ * conflict detection, if not yet.
+ */
+static void
+create_conflict_slot_if_not_exists(void)
+{
+	TransactionId xmin_horizon;
+
+	/* Exit early if the replication slot is already created and acquired */
+	if (MyReplicationSlot)
+		return;
+
+	/* If the replication slot exists, acquire it and exit */
+	if (SearchNamedReplicationSlot(CONFLICT_DETECTION_SLOT, true))
+	{
+		ReplicationSlotAcquire(CONFLICT_DETECTION_SLOT, true, false);
+		return;
+	}
+
+	ReplicationSlotCreate(CONFLICT_DETECTION_SLOT, false,
+						  RS_PERSISTENT, false, false, false);
+
+	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
+
+	xmin_horizon = GetOldestSafeDecodingTransactionId(false);
+
+	SpinLockAcquire(&MyReplicationSlot->mutex);
+	MyReplicationSlot->effective_xmin = xmin_horizon;
+	MyReplicationSlot->data.xmin = xmin_horizon;
+	SpinLockRelease(&MyReplicationSlot->mutex);
+
+	ReplicationSlotsComputeRequiredXmin(true);
+
+	LWLockRelease(ProcArrayLock);
+
+	/* Write this slot to disk */
+	ReplicationSlotMarkDirty();
+	ReplicationSlotSave();
+}
+
+/*
+ * Attempt to advance the xmin value of the replication slot used to retain
+ * dead tuples for conflict detection.
+ */
+static void
+advance_conflict_slot_xmin(FullTransactionId new_xmin)
+{
+	FullTransactionId full_xmin;
+	FullTransactionId next_full_xid;
+
+	Assert(MyReplicationSlot);
+	Assert(FullTransactionIdIsValid(new_xmin));
+
+	next_full_xid = ReadNextFullTransactionId();
+
+	/*
+	 * Compute FullTransactionId for the current xmin. This handles the case
+	 * where transaction ID wraparound has occurred.
+	 */
+	full_xmin = FullTransactionIdFromAllowableAt(next_full_xid,
+												 MyReplicationSlot->data.xmin);
+
+	if (FullTransactionIdPrecedesOrEquals(new_xmin, full_xmin))
+		return;
+
+	SpinLockAcquire(&MyReplicationSlot->mutex);
+	MyReplicationSlot->data.xmin = XidFromFullTransactionId(new_xmin);
+	SpinLockRelease(&MyReplicationSlot->mutex);
+
+	/* first write new xmin to disk, so we know what's up after a crash */
+
+	ReplicationSlotMarkDirty();
+	ReplicationSlotSave();
+	elog(DEBUG1, "updated xmin: %u", MyReplicationSlot->data.xmin);
+
+	/*
+	 * Now the new xmin is safely on disk, we can let the global value
+	 * advance. We do not take ProcArrayLock or similar since we only advance
+	 * xmin here and there's not much harm done by a concurrent computation
+	 * missing that.
+	 */
+	SpinLockAcquire(&MyReplicationSlot->mutex);
+	MyReplicationSlot->effective_xmin = MyReplicationSlot->data.xmin;
+	SpinLockRelease(&MyReplicationSlot->mutex);
+
+	ReplicationSlotsComputeRequiredXmin(false);
+
+	return;
+}
+
+/*
+ * Drop the replication slot used to retain dead tuples for conflict detection,
+ * if it exists.
+ */
+static void
+drop_conflict_slot_if_exists(void)
+{
+	if (MyReplicationSlot)
+		ReplicationSlotDropAcquired();
+	else if (SearchNamedReplicationSlot(CONFLICT_DETECTION_SLOT, true))
+		ReplicationSlotDrop(CONFLICT_DETECTION_SLOT, true);
+}
+
 /*
  * Is current process the logical replication launcher?
  */
diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index 67655111875..85239f6c316 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -4787,7 +4787,7 @@ StartupReorderBuffer(void)
 			continue;
 
 		/* if it cannot be a slot, skip the directory */
-		if (!ReplicationSlotValidateName(logical_de->d_name, DEBUG2))
+		if (!ReplicationSlotValidateName(logical_de->d_name, true, DEBUG2))
 			continue;
 
 		/*
diff --git a/src/backend/replication/logical/worker.c b/src/backend/replication/logical/worker.c
index ffbd4e3a02c..4922104b018 100644
--- a/src/backend/replication/logical/worker.c
+++ b/src/backend/replication/logical/worker.c
@@ -4357,6 +4357,9 @@ wait_for_local_flush(RetainConflictInfoData *data)
 		 LSN_FORMAT_ARGS(data->remote_lsn),
 		 XidFromFullTransactionId(data->candidate_xid));
 
+	/* Notify launcher to update the xmin of the conflict slot */
+	ApplyLauncherWakeup();
+
 	/*
 	 * Reset all data fields except those used to determine the timing for the
 	 * next round of transaction ID advancement.
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 600b87fa9cb..668279cb4e8 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -47,6 +47,7 @@
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "postmaster/interrupt.h"
+#include "replication/logicallauncher.h"
 #include "replication/slotsync.h"
 #include "replication/slot.h"
 #include "replication/walsender_private.h"
@@ -172,6 +173,7 @@ static SyncStandbySlotsConfigData *synchronized_standby_slots_config;
 static XLogRecPtr ss_oldest_flush_lsn = InvalidXLogRecPtr;
 
 static void ReplicationSlotShmemExit(int code, Datum arg);
+static bool IsReservedSlotName(const char *name);
 static void ReplicationSlotDropPtr(ReplicationSlot *slot);
 
 /* internal persistency functions */
@@ -258,13 +260,17 @@ ReplicationSlotShmemExit(int code, Datum arg)
 /*
  * Check whether the passed slot name is valid and report errors at elevel.
  *
+ * An error will be reported for a reserved replication slot name if
+ * allow_reserved_name is set to false.
+ *
  * Slot names may consist out of [a-z0-9_]{1,NAMEDATALEN-1} which should allow
  * the name to be used as a directory name on every supported OS.
  *
  * Returns whether the directory name is valid or not if elevel < ERROR.
  */
 bool
-ReplicationSlotValidateName(const char *name, int elevel)
+ReplicationSlotValidateName(const char *name, bool allow_reserved_name,
+							int elevel)
 {
 	const char *cp;
 
@@ -300,9 +306,29 @@ ReplicationSlotValidateName(const char *name, int elevel)
 			return false;
 		}
 	}
+
+	if (!allow_reserved_name && IsReservedSlotName(name))
+	{
+		ereport(elevel,
+				errcode(ERRCODE_RESERVED_NAME),
+				errmsg("replication slot name \"%s\" is reserved",
+					   name));
+
+		return false;
+	}
+
 	return true;
 }
 
+/*
+ * Return true if the replication slot name is "pg_conflict_detection".
+ */
+static bool
+IsReservedSlotName(const char *name)
+{
+	return (strcmp(name, CONFLICT_DETECTION_SLOT) == 0);
+}
+
 /*
  * Create a new replication slot and mark it as used by this backend.
  *
@@ -330,7 +356,11 @@ ReplicationSlotCreate(const char *name, bool db_specific,
 
 	Assert(MyReplicationSlot == NULL);
 
-	ReplicationSlotValidateName(name, ERROR);
+	/*
+	 * The logical launcher might be creating an internal slot, so using a
+	 * reserved name is allowed in this case.
+	 */
+	ReplicationSlotValidateName(name, IsLogicalLauncher(), ERROR);
 
 	if (failover)
 	{
diff --git a/src/include/replication/logicallauncher.h b/src/include/replication/logicallauncher.h
index 82b202f3305..7b29f1814db 100644
--- a/src/include/replication/logicallauncher.h
+++ b/src/include/replication/logicallauncher.h
@@ -25,6 +25,7 @@ extern void ApplyLauncherShmemInit(void);
 extern void ApplyLauncherForgetWorkerStartTime(Oid subid);
 
 extern void ApplyLauncherWakeupAtCommit(void);
+extern void ApplyLauncherWakeup(void);
 extern void AtEOXact_ApplyLauncher(bool isCommit);
 
 extern bool IsLogicalLauncher(void);
diff --git a/src/include/replication/slot.h b/src/include/replication/slot.h
index eb0b93b1114..e03e123a2ff 100644
--- a/src/include/replication/slot.h
+++ b/src/include/replication/slot.h
@@ -20,6 +20,13 @@
 /* directory to store replication slot data in */
 #define PG_REPLSLOT_DIR     "pg_replslot"
 
+/*
+ * The reserved name for a replication slot used to retain dead tuples for
+ * conflict detection in logical replication. See
+ * maybe_advance_nonremovable_xid() for detail.
+ */
+#define CONFLICT_DETECTION_SLOT "pg_conflict_detection"
+
 /*
  * Behaviour of replication slots, upon release or crash.
  *
@@ -284,7 +291,9 @@ extern void ReplicationSlotMarkDirty(void);
 
 /* misc stuff */
 extern void ReplicationSlotInitialize(void);
-extern bool ReplicationSlotValidateName(const char *name, int elevel);
+extern bool ReplicationSlotValidateName(const char *name,
+										bool allow_reserved_name,
+										int elevel);
 extern void ReplicationSlotReserveWal(void);
 extern void ReplicationSlotsComputeRequiredXmin(bool already_locked);
 extern void ReplicationSlotsComputeRequiredLSN(void);
-- 
2.30.0.windows.2