From 24bdb1df051e33c42f1cb2da86f329eae9d4ff6d Mon Sep 17 00:00:00 2001
From: Masahiko Sawada <sawada.mshk@gmail.com>
Date: Mon, 25 Mar 2024 11:29:21 +0900
Subject: [PATCH] draft fix.

---
 contrib/test_decoding/Makefile                |   3 +-
 .../expected/skip_snapshot_restore.out        | 129 ++++++++++++++++++
 .../specs/skip_snapshot_restore.spec          |  58 ++++++++
 src/backend/replication/logical/logical.c     |  13 +-
 src/backend/replication/logical/slotsync.c    |   3 +-
 src/backend/replication/logical/snapbuild.c   |  16 ++-
 src/backend/storage/ipc/procarray.c           |   9 +-
 src/include/replication/snapbuild.h           |   4 +-
 src/include/storage/procarray.h               |   3 +-
 9 files changed, 228 insertions(+), 10 deletions(-)
 create mode 100644 contrib/test_decoding/expected/skip_snapshot_restore.out
 create mode 100644 contrib/test_decoding/specs/skip_snapshot_restore.spec

diff --git a/contrib/test_decoding/Makefile b/contrib/test_decoding/Makefile
index c7ce603706..a4ba1a509a 100644
--- a/contrib/test_decoding/Makefile
+++ b/contrib/test_decoding/Makefile
@@ -8,7 +8,8 @@ REGRESS = ddl xact rewrite toast permissions decoding_in_xact \
 	spill slot truncate stream stats twophase twophase_stream
 ISOLATION = mxact delayed_startup ondisk_startup concurrent_ddl_dml \
 	oldest_xmin snapshot_transfer subxact_without_top concurrent_stream \
-	twophase_snapshot slot_creation_error catalog_change_snapshot
+	twophase_snapshot slot_creation_error catalog_change_snapshot \
+	skip_snapshot_restore
 
 REGRESS_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf
 ISOLATION_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf
diff --git a/contrib/test_decoding/expected/skip_snapshot_restore.out b/contrib/test_decoding/expected/skip_snapshot_restore.out
new file mode 100644
index 0000000000..fa4498fb11
--- /dev/null
+++ b/contrib/test_decoding/expected/skip_snapshot_restore.out
@@ -0,0 +1,129 @@
+Parsed test spec with 3 sessions
+
+starting permutation: s0_init s0_begin s0_insert1 s1_init s2_checkpoint s2_get_changes_slot0 s0_insert2 s0_commit s1_get_changes_slot0 s1_get_changes_slot1
+step s0_init: SELECT 'init' FROM pg_create_logical_replication_slot('slot0', 'test_decoding');
+?column?
+--------
+init    
+(1 row)
+
+step s0_begin: BEGIN;
+step s0_insert1: INSERT INTO tbl VALUES (1);
+step s1_init: SELECT 'init' FROM pg_create_logical_replication_slot('slot1', 'test_decoding'); <waiting ...>
+step s2_checkpoint: CHECKPOINT;
+step s2_get_changes_slot0: SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
+data
+----
+(0 rows)
+
+step s0_insert2: INSERT INTO tbl VALUES (2);
+step s0_commit: COMMIT;
+step s1_init: <... completed>
+?column?
+--------
+init    
+(1 row)
+
+step s1_get_changes_slot0: SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
+data                                     
+-----------------------------------------
+BEGIN                                    
+table public.tbl: INSERT: val1[integer]:1
+table public.tbl: INSERT: val1[integer]:2
+COMMIT                                   
+(4 rows)
+
+step s1_get_changes_slot1: SELECT data FROM pg_logical_slot_get_changes('slot1', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
+data
+----
+(0 rows)
+
+?column?
+--------
+stop    
+(1 row)
+
+
+starting permutation: s0_init s0_insert1 s0_begin s0_insert1 s1_init s2_checkpoint s2_get_changes_slot0 s0_insert2 s0_commit s1_get_changes_slot0 s1_get_changes_slot1
+step s0_init: SELECT 'init' FROM pg_create_logical_replication_slot('slot0', 'test_decoding');
+?column?
+--------
+init    
+(1 row)
+
+step s0_insert1: INSERT INTO tbl VALUES (1);
+step s0_begin: BEGIN;
+step s0_insert1: INSERT INTO tbl VALUES (1);
+step s1_init: SELECT 'init' FROM pg_create_logical_replication_slot('slot1', 'test_decoding'); <waiting ...>
+step s2_checkpoint: CHECKPOINT;
+step s2_get_changes_slot0: SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
+data                                     
+-----------------------------------------
+BEGIN                                    
+table public.tbl: INSERT: val1[integer]:1
+COMMIT                                   
+(3 rows)
+
+step s0_insert2: INSERT INTO tbl VALUES (2);
+step s0_commit: COMMIT;
+step s1_init: <... completed>
+?column?
+--------
+init    
+(1 row)
+
+step s1_get_changes_slot0: SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
+data                                     
+-----------------------------------------
+BEGIN                                    
+table public.tbl: INSERT: val1[integer]:1
+table public.tbl: INSERT: val1[integer]:2
+COMMIT                                   
+(4 rows)
+
+step s1_get_changes_slot1: SELECT data FROM pg_logical_slot_get_changes('slot1', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
+data
+----
+(0 rows)
+
+?column?
+--------
+stop    
+(1 row)
+
+
+starting permutation: s0_init s0_begin s0_savepoint s0_insert1 s1_init s2_checkpoint s2_get_changes_slot0 s0_insert_cat s0_commit s1_get_changes_slot1
+step s0_init: SELECT 'init' FROM pg_create_logical_replication_slot('slot0', 'test_decoding');
+?column?
+--------
+init    
+(1 row)
+
+step s0_begin: BEGIN;
+step s0_savepoint: SAVEPOINT sp0;
+step s0_insert1: INSERT INTO tbl VALUES (1);
+step s1_init: SELECT 'init' FROM pg_create_logical_replication_slot('slot1', 'test_decoding'); <waiting ...>
+step s2_checkpoint: CHECKPOINT;
+step s2_get_changes_slot0: SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
+data
+----
+(0 rows)
+
+step s0_insert_cat: INSERT INTO tbl VALUES (1);
+step s0_commit: COMMIT;
+step s1_init: <... completed>
+?column?
+--------
+init    
+(1 row)
+
+step s1_get_changes_slot1: SELECT data FROM pg_logical_slot_get_changes('slot1', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0');
+data
+----
+(0 rows)
+
+?column?
+--------
+stop    
+(1 row)
+
diff --git a/contrib/test_decoding/specs/skip_snapshot_restore.spec b/contrib/test_decoding/specs/skip_snapshot_restore.spec
new file mode 100644
index 0000000000..81c6a66039
--- /dev/null
+++ b/contrib/test_decoding/specs/skip_snapshot_restore.spec
@@ -0,0 +1,58 @@
+# Test that a slot creation skips to restore serialized snapshot to reach
+# the consistent state.
+
+setup
+{
+    DROP TABLE IF EXISTS tbl;
+    DROP TABLE IF EXISTS user_cat;
+    CREATE TABLE tbl (val1 integer);
+    CREATE TABLE user_cat (val1 integer) WITH (user_catalog_table = true);
+}
+
+teardown
+{
+    DROP TABLE tbl;
+    DROP TABLE user_cat;
+    SELECT 'stop' FROM pg_drop_replication_slot('slot0');
+    SELECT 'stop' FROM pg_drop_replication_slot('slot1');
+}
+
+session "s0"
+setup { SET synchronous_commit = on; }
+step "s0_init" { SELECT 'init' FROM pg_create_logical_replication_slot('slot0', 'test_decoding'); }
+step "s0_begin" { BEGIN; }
+step "s0_insert1" { INSERT INTO tbl VALUES (1); }
+step "s0_insert2" { INSERT INTO tbl VALUES (2); }
+step "s0_insert_cat" { INSERT INTO tbl VALUES (1); }
+step "s0_savepoint" { SAVEPOINT sp0; }
+step "s0_commit" { COMMIT; }
+
+session "s1"
+setup { SET synchronous_commit = on; }
+step "s1_init" { SELECT 'init' FROM pg_create_logical_replication_slot('slot1', 'test_decoding'); }
+step "s1_get_changes_slot0" { SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0'); }
+step "s1_get_changes_slot1" { SELECT data FROM pg_logical_slot_get_changes('slot1', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0'); }
+
+session "s2"
+setup { SET synchronous_commit = on ;}
+step "s2_checkpoint" { CHECKPOINT; }
+step "s2_get_changes_slot0" { SELECT data FROM pg_logical_slot_get_changes('slot0', NULL, NULL, 'skip-empty-xacts', '1', 'include-xids', '0'); }
+
+
+# While 'slot1' creation by "s1_init" waits for s0-transaction to commit, the
+# RUNNING_XACTS record is written by "s2_checkpoint" and "s2_get_changes_slot1"
+# serializes consistent snapshots to the disk at LSNs where are before
+# s0-transaction's commit. After s0-transaction commits, "s1_init" resumes but
+# must not restore any serialized snapshots and reach the consistent state until
+# decoding s0-transaction's commit recrod. We check if the get_changes on 'slot1'
+# will not return any s0-transaction's changes as its confirmed_flush_lsn will
+# be after the s0-transaction's commit record.
+permutation "s0_init" "s0_begin" "s0_insert1" "s1_init" "s2_checkpoint" "s2_get_changes_slot0" "s0_insert2" "s0_commit" "s1_get_changes_slot0" "s1_get_changes_slot1"
+permutation "s0_init" "s0_insert1" "s0_begin" "s0_insert1" "s1_init" "s2_checkpoint" "s2_get_changes_slot0" "s0_insert2" "s0_commit" "s1_get_changes_slot0" "s1_get_changes_slot1"
+
+# The last decoding restarts from the NEW_CID record in the subtransaction.
+# While processing it, a same ReorderBufferChange entry would be associated
+# with both the top and the sub transaction, as the first entry. This breaks
+# an assumption in AssertTXNLsnOrder() which the first_lsn of entries must be
+# strictly higher than previous.
+permutation "s0_init" "s0_begin" "s0_savepoint" "s0_insert1" "s1_init" "s2_checkpoint" "s2_get_changes_slot0" "s0_insert_cat" "s0_commit" "s1_get_changes_slot1"
diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 51ffb623c0..bf1e9bf4e4 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -149,6 +149,7 @@ static LogicalDecodingContext *
 StartupDecodingContext(List *output_plugin_options,
 					   XLogRecPtr start_lsn,
 					   TransactionId xmin_horizon,
+					   TransactionId oldest_running,
 					   bool need_full_snapshot,
 					   bool fast_forward,
 					   XLogReaderRoutine *xl_routine,
@@ -210,8 +211,8 @@ StartupDecodingContext(List *output_plugin_options,
 
 	ctx->reorder = ReorderBufferAllocate();
 	ctx->snapshot_builder =
-		AllocateSnapshotBuilder(ctx->reorder, xmin_horizon, start_lsn,
-								need_full_snapshot, slot->data.two_phase_at);
+		AllocateSnapshotBuilder(ctx->reorder, xmin_horizon, oldest_running,
+								start_lsn, need_full_snapshot, slot->data.two_phase_at);
 
 	ctx->reorder->private_data = ctx;
 
@@ -335,6 +336,7 @@ CreateInitDecodingContext(const char *plugin,
 						  LogicalOutputPluginWriterUpdateProgress update_progress)
 {
 	TransactionId xmin_horizon = InvalidTransactionId;
+	TransactionId oldest_running = InvalidTransactionId;
 	ReplicationSlot *slot;
 	NameData	plugin_name;
 	LogicalDecodingContext *ctx;
@@ -420,7 +422,8 @@ CreateInitDecodingContext(const char *plugin,
 	 */
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
-	xmin_horizon = GetOldestSafeDecodingTransactionId(!need_full_snapshot);
+	xmin_horizon = GetOldestSafeDecodingTransactionId(!need_full_snapshot,
+													  &oldest_running);
 
 	SpinLockAcquire(&slot->mutex);
 	slot->effective_catalog_xmin = xmin_horizon;
@@ -437,6 +440,7 @@ CreateInitDecodingContext(const char *plugin,
 	ReplicationSlotSave();
 
 	ctx = StartupDecodingContext(NIL, restart_lsn, xmin_horizon,
+								 oldest_running,
 								 need_full_snapshot, false,
 								 xl_routine, prepare_write, do_write,
 								 update_progress);
@@ -584,7 +588,8 @@ CreateDecodingContext(XLogRecPtr start_lsn,
 	}
 
 	ctx = StartupDecodingContext(output_plugin_options,
-								 start_lsn, InvalidTransactionId, false,
+								 start_lsn, InvalidTransactionId,
+								 InvalidTransactionId, false,
 								 fast_forward, xl_routine, prepare_write,
 								 do_write, update_progress);
 
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 30480960c5..d46b07c8b5 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -592,6 +592,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 	{
 		NameData	plugin_name;
 		TransactionId xmin_horizon = InvalidTransactionId;
+		TransactionId tmp = InvalidTransactionId;
 
 		/* Skip creating the local slot if remote_slot is invalidated already */
 		if (remote_slot->invalidated != RS_INVAL_NONE)
@@ -623,7 +624,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
 		reserve_wal_for_local_slot(remote_slot->restart_lsn);
 
 		LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
-		xmin_horizon = GetOldestSafeDecodingTransactionId(true);
+		xmin_horizon = GetOldestSafeDecodingTransactionId(true, &tmp);
 		SpinLockAcquire(&slot->mutex);
 		slot->effective_catalog_xmin = xmin_horizon;
 		slot->data.catalog_xmin = xmin_horizon;
diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index ac24b51860..94def02f11 100644
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -185,6 +185,7 @@ struct SnapBuild
 	 * indicates there are no running xids with an xid smaller than this.
 	 */
 	TransactionId initial_xmin_horizon;
+	TransactionId initial_oldest_running;
 
 	/* Indicates if we are building full snapshot or just catalog one. */
 	bool		building_full_snapshot;
@@ -315,6 +316,7 @@ static void SnapBuildRestoreContents(int fd, char *dest, Size size, const char *
 SnapBuild *
 AllocateSnapshotBuilder(ReorderBuffer *reorder,
 						TransactionId xmin_horizon,
+						TransactionId oldest_running,
 						XLogRecPtr start_lsn,
 						bool need_full_snapshot,
 						XLogRecPtr two_phase_at)
@@ -346,6 +348,7 @@ AllocateSnapshotBuilder(ReorderBuffer *reorder,
 	builder->catchange.xip = NULL;
 
 	builder->initial_xmin_horizon = xmin_horizon;
+	builder->initial_oldest_running = oldest_running;
 	builder->start_decoding_at = start_lsn;
 	builder->building_full_snapshot = need_full_snapshot;
 	builder->two_phase_at = two_phase_at;
@@ -572,6 +575,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
 	TransactionId xid;
 	TransactionId safeXid;
 	TransactionId *newxip;
+	TransactionId tmp;
 	int			newxcnt = 0;
 
 	Assert(XactIsoLevel == XACT_REPEATABLE_READ);
@@ -605,7 +609,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
 	 * the horizon is enforced.
 	 */
 	LWLockAcquire(ProcArrayLock, LW_SHARED);
-	safeXid = GetOldestSafeDecodingTransactionId(false);
+	safeXid = GetOldestSafeDecodingTransactionId(false, &tmp);
 	LWLockRelease(ProcArrayLock);
 
 	if (TransactionIdFollows(safeXid, snap->xmin))
@@ -1305,6 +1309,15 @@ SnapBuildProcessRunningXacts(SnapBuild *builder, XLogRecPtr lsn, xl_running_xact
 											  builder->last_serialized_snapshot);
 }
 
+static bool
+xxx_restore_ok(SnapBuild *builder, xl_running_xacts *running)
+{
+	if (!TransactionIdIsValid(builder->initial_oldest_running))
+		return true;
+
+	return TransactionIdPrecedes(builder->initial_oldest_running,
+								 running->oldestRunningXid);
+}
 
 /*
  * Build the start of a snapshot that's capable of decoding the catalog.
@@ -1397,6 +1410,7 @@ SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *runn
 	}
 	/* b) valid on disk state and not building full snapshot */
 	else if (!builder->building_full_snapshot &&
+			 xxx_restore_ok(builder, running) &&
 			 SnapBuildRestore(builder, lsn))
 	{
 		/* there won't be any state to cleanup */
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index b3cd248fb6..5fbdec69ba 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -2929,10 +2929,11 @@ GetOldestActiveTransactionId(void)
  * that the caller will immediately use the xid to peg the xmin horizon.
  */
 TransactionId
-GetOldestSafeDecodingTransactionId(bool catalogOnly)
+GetOldestSafeDecodingTransactionId(bool catalogOnly, TransactionId *oldestActiveXid_p)
 {
 	ProcArrayStruct *arrayP = procArray;
 	TransactionId oldestSafeXid;
+	TransactionId oldestActiveXid;
 	int			index;
 	bool		recovery_in_progress = RecoveryInProgress();
 
@@ -2948,6 +2949,7 @@ GetOldestSafeDecodingTransactionId(bool catalogOnly)
 	 */
 	LWLockAcquire(XidGenLock, LW_SHARED);
 	oldestSafeXid = XidFromFullTransactionId(TransamVariables->nextXid);
+	oldestActiveXid = oldestSafeXid;
 
 	/*
 	 * If there's already a slot pegging the xmin horizon, we can start with
@@ -2998,11 +3000,16 @@ GetOldestSafeDecodingTransactionId(bool catalogOnly)
 
 			if (TransactionIdPrecedes(xid, oldestSafeXid))
 				oldestSafeXid = xid;
+
+			if (TransactionIdPrecedes(xid, oldestActiveXid))
+				oldestActiveXid = xid;
 		}
 	}
 
 	LWLockRelease(XidGenLock);
 
+	*oldestActiveXid_p = oldestActiveXid;
+
 	return oldestSafeXid;
 }
 
diff --git a/src/include/replication/snapbuild.h b/src/include/replication/snapbuild.h
index fbdf362396..63a380b692 100644
--- a/src/include/replication/snapbuild.h
+++ b/src/include/replication/snapbuild.h
@@ -60,7 +60,9 @@ struct xl_running_xacts;
 extern void CheckPointSnapBuild(void);
 
 extern SnapBuild *AllocateSnapshotBuilder(struct ReorderBuffer *reorder,
-										  TransactionId xmin_horizon, XLogRecPtr start_lsn,
+										  TransactionId xmin_horizon,
+										  TransactionId oldest_running,
+										  XLogRecPtr start_lsn,
 										  bool need_full_snapshot,
 										  XLogRecPtr two_phase_at);
 extern void FreeSnapshotBuilder(SnapBuild *builder);
diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index 8ca6050462..d060941e4f 100644
--- a/src/include/storage/procarray.h
+++ b/src/include/storage/procarray.h
@@ -57,7 +57,8 @@ extern bool TransactionIdIsActive(TransactionId xid);
 extern TransactionId GetOldestNonRemovableTransactionId(Relation rel);
 extern TransactionId GetOldestTransactionIdConsideredRunning(void);
 extern TransactionId GetOldestActiveTransactionId(void);
-extern TransactionId GetOldestSafeDecodingTransactionId(bool catalogOnly);
+extern TransactionId GetOldestSafeDecodingTransactionId(bool catalogOnly,
+														TransactionId *oldestActiveXid_p);
 extern void GetReplicationHorizons(TransactionId *xmin, TransactionId *catalog_xmin);
 
 extern VirtualTransactionId *GetVirtualXIDsDelayingChkpt(int *nvxids, int type);
-- 
2.39.3

