Fix data loss in logical replication

Previously, logical replication could lose data if one user modified a
publication to add a table while another user concurrently modified that table
and commit later than the publication modification transaction. The issue
arised during the decoding of transactions modifying the table: if the initial
catalog cache was built using a snapshot taken before the publication DDL
execution, all subsequent changes to that table were decoded with outdated
catalog cache, which caused them to be filtered from replication. This happened
because invalidation messages were only present in the publication modification
transaction, which was decoded before these subsequent changes.

This issue is not limited to publication DDLs; similar problems can occur with
ALTER TYPE statements executed concurrently with DMLs, leading to incorrect
decoding under outdated type contexts.

To address this, the commit improves logical decoding by ensuring that
invalidation messages from catalog-modifying transactions are distributed to
all concurrent in-progress transactions. This allows the necessary rebuild of
the catalog cache when decoding new changes, similar to handling historic
catalog snapshots (see SnapBuildDistributeNewCatalogSnapshot()).

Following this change, some performance regression is observed, primarily
during frequent execution of publication DDL statements that modify published
tables. This is an expected trade-off due to cache rebuild and distribution
overhead. The regression is minor or nearly nonexistent when DDLs do not affect
published tables or occur infrequently, making this a worthwhile cost to
resolve a longstanding data loss issue.

An alternative approach considered was to take a strong lock on each affected
table during publication modification. However, this would only address issues
related to publication DDLs and require locking every relation in the database
for publications created as FOR ALL TABLES, which is impractical. Thus, this
commit chooses to distribute invalidation messages as outlined above.

Author: skkyal

contrib/test_decoding/Makefile

@@ -9,7 +9,7 @@ REGRESS = ddl xact rewrite toast permissions decoding_in_xact \
 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 \
-	skip_snapshot_restore
+	skip_snapshot_restore invalidation_distrubution
 
 REGRESS_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf
 ISOLATION_OPTS = --temp-config $(top_srcdir)/contrib/test_decoding/logical.conf

contrib/test_decoding/expected/invalidation_distrubution.out

@@ -0,0 +1,20 @@
+Parsed test spec with 2 sessions
+
+starting permutation: s1_insert_tbl1 s1_begin s1_insert_tbl1 s2_alter_pub_add_tbl s1_commit s1_insert_tbl1 s2_get_binary_changes
+step s1_insert_tbl1: INSERT INTO tbl1 (val1, val2) VALUES (1, 1);
+step s1_begin: BEGIN;
+step s1_insert_tbl1: INSERT INTO tbl1 (val1, val2) VALUES (1, 1);
+step s2_alter_pub_add_tbl: ALTER PUBLICATION pub ADD TABLE tbl1;
+step s1_commit: COMMIT;
+step s1_insert_tbl1: INSERT INTO tbl1 (val1, val2) VALUES (1, 1);
+step s2_get_binary_changes: SELECT count(data) FROM pg_logical_slot_get_binary_changes('isolation_slot', NULL, NULL, 'proto_version', '4', 'publication_names', 'pub') WHERE get_byte(data, 0) = 73;
+count
+-----
+    1
+(1 row)
+
+?column?
+--------
+stop    
+(1 row)
+

contrib/test_decoding/meson.build

@@ -63,6 +63,7 @@ tests += {
       'twophase_snapshot',
       'slot_creation_error',
       'skip_snapshot_restore',
+      'invalidation_distrubution',
     ],
     'regress_args': [
       '--temp-config', files('logical.conf'),

contrib/test_decoding/specs/invalidation_distrubution.spec

@@ -0,0 +1,32 @@
+# Test that catalog cache invalidation messages are distributed to ongoing
+# transactions, ensuring they can access the updated catalog content after
+# processing these messages.
+setup
+{
+    SELECT 'init' FROM pg_create_logical_replication_slot('isolation_slot', 'pgoutput');
+    CREATE TABLE tbl1(val1 integer, val2 integer);
+    CREATE PUBLICATION pub;
+}
+
+teardown
+{
+    DROP TABLE tbl1;
+    DROP PUBLICATION pub;
+    SELECT 'stop' FROM pg_drop_replication_slot('isolation_slot');
+}
+
+session "s1"
+setup { SET synchronous_commit=on; }
+
+step "s1_begin" { BEGIN; }
+step "s1_insert_tbl1" { INSERT INTO tbl1 (val1, val2) VALUES (1, 1); }
+step "s1_commit" { COMMIT; }
+
+session "s2"
+setup { SET synchronous_commit=on; }
+
+step "s2_alter_pub_add_tbl" { ALTER PUBLICATION pub ADD TABLE tbl1; }
+step "s2_get_binary_changes" { SELECT count(data) FROM pg_logical_slot_get_binary_changes('isolation_slot', NULL, NULL, 'proto_version', '4', 'publication_names', 'pub') WHERE get_byte(data, 0) = 73; }
+
+# Expect to get one insert change. LOGICAL_REP_MSG_INSERT = 'I'
+permutation "s1_insert_tbl1" "s1_begin" "s1_insert_tbl1" "s2_alter_pub_add_tbl" "s1_commit" "s1_insert_tbl1" "s2_get_binary_changes"

src/backend/replication/logical/reorderbuffer.c

@@ -5460,3 +5460,26 @@ ResolveCminCmaxDuringDecoding(HTAB *tuplecid_data,
 		*cmax = ent->cmax;
 	return true;
 }
+
+/*
+ * Count invalidation messages of specified transaction.
+ *
+ * Returns number of messages, and msgs is set to the pointer of the linked
+ * list for the messages.
+ */
+uint32
+ReorderBufferGetInvalidations(ReorderBuffer *rb, TransactionId xid,
+							  SharedInvalidationMessage **msgs)
+{
+	ReorderBufferTXN *txn;
+
+	txn = ReorderBufferTXNByXid(rb, xid, false, NULL, InvalidXLogRecPtr,
+								false);
+
+	if (txn == NULL)
+		return 0;
+
+	*msgs = txn->invalidations;
+
+	return txn->ninvalidations;
+}

src/backend/replication/logical/snapbuild.c

@@ -161,7 +161,7 @@ static void SnapBuildFreeSnapshot(Snapshot snap);
 
 static void SnapBuildSnapIncRefcount(Snapshot snap);
 
-static void SnapBuildDistributeNewCatalogSnapshot(SnapBuild *builder, XLogRecPtr lsn);
+static void SnapBuildDistributeSnapshotAndInval(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid);
 
 static inline bool SnapBuildXidHasCatalogChanges(SnapBuild *builder, TransactionId xid,
 												 uint32 xinfo);
@@ -720,23 +720,24 @@ SnapBuildProcessNewCid(SnapBuild *builder, TransactionId xid,
 }
 
 /*
- * Add a new Snapshot to all transactions we're decoding that currently are
- * in-progress so they can see new catalog contents made by the transaction
- * that just committed. This is necessary because those in-progress
- * transactions will use the new catalog's contents from here on (at the very
- * least everything they do needs to be compatible with newer catalog
- * contents).
+ * Add a new Snapshot and invalidation messages to all transactions we're
+ * decoding that currently are in-progress so they can see new catalog contents
+ * made by the transaction that just committed. This is necessary because those
+ * in-progress transactions will use the new catalog's contents from here on
+ * (at the very least everything they do needs to be compatible with newer
+ * catalog contents).
  */
 static void
-SnapBuildDistributeNewCatalogSnapshot(SnapBuild *builder, XLogRecPtr lsn)
+SnapBuildDistributeSnapshotAndInval(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid)
 {
 	dlist_iter	txn_i;
 	ReorderBufferTXN *txn;
 
 	/*
 	 * Iterate through all toplevel transactions. This can include
 	 * subtransactions which we just don't yet know to be that, but that's
-	 * fine, they will just get an unnecessary snapshot queued.
+	 * fine, they will just get an unnecessary snapshot and invalidations
+	 * queued.
 	 */
 	dlist_foreach(txn_i, &builder->reorder->toplevel_by_lsn)
 	{
@@ -749,6 +750,14 @@ SnapBuildDistributeNewCatalogSnapshot(SnapBuild *builder, XLogRecPtr lsn)
 		 * transaction which in turn implies we don't yet need a snapshot at
 		 * all. We'll add a snapshot when the first change gets queued.
 		 *
+		 * Similarly, we don't need to add invalidations to a transaction whose
+		 * base snapshot is not yet set. Once a base snapshot is built, it will
+		 * include the xids of committed transactions that have modified the
+		 * catalog, thus reflecting the new catalog contents. The existing
+		 * catalog cache will have already been invalidated after processing
+		 * the invalidations in the transaction that modified catalogs,
+		 * ensuring that a fresh cache is constructed during decoding.
+		 *
 		 * NB: This works correctly even for subtransactions because
 		 * ReorderBufferAssignChild() takes care to transfer the base snapshot
 		 * to the top-level transaction, and while iterating the changequeue
@@ -758,13 +767,13 @@ SnapBuildDistributeNewCatalogSnapshot(SnapBuild *builder, XLogRecPtr lsn)
 			continue;
 
 		/*
-		 * We don't need to add snapshot to prepared transactions as they
-		 * should not see the new catalog contents.
+		 * We don't need to add snapshot or invalidations to prepared
+		 * transactions as they should not see the new catalog contents.
 		 */
 		if (rbtxn_is_prepared(txn))
 			continue;
 
-		elog(DEBUG2, "adding a new snapshot to %u at %X/%X",
+		elog(DEBUG2, "adding a new snapshot and invalidations to %u at %X/%X",
 			 txn->xid, LSN_FORMAT_ARGS(lsn));
 
 		/*
@@ -774,6 +783,33 @@ SnapBuildDistributeNewCatalogSnapshot(SnapBuild *builder, XLogRecPtr lsn)
 		SnapBuildSnapIncRefcount(builder->snapshot);
 		ReorderBufferAddSnapshot(builder->reorder, txn->xid, lsn,
 								 builder->snapshot);
+
+		/*
+		 * Add invalidation messages to the reorder buffer of in-progress
+		 * transactions except the current committed transaction, for which we
+		 * will execute invalidations at the end.
+		 *
+		 * It is required, otherwise, we will end up using the stale catcache
+		 * contents built by the current transaction even after its decoding,
+		 * which should have been invalidated due to concurrent catalog
+		 * changing transaction.
+		 */
+		if (txn->xid != xid)
+		{
+			uint32 ninvalidations;
+			SharedInvalidationMessage *msgs = NULL;
+
+			ninvalidations = ReorderBufferGetInvalidations(builder->reorder,
+														   xid, &msgs);
+
+			if (ninvalidations > 0)
+			{
+				Assert(msgs != NULL);
+
+				ReorderBufferAddInvalidations(builder->reorder, txn->xid, lsn,
+											  ninvalidations, msgs);
+			}
+		}
 	}
 }
 
@@ -1045,8 +1081,11 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 		/* refcount of the snapshot builder for the new snapshot */
 		SnapBuildSnapIncRefcount(builder->snapshot);
 
-		/* add a new catalog snapshot to all currently running transactions */
-		SnapBuildDistributeNewCatalogSnapshot(builder, lsn);
+		/*
+		 * Add a new catalog snapshot and invalidations messages to all
+		 * currently running transactions.
+		 */
+		SnapBuildDistributeSnapshotAndInval(builder, lsn, xid);
 	}
 }
 

src/include/replication/reorderbuffer.h

@@ -758,6 +758,10 @@ extern TransactionId *ReorderBufferGetCatalogChangesXacts(ReorderBuffer *rb);
 
 extern void ReorderBufferSetRestartPoint(ReorderBuffer *rb, XLogRecPtr ptr);
 
+extern uint32 ReorderBufferGetInvalidations(ReorderBuffer *rb,
+											TransactionId xid,
+											SharedInvalidationMessage **msgs);
+
 extern void StartupReorderBuffer(void);
 
 #endif