Fix fetching default toast value during decoding of in-progress transactions.

Amit Kapila · Amit Kapila · commit 91810775904f · 2024-10-07T15:15:05.000+05:30
During logical decoding of in-progress transactions, we perform the toast table scan while fetching the default toast value for an attribute. We forgot to initialize the flag during this scan to indicate that the system table scan is in progress. We need this flag to ensure that during logical decoding we never directly access the tableam or heap APIs because we check for concurrent aborts only in systable_* APIs. Reported-by: Alexander Lakhin Author: Takeshi Ideriha, Hou Zhijie Reviewed-by: Amit Kapila, Hou Zhijie Backpatch-through: 14 Discussion: https://postgr.es/m/18641-6687273b7f15269d@postgresql.org
diff --git a/contrib/test_decoding/expected/stream.out b/contrib/test_decoding/expected/stream.out
@@ -128,6 +128,28 @@ SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL,
      5
 (1 row)
 
+-- Test that accessing a TOAST table in streaming mode is allowed.
+-- Create a table with a column that uses a TOASTed default value.
+-- (temporarily hide query, to avoid the long CREATE TABLE stmt)
+\set ECHO none
+SET debug_logical_replication_streaming = immediate;
+BEGIN;
+INSERT INTO test_tab VALUES(1);
+-- Force WAL flush, so that the above changes will be streamed.
+SELECT 'force flush' FROM pg_switch_wal();
+  ?column?   
+-------------
+ force flush
+(1 row)
+
+SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
+ count 
+-------
+     3
+(1 row)
+
+COMMIT;
+RESET debug_logical_replication_streaming;
 DROP TABLE stream_test;
 SELECT pg_drop_replication_slot('regression_slot');
  pg_drop_replication_slot 
diff --git a/contrib/test_decoding/sql/stream.sql b/contrib/test_decoding/sql/stream.sql
@@ -59,5 +59,27 @@ ROLLBACK TO s1;
 COMMIT;
 SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
 
+-- Test that accessing a TOAST table in streaming mode is allowed.
+
+-- Create a table with a column that uses a TOASTed default value.
+-- (temporarily hide query, to avoid the long CREATE TABLE stmt)
+\set ECHO none
+SELECT 'CREATE TABLE test_tab (a text DEFAULT ''' || string_agg('toast value', '') || ''');' FROM generate_series(1, 4000)
+\gexec
+\set ECHO all
+
+SET debug_logical_replication_streaming = immediate;
+
+BEGIN;
+INSERT INTO test_tab VALUES(1);
+
+-- Force WAL flush, so that the above changes will be streamed.
+SELECT 'force flush' FROM pg_switch_wal();
+
+SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
+COMMIT;
+
+RESET debug_logical_replication_streaming;
+
 DROP TABLE stream_test;
 SELECT pg_drop_replication_slot('regression_slot');
diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c
@@ -702,6 +702,14 @@ systable_beginscan_ordered(Relation heapRelation,
 	index_rescan(sysscan->iscan, key, nkeys, NULL, 0);
 	sysscan->scan = NULL;
 
+	/*
+	 * If CheckXidAlive is set then set a flag to indicate that system table
+	 * scan is in-progress.  See detailed comments in xact.c where these
+	 * variables are declared.
+	 */
+	if (TransactionIdIsValid(CheckXidAlive))
+		bsysscan = true;
+
 	return sysscan;
 }
 
@@ -746,6 +754,14 @@ systable_endscan_ordered(SysScanDesc sysscan)
 	index_endscan(sysscan->iscan);
 	if (sysscan->snapshot)
 		UnregisterSnapshot(sysscan->snapshot);
+
+	/*
+	 * Reset the bsysscan flag at the end of the systable scan.  See detailed
+	 * comments in xact.c where these variables are declared.
+	 */
+	if (TransactionIdIsValid(CheckXidAlive))
+		bsysscan = false;
+
 	pfree(sysscan);
 }
 
