Relax overly strict assertion

alvherre · alvherre · commit 350cdcd5e6dd · 2019-02-12T18:42:37.000-03:00
Ever since its birth, ReorderBufferBuildTupleCidHash() has contained an assertion that a catalog tuple cannot change Cmax after acquiring one. But that's wrong: if a subtransaction executes DDL that affects that catalog tuple, and later aborts and another DDL affects the same tuple, it will change Cmax. Relax the assertion to merely verify that the Cmax remains valid and monotonically increasing, instead. Add a test that tickles the relevant code. Diagnosed by, and initial patch submitted by: Arseny Sher Co-authored-by: Arseny Sher Discussion: https://postgr.es/m/874l9p8hyw.fsf@ars-thinkpad
diff --git a/contrib/test_decoding/expected/ddl.out b/contrib/test_decoding/expected/ddl.out
@@ -338,6 +338,24 @@ SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'inc
  COMMIT
 (6 rows)
 
+-- check that DDL in aborted subtransactions handled correctly
+CREATE TABLE tr_sub_ddl(data int);
+BEGIN;
+SAVEPOINT a;
+ALTER TABLE tr_sub_ddl ALTER COLUMN data TYPE text;
+INSERT INTO tr_sub_ddl VALUES ('blah-blah');
+ROLLBACK TO SAVEPOINT a;
+ALTER TABLE tr_sub_ddl ALTER COLUMN data TYPE bigint;
+INSERT INTO tr_sub_ddl VALUES(43);
+COMMIT;
+SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+                       data                       
+--------------------------------------------------
+ BEGIN
+ table public.tr_sub_ddl: INSERT: data[bigint]:43
+ COMMIT
+(3 rows)
+
 /*
  * Check whether treating a table as a catalog table works somewhat
  */
diff --git a/contrib/test_decoding/sql/ddl.sql b/contrib/test_decoding/sql/ddl.sql
@@ -215,6 +215,19 @@ INSERT INTO tr_sub(path) VALUES ('5-top-1-#1');
 COMMIT;
 
 
+SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+
+-- check that DDL in aborted subtransactions handled correctly
+CREATE TABLE tr_sub_ddl(data int);
+BEGIN;
+SAVEPOINT a;
+ALTER TABLE tr_sub_ddl ALTER COLUMN data TYPE text;
+INSERT INTO tr_sub_ddl VALUES ('blah-blah');
+ROLLBACK TO SAVEPOINT a;
+ALTER TABLE tr_sub_ddl ALTER COLUMN data TYPE bigint;
+INSERT INTO tr_sub_ddl VALUES(43);
+COMMIT;
+
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
 
 
diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
@@ -1326,15 +1326,19 @@ ReorderBufferBuildTupleCidHash(ReorderBuffer *rb, ReorderBufferTXN *txn)
 		}
 		else
 		{
+			/*
+			 * Maybe we already saw this tuple before in this transaction,
+			 * but if so it must have the same cmin.
+			 */
 			Assert(ent->cmin == change->data.tuplecid.cmin);
-			Assert(ent->cmax == InvalidCommandId ||
-				   ent->cmax == change->data.tuplecid.cmax);
 
 			/*
-			 * if the tuple got valid in this transaction and now got deleted
-			 * we already have a valid cmin stored. The cmax will be
-			 * InvalidCommandId though.
+			 * cmax may be initially invalid, but once set it can only grow,
+			 * and never become invalid again.
 			 */
+			Assert((ent->cmax == InvalidCommandId) ||
+				   ((change->data.tuplecid.cmax != InvalidCommandId) &&
+					(change->data.tuplecid.cmax > ent->cmax)));
 			ent->cmax = change->data.tuplecid.cmax;
 		}
 	}

Original file line number	Diff line number	Diff line change
`@@ -1326,15 +1326,19 @@ ReorderBufferBuildTupleCidHash(ReorderBuffer rb, ReorderBufferTXN txn)`
`1326`	`1326`	`}`
`1327`	`1327`	`else`
`1328`	`1328`	`{`
	`1329`	`+ /*`
	`1330`	`+ * Maybe we already saw this tuple before in this transaction,`
	`1331`	`+ * but if so it must have the same cmin.`
	`1332`	`+ */`
`1329`	`1333`	`Assert(ent->cmin == change->data.tuplecid.cmin);`
`1330`		`- Assert(ent->cmax == InvalidCommandId \|\|`
`1331`		`- ent->cmax == change->data.tuplecid.cmax);`
`1332`	`1334`
`1333`	`1335`	`/*`
`1334`		`- * if the tuple got valid in this transaction and now got deleted`
`1335`		`- * we already have a valid cmin stored. The cmax will be`
`1336`		`- * InvalidCommandId though.`
	`1336`	`+ * cmax may be initially invalid, but once set it can only grow,`
	`1337`	`+ * and never become invalid again.`
`1337`	`1338`	`*/`
	`1339`	`+ Assert((ent->cmax == InvalidCommandId) \|\|`
	`1340`	`+ ((change->data.tuplecid.cmax != InvalidCommandId) &&`
	`1341`	`+ (change->data.tuplecid.cmax > ent->cmax)));`
`1338`	`1342`	`ent->cmax = change->data.tuplecid.cmax;`
`1339`	`1343`	`}`
`1340`	`1344`	`}`