Reinstate HEAP_XMAX_LOCK_ONLY|HEAP_KEYS_UPDATED as allowed

Commit 866e24d47db1 added an assert that HEAP_XMAX_LOCK_ONLY and
HEAP_KEYS_UPDATED cannot appear together, on the faulty assumption that
the latter necessarily referred to an update and not a tuple lock; but
that's wrong, because SELECT FOR UPDATE can use precisely that
combination, as evidenced by the amcheck test case added here.

Remove the Assert(), and also patch amcheck's verify_heapam.c to not
complain if the combination is found.  Also, out of overabundance of
caution, update (across all branches) README.tuplock to be more explicit
about this.

Author: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Mahendra Singh Thalor <mahi6run@gmail.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Discussion: https://postgr.es/m/20210124061758.GA11756@nol

contrib/amcheck/t/001_verify_heapam.pl

@@ -4,7 +4,7 @@ use warnings;
 use PostgresNode;
 use TestLib;
 
-use Test::More tests => 79;
+use Test::More tests => 80;
 
 my ($node, $result);
 
@@ -47,6 +47,9 @@ detects_heap_corruption(
 #
 fresh_test_table('test');
 $node->safe_psql('postgres', q(VACUUM (FREEZE, DISABLE_PAGE_SKIPPING) test));
+detects_no_corruption(
+	"verify_heapam('test')",
+	"all-frozen not corrupted table");
 corrupt_first_page('test');
 detects_heap_corruption("verify_heapam('test')",
 	"all-frozen corrupted table");
@@ -92,6 +95,15 @@ sub fresh_test_table
 		ALTER TABLE $relname ALTER b SET STORAGE external;
 		INSERT INTO $relname (a, b)
 			(SELECT gs, repeat('b',gs*10) FROM generate_series(1,1000) gs);
+		BEGIN;
+		SAVEPOINT s1;
+		SELECT 1 FROM $relname WHERE a = 42 FOR UPDATE;
+		UPDATE $relname SET b = b WHERE a = 42;
+		RELEASE s1;
+		SAVEPOINT s1;
+		SELECT 1 FROM $relname WHERE a = 42 FOR UPDATE;
+		UPDATE $relname SET b = b WHERE a = 42;
+		COMMIT;
 	));
 }
 

contrib/amcheck/verify_heapam.c

@@ -608,13 +608,6 @@ check_tuple_header_and_visibilty(HeapTupleHeader tuphdr, HeapCheckContext *ctx)
 								   ctx->tuphdr->t_hoff, ctx->lp_len));
 		header_garbled = true;
 	}
-	if ((ctx->tuphdr->t_infomask & HEAP_XMAX_LOCK_ONLY) &&
-		(ctx->tuphdr->t_infomask2 & HEAP_KEYS_UPDATED))
-	{
-		report_corruption(ctx,
-						  pstrdup("tuple is marked as only locked, but also claims key columns were updated"));
-		header_garbled = true;
-	}
 
 	if ((ctx->tuphdr->t_infomask & HEAP_XMAX_COMMITTED) &&
 		(ctx->tuphdr->t_infomask & HEAP_XMAX_IS_MULTI))

src/backend/access/heap/README.tuplock

@@ -146,9 +146,10 @@ The following infomask bits are applicable:
   FOR UPDATE; this is implemented by the HEAP_KEYS_UPDATED bit.
 
 - HEAP_KEYS_UPDATED
-  This bit lives in t_infomask2.  If set, indicates that the XMAX updated
+  This bit lives in t_infomask2.  If set, indicates that the operation(s) done
-  this tuple and changed the key values, or it deleted the tuple.
+  by the XMAX compromise the tuple key, such as a SELECT FOR UPDATE, an UPDATE
-  It's set regardless of whether the XMAX is a TransactionId or a MultiXactId.
+  that modifies the columns of the key, or a DELETE.  It's set regardless of
+  whether the XMAX is a TransactionId or a MultiXactId.
 
 We currently never set the HEAP_XMAX_COMMITTED when the HEAP_XMAX_IS_MULTI bit
 is set.

src/backend/access/heap/hio.c

@@ -49,12 +49,10 @@ RelationPutHeapTuple(Relation relation,
 
 	/*
 	 * Do not allow tuples with invalid combinations of hint bits to be placed
-	 * on a page.  These combinations are detected as corruption by the
+	 * on a page.  This combination is detected as corruption by the
-	 * contrib/amcheck logic, so if you disable one or both of these
+	 * contrib/amcheck logic, so if you disable this assertion, make
-	 * assertions, make corresponding changes there.
+	 * corresponding changes there.
 	 */
-	Assert(!((tuple->t_data->t_infomask & HEAP_XMAX_LOCK_ONLY) &&
-			 (tuple->t_data->t_infomask2 & HEAP_KEYS_UPDATED)));
 	Assert(!((tuple->t_data->t_infomask & HEAP_XMAX_COMMITTED) &&
 			 (tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI)));