Add test for row-locking and multixids with prepared transactions

This is a repro for the issue fixed in commit ccae90abdb. Backpatch to
v17 like that commit, although that's a little arbitrary as this test
would work on older versions too.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAA5RZ0twq5bNMq0r0QNoopQnAEv+J3qJNCrLs7HVqTEntBhJ=g@mail.gmail.com
Backpatch-through: 17

src/test/regress/expected/prepared_xacts.out

@@ -267,7 +267,37 @@ SELECT gid FROM pg_prepared_xacts WHERE gid ~ '^regress_' ORDER BY gid;
 -----
 (0 rows)
 
+-- Test row-level locks held by prepared transactions
+CREATE TABLE pxtest_rowlock (id int PRIMARY KEY, data text);
+INSERT INTO pxtest_rowlock VALUES (1, 'test data');
+BEGIN;
+SELECT * FROM pxtest_rowlock WHERE id = 1 FOR SHARE;
+ id |   data    
+----+-----------
+  1 | test data
+(1 row)
+
+PREPARE TRANSACTION 'regress_p1';
+-- Should fail because the row is locked
+SELECT * FROM pxtest_rowlock WHERE id = 1 FOR UPDATE NOWAIT;
+ERROR:  could not obtain lock on row in relation "pxtest_rowlock"
+-- Test prepared transactions that participate in multixacts. For
+-- that, lock the same row again, creating a multixid.
+BEGIN;
+SELECT * FROM pxtest_rowlock WHERE id = 1 FOR SHARE;
+ id |   data    
+----+-----------
+  1 | test data
+(1 row)
+
+PREPARE TRANSACTION 'regress_p2';
+-- Should fail because the row is locked
+SELECT * FROM pxtest_rowlock WHERE id = 1 FOR UPDATE NOWAIT;
+ERROR:  could not obtain lock on row in relation "pxtest_rowlock"
+ROLLBACK PREPARED 'regress_p1';
+ROLLBACK PREPARED 'regress_p2';
 -- Clean up
 DROP TABLE pxtest2;
 -- pxtest3 was already dropped
 DROP TABLE pxtest4;
+DROP TABLE pxtest_rowlock;

src/test/regress/sql/prepared_xacts.sql

@@ -163,7 +163,32 @@ SELECT * FROM pxtest3;
 -- There should be no prepared transactions
 SELECT gid FROM pg_prepared_xacts WHERE gid ~ '^regress_' ORDER BY gid;
 
+
+-- Test row-level locks held by prepared transactions
+CREATE TABLE pxtest_rowlock (id int PRIMARY KEY, data text);
+INSERT INTO pxtest_rowlock VALUES (1, 'test data');
+
+BEGIN;
+SELECT * FROM pxtest_rowlock WHERE id = 1 FOR SHARE;
+PREPARE TRANSACTION 'regress_p1';
+
+-- Should fail because the row is locked
+SELECT * FROM pxtest_rowlock WHERE id = 1 FOR UPDATE NOWAIT;
+
+-- Test prepared transactions that participate in multixacts. For
+-- that, lock the same row again, creating a multixid.
+BEGIN;
+SELECT * FROM pxtest_rowlock WHERE id = 1 FOR SHARE;
+PREPARE TRANSACTION 'regress_p2';
+
+-- Should fail because the row is locked
+SELECT * FROM pxtest_rowlock WHERE id = 1 FOR UPDATE NOWAIT;
+
+ROLLBACK PREPARED 'regress_p1';
+ROLLBACK PREPARED 'regress_p2';
+
 -- Clean up
 DROP TABLE pxtest2;
 -- pxtest3 was already dropped
 DROP TABLE pxtest4;
+DROP TABLE pxtest_rowlock;