MDEV-27962 Instant DDL downgrades the MDL when table is empty

 - Server incorrectly downgrading the MDL after prepare phase when
table is empty. mdl_exclusive_after_prepare is being set in
prepare phase only. But mdl_exclusive_after_prepare condition was
misplaced and checked before prepare phase by
commit d270525dfde86bcb92a2327234a0954083e14a94 and it is now
changed to check after prepare phase.

 - main.innodb_mysql_sync test case was changed to avoid locking
optimization when table is empty.

1 files changed, 8 insertions, 4 deletions

diff --git a/mysql-test/main/innodb_mysql_sync.result b/mysql-test/main/innodb_mysql_sync.result
index 3f284edde86..5e1f60d3e25 100644
--- a/mysql-test/main/innodb_mysql_sync.result
+++ b/mysql-test/main/innodb_mysql_sync.result
@@ -131,6 +131,7 @@ connection default;
 DROP DATABASE db1;
 # Test 2: Primary index (implicit), should block writes.
 CREATE TABLE t1(a INT NOT NULL, b INT NOT NULL) engine=innodb;
+INSERT INTO t1 VALUES(1, 2);
 SET DEBUG_SYNC= "alter_table_inplace_after_lock_downgrade SIGNAL manage WAIT_FOR query";
 # Sending:
 ALTER TABLE t1 ADD UNIQUE INDEX(a), LOCK=SHARED;
@@ -139,15 +140,16 @@ SET DEBUG_SYNC= "now WAIT_FOR manage";
 USE test;
 SELECT * FROM t1;
 a	b
+1	2
 # Sending:
-UPDATE t1 SET a=NULL;
+UPDATE t1 SET a=3;
 connection con2;
 # Waiting for SELECT to be blocked by the metadata lock on t1
 SET DEBUG_SYNC= "now SIGNAL query";
 connection default;
 # Reaping: ALTER TABLE t1 ADD UNIQUE INDEX(a)
 connection con1;
-# Reaping: UPDATE t1 SET a=NULL
+# Reaping: UPDATE t1 SET a=3
 # Test 3: Primary index (explicit), should block writes.
 connection default;
 ALTER TABLE t1 DROP INDEX a;
@@ -158,15 +160,16 @@ connection con1;
 SET DEBUG_SYNC= "now WAIT_FOR manage";
 SELECT * FROM t1;
 a	b
+3	2
 # Sending:
-UPDATE t1 SET a=NULL;
+UPDATE t1 SET a=4;
 connection con2;
 # Waiting for SELECT to be blocked by the metadata lock on t1
 SET DEBUG_SYNC= "now SIGNAL query";
 connection default;
 # Reaping: ALTER TABLE t1 ADD PRIMARY KEY (a)
 connection con1;
-# Reaping: UPDATE t1 SET a=NULL
+# Reaping: UPDATE t1 SET a=4
 # Test 4: Secondary unique index, should not block reads.
 connection default;
 SET DEBUG_SYNC= "alter_table_inplace_after_lock_downgrade SIGNAL manage WAIT_FOR query";
@@ -176,6 +179,7 @@ connection con1;
 SET DEBUG_SYNC= "now WAIT_FOR manage";
 SELECT * FROM t1;
 a	b
+4	2
 SET DEBUG_SYNC= "now SIGNAL query";
 connection default;
 # Reaping: ALTER TABLE t1 ADD UNIQUE (b)