Bug #57002 Assert in upgrade_shared_lock_to_exclusive()

           for ALTER TABLE + MERGE tables

The patch for Bug#56292 changed how metadata locks are taken for MERGE
tables. After the patch, locking the MERGE table will also lock the
children tables with the same metadata lock type. This means that 
LOCK TABLES on a MERGE table also will implicitly do LOCK TABLES on
the children tables.

A consequence of this change, is that it is possible to do LOCK TABLES
on a child table both explicitly and implicitly with the same statement
and that these two locks can be of different strength. For example,
LOCK TABLES child READ, merge WRITE.

In LOCK TABLES mode, we are not allowed to take new locks and each
statement must therefore try to find an existing TABLE instance with
a suitable lock. The code that searched for a suitable TABLE instance,
only considered table level locks. If a child table was locked twice,
it was therefore possible for this code to find a TABLE instance with
suitable table level locks but without suitable metadata lock.

This problem caused the assert in upgrade_shared_lock_to_exclusive()
to be triggered as it tried to upgrade a MDL_SHARED lock to
EXCLUSIVE. The problem was a regression caused by the patch for
Bug#56292.

This patch fixes the problem by partially reverting the changes
done by Bug#56292. Now, the children tables will only use the
same metadata lock as the MERGE table for MDL_SHARED_NO_WRITE
when not in locked tables mode. This means that LOCK TABLE
on a MERGE table will not implicitly lock the children tables.
This still fixes the original problem in Bug#56292 without
causing a regression.

Test case added to merge.test.

1 files changed, 23 insertions, 1 deletions

diff --git a/mysql-test/t/merge.test b/mysql-test/t/merge.test
index 34d0b5bf237..07839257a15 100644
--- a/mysql-test/t/merge.test
+++ b/mysql-test/t/merge.test
@@ -2600,11 +2600,12 @@ UNLOCK TABLES;
 DROP TABLE m1, t1;
 
 --echo #
---echo # Locking the merge table will implicitly lock children.
+--echo # Locking the merge table won't implicitly lock children.
 --echo #
 CREATE TABLE t1 (c1 INT);
 CREATE TABLE m1 (c1 INT) ENGINE=MRG_MyISAM UNION=(t1);
 LOCK TABLE m1 WRITE;
+--error ER_TABLE_NOT_LOCKED_FOR_WRITE
 ALTER TABLE t1 ADD INDEX (c1);
 LOCK TABLE m1 WRITE, t1 WRITE;
 ALTER TABLE t1 ADD INDEX (c1);
@@ -2776,6 +2777,27 @@ REPAIR TABLE t2 USE_FRM;
 DROP TABLE t1, t2;
 
 
+--echo #
+--echo # Bug#57002 Assert in upgrade_shared_lock_to_exclusive()
+--echo #           for ALTER TABLE + MERGE tables
+--echo #
+
+--disable_warnings
+DROP TABLE IF EXISTS t1, m1;
+--enable_warnings
+
+CREATE TABLE t1(a INT) engine=myisam;
+CREATE TABLE m1(a INT) engine=merge UNION(t1);
+LOCK TABLES t1 READ, m1 WRITE;
+
+# This caused an assert
+--error ER_TABLE_NOT_LOCKED_FOR_WRITE
+ALTER TABLE t1 engine=myisam;
+
+UNLOCK TABLES;
+DROP TABLE m1, t1;
+
+
 --echo End of 6.0 tests
 
 --disable_result_log