Bug#46339 - crash on REPAIR TABLE merge table USE_FRM

REPAIR TABLE ... USE_FRM crashed debug servers.

A wrong assert assumed that this operation would not be executed
for MERGE tables.

Removed the assert.

mysql-test/r/merge.result

@@ -2298,4 +2298,45 @@ t2 WHERE b SOUNDS LIKE e AND d = 1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 DROP TABLE t2, t1;
+#
+# Bug#46339 - crash on REPAIR TABLE merge table USE_FRM
+#
+DROP TABLE IF EXISTS m1, t1;
+CREATE TABLE t1 (c1 INT) ENGINE=MYISAM;
+CREATE TABLE m1 (c1 INT) ENGINE=MRG_MyISAM UNION=(t1) INSERT_METHOD=LAST;
+LOCK TABLE m1 READ;
+REPAIR TABLE m1 USE_FRM;
+Table	Op	Msg_type	Msg_text
+test.m1	repair	note	The storage engine for the table doesn't support repair
+UNLOCK TABLES;
+REPAIR TABLE m1 USE_FRM;
+Table	Op	Msg_type	Msg_text
+test.m1	repair	note	The storage engine for the table doesn't support repair
+DROP TABLE m1,t1;
+CREATE TABLE m1 (f1 BIGINT) ENGINE=MRG_MyISAM UNION(t1);
+REPAIR TABLE m1 USE_FRM;
+Table	Op	Msg_type	Msg_text
+test.m1	repair	Error	Can't open table
+test.m1	repair	error	Corrupt
+CREATE TABLE t1 (f1 BIGINT) ENGINE = MyISAM;
+REPAIR TABLE m1 USE_FRM;
+Table	Op	Msg_type	Msg_text
+test.m1	repair	note	The storage engine for the table doesn't support repair
+REPAIR TABLE m1;
+Table	Op	Msg_type	Msg_text
+test.m1	repair	note	The storage engine for the table doesn't support repair
+DROP TABLE m1, t1;
+CREATE TEMPORARY TABLE m1 (f1 BIGINT) ENGINE=MRG_MyISAM UNION(t1);
+REPAIR TABLE m1 USE_FRM;
+Table	Op	Msg_type	Msg_text
+test.m1	repair	Error	Table 'test.m1' doesn't exist
+test.m1	repair	error	Corrupt
+CREATE TEMPORARY TABLE t1 (f1 BIGINT) ENGINE=MyISAM;
+REPAIR TABLE m1 USE_FRM;
+Table	Op	Msg_type	Msg_text
+m1	repair	error	Cannot repair temporary table from .frm file
+REPAIR TABLE m1;
+Table	Op	Msg_type	Msg_text
+test.m1	repair	note	The storage engine for the table doesn't support repair
+DROP TABLE m1, t1;
 End of 5.1 tests

mysql-test/t/merge.test

@@ -1705,4 +1705,82 @@ t2 WHERE b SOUNDS LIKE e AND d = 1;
 
 DROP TABLE t2, t1;
 
+--echo #
+--echo # Bug#46339 - crash on REPAIR TABLE merge table USE_FRM
+--echo #
+--disable_warnings
+DROP TABLE IF EXISTS m1, t1;
+--enable_warnings
+#
+# Test derived from a proposal of Shane Bester.
+#
+CREATE TABLE t1 (c1 INT) ENGINE=MYISAM;
+CREATE TABLE m1 (c1 INT) ENGINE=MRG_MyISAM UNION=(t1) INSERT_METHOD=LAST;
+#
+# REPAIR ... USE_FRM with LOCK TABLES.
+#
+LOCK TABLE m1 READ;
+REPAIR TABLE m1 USE_FRM;
+UNLOCK TABLES;
+#
+# REPAIR ... USE_FRM without LOCK TABLES.
+#
+# This statement crashed the server (Bug#46339).
+#
+REPAIR TABLE m1 USE_FRM;
+#
+DROP TABLE m1,t1;
+#
+# Test derived from a proposal of Matthias Leich.
+#
+# Base table is missing.
+#
+CREATE TABLE m1 (f1 BIGINT) ENGINE=MRG_MyISAM UNION(t1);
+#
+# This statement crashed the server (Bug#46339).
+#
+REPAIR TABLE m1 USE_FRM;
+#
+# Create base table.
+#
+CREATE TABLE t1 (f1 BIGINT) ENGINE = MyISAM;
+#
+# This statement crashed the server (Bug#46339).
+#
+REPAIR TABLE m1 USE_FRM;
+#
+# Normal repair as reference.
+#
+REPAIR TABLE m1;
+#
+# Cleanup.
+#
+DROP TABLE m1, t1;
+#
+# Same with temporary tables.
+#
+# Base table is missing.
+#
+CREATE TEMPORARY TABLE m1 (f1 BIGINT) ENGINE=MRG_MyISAM UNION(t1);
+#
+# This statement crashed the server (Bug#46339).
+#
+REPAIR TABLE m1 USE_FRM;
+#
+# Create base table.
+#
+CREATE TEMPORARY TABLE t1 (f1 BIGINT) ENGINE=MyISAM;
+#
+# This statement crashed the server (Bug#46339).
+#
+REPAIR TABLE m1 USE_FRM;
+#
+# Normal repair as reference.
+#
+REPAIR TABLE m1;
+#
+# Cleanup.
+#
+DROP TABLE m1, t1;
+
 --echo End of 5.1 tests

sql/sql_table.cc

@@ -4384,9 +4384,6 @@ static int prepare_for_repair(THD *thd, TABLE_LIST *table_list,
     pthread_mutex_unlock(&LOCK_open);
   }
 
-  /* A MERGE table must not come here. */
-  DBUG_ASSERT(!table->child_l);
-
   /*
     REPAIR TABLE ... USE_FRM for temporary tables makes little sense.
   */