Bug#20748660 THD->MDL_CONTEXT.OWNS_EQUAL_OR_STRONGER_LOCK | CREATE_INFO->TABLESPACE

A failing DISCARD/IMPORT TABLESPACE may leave the flag
THD::tablespace_op in an incorrect state, causing a
succeeding statement to fail. This patch introduces
a class utilizing RAII to make sure the flag is set
and reset correctly by allocating a local instance
of the class on the stack. The flag is reset by the
destructor when the instance goes out of scope.

Within the server, the THD::tablespace_op flag is used
only in 'lock_table_names()' to avoid locking the
tablespace name if this is a DISCARD or IMPORT tablespace
command (which only support file-per-table tablespaces).
This is the situation caught by this bug report.

The THD::tablespace_op flag is exported by means of the
'thd_tablespace_op()' function, which is part of the plugin
API. This function is used by the SEs Archive, Blackhole,
and InnoDB (partitioned and non-partitioned).

The usage in Archive and Blackhole is in the overriding
'handler::store_lock()' functions.

Within InnoDB, the 'thd_tablespace_op()' function is used
when opening a table, to suppress errors normally being
thrown when attempting to open a table with no tablespace.

Author: ssorumgard

mysql-test/r/alter_table.result

@@ -3345,3 +3345,23 @@ SELECT MERGE_THRESHOLD from INFORMATION_SCHEMA.INNODB_SYS_INDEXES WHERE NAME='ke
 MERGE_THRESHOLD
 45
 DROP TABLE t1;
+#
+# Bug#20748660 THD->MDL_CONTEXT.OWNS_EQUAL_OR_STRONGER_LOCK | CREATE_INFO->TABLESPACE
+#
+# A failing DISCARD/IMPORT TABLESPACE may leave the flag THD::tablespace_op
+# in an incorrect state, causing a succeeding statement to fail.
+
+# Create a tablespace with a table.
+CREATE TABLESPACE s ADD DATAFILE 's.ibd' ENGINE InnoDB;
+CREATE TABLE t (i int) TABLESPACE s ENGINE InnoDB;
+# The following statent will fail because the table is not partitioned.
+# Without the fix, the THD::tablespace_op flag will be left set.
+ALTER TABLE t DISCARD PARTITION p TABLESPACE;
+ERROR HY000: Partition management on a not partitioned table is not possible
+# Without the patch, this statement will inherit the tablespace_op flag
+# from the previous statement, causing the statement below to fail due
+# to not acquiring the required MDL lock.
+ALTER TABLE t TABLESPACE s;
+# Drop table and tablespace.
+DROP TABLE t;
+DROP TABLESPACE s ENGINE InnoDB;

mysql-test/t/alter_table.test

@@ -2638,3 +2638,27 @@ ALTER TABLE t1 DROP INDEX key1, ADD INDEX key1(fld1) COMMENT
 SHOW INDEX FROM t1;
 SELECT MERGE_THRESHOLD from INFORMATION_SCHEMA.INNODB_SYS_INDEXES WHERE NAME='key1';
 DROP TABLE t1;
+
+--echo #
+--echo # Bug#20748660 THD->MDL_CONTEXT.OWNS_EQUAL_OR_STRONGER_LOCK | CREATE_INFO->TABLESPACE
+--echo #
+--echo # A failing DISCARD/IMPORT TABLESPACE may leave the flag THD::tablespace_op
+--echo # in an incorrect state, causing a succeeding statement to fail.
+--echo
+--echo # Create a tablespace with a table.
+CREATE TABLESPACE s ADD DATAFILE 's.ibd' ENGINE InnoDB;
+CREATE TABLE t (i int) TABLESPACE s ENGINE InnoDB;
+
+--echo # The following statent will fail because the table is not partitioned.
+--echo # Without the fix, the THD::tablespace_op flag will be left set.
+--error ER_PARTITION_MGMT_ON_NONPARTITIONED
+ALTER TABLE t DISCARD PARTITION p TABLESPACE;
+
+--echo # Without the patch, this statement will inherit the tablespace_op flag
+--echo # from the previous statement, causing the statement below to fail due
+--echo # to not acquiring the required MDL lock.
+ALTER TABLE t TABLESPACE s;
+
+--echo # Drop table and tablespace.
+DROP TABLE t;
+DROP TABLESPACE s ENGINE InnoDB;

sql/sql_table.cc

@@ -5669,6 +5669,28 @@ bool mysql_create_like_table(THD* thd, TABLE_LIST* table, TABLE_LIST* src_table,
 }
 
 
+/**
+  Class utilizing RAII for correct set/reset of the
+  THD::tablespace_op flag. The destructor will reset
+  the flag when a stack allocated instance goes out
+  of scope.
+ */
+
+class Tablespace_op_flag_handler
+{
+private:
+  THD *m_thd;
+public:
+  Tablespace_op_flag_handler(THD *thd): m_thd(thd)
+  {
+    m_thd->tablespace_op= true;
+  }
+  ~Tablespace_op_flag_handler()
+  {
+    m_thd->tablespace_op= false;
+  }
+};
+
 /* table_list should contain just one table */
 int mysql_discard_or_import_tablespace(THD *thd,
                                        TABLE_LIST *table_list,
@@ -5685,11 +5707,13 @@ int mysql_discard_or_import_tablespace(THD *thd,
 
   THD_STAGE_INFO(thd, stage_discard_or_import_tablespace);
 
- /*
-   We set this flag so that ha_innobase::open and ::external_lock() do
-   not complain when we lock the table
- */
-  thd->tablespace_op= true;
+  /*
+    Set thd->tablespace_op, and reset it when the variable leaves scope.
+    We set this flag so that ha_innobase::open and ::external_lock() do
+    not complain when we lock the table
+  */
+  Tablespace_op_flag_handler set_tablespace_op(thd);
+
   /*
     Adjust values of table-level and metadata which was set in parser
     for the case general ALTER TABLE.
@@ -5702,7 +5726,6 @@ int mysql_discard_or_import_tablespace(THD *thd,
   if (open_and_lock_tables(thd, table_list, 0, &alter_prelocking_strategy))
   {
     /* purecov: begin inspected */
-    thd->tablespace_op= false;
     DBUG_RETURN(-1);
     /* purecov: end */
   }
@@ -5747,7 +5770,6 @@ int mysql_discard_or_import_tablespace(THD *thd,
                                            MDL_EXCLUSIVE,
                                            thd->variables.lock_wait_timeout))
   {
-    thd->tablespace_op= false;
     DBUG_RETURN(-1);
   }
 
@@ -5780,8 +5802,6 @@ int mysql_discard_or_import_tablespace(THD *thd,
     table_list->table->mdl_ticket->downgrade_lock(MDL_SHARED_NO_READ_WRITE);
   }
 
-  thd->tablespace_op= false;
-
   if (error == 0)
   {
     my_ok(thd);