MDEV-5309 - RENAME TABLE does not check for existence of the table's engine

When RENAME TABLE is executed, it apparently does not check whether the engine
is available (unlike ALTER TABLE .. RENAME, which does). It means that if the
engine in question was not loaded on some reason, the table might become
unusable, since the engine won't know about the change.

With this patch RENAME TABLE fails if storage engine is not available.

mysql-test/r/plugin.result

@@ -321,3 +321,13 @@ UNUSABLE
 uninstall soname 'ha_example';
 select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
 plugin_name
+#
+# MDEV-5309 - RENAME TABLE does not check for existence of the table's
+#             engine
+#
+INSTALL PLUGIN example SONAME 'ha_example';
+CREATE TABLE t1(a INT) ENGINE=EXAMPLE;
+UNINSTALL PLUGIN example;
+RENAME TABLE t1 TO t2;
+ERROR 42S02: Table 'test.t1' doesn't exist
+DROP TABLE t1;

mysql-test/t/plugin.test

@@ -252,3 +252,14 @@ select plugin_name from information_schema.plugins where plugin_library like 'ha
 uninstall soname 'ha_example';
 select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
 
+
+--echo #
+--echo # MDEV-5309 - RENAME TABLE does not check for existence of the table's
+--echo #             engine
+--echo #
+INSTALL PLUGIN example SONAME 'ha_example';
+CREATE TABLE t1(a INT) ENGINE=EXAMPLE;
+UNINSTALL PLUGIN example;
+--error ER_NO_SUCH_TABLE
+RENAME TABLE t1 TO t2;
+DROP TABLE t1;

sql/sql_rename.cc

@@ -238,7 +238,6 @@ do_rename(THD *thd, TABLE_LIST *ren_table, char *new_db, char *new_table_name,
 {
   int rc= 1;
   handlerton *hton;
-  bool new_exists, old_exists;
   const char *new_alias, *old_alias;
   DBUG_ENTER("do_rename");
 
@@ -254,17 +253,13 @@ do_rename(THD *thd, TABLE_LIST *ren_table, char *new_db, char *new_table_name,
   }
   DBUG_ASSERT(new_alias);
 
-  new_exists= ha_table_exists(thd, new_db, new_alias);
-
-  if (new_exists)
+  if (ha_table_exists(thd, new_db, new_alias))
   {
     my_error(ER_TABLE_EXISTS_ERROR, MYF(0), new_alias);
     DBUG_RETURN(1);                     // This can't be skipped
   }
 
-  old_exists= ha_table_exists(thd, ren_table->db, old_alias, &hton);
-
-  if (old_exists)
+  if (ha_table_exists(thd, ren_table->db, old_alias, &hton) && hton)
   {
     DBUG_ASSERT(!thd->locked_tables_mode);
     tdc_remove_table(thd, TDC_RT_REMOVE_ALL,

sql/sql_table.cc

@@ -5149,6 +5149,7 @@ mysql_rename_table(handlerton *base, const char *old_db,
   ulonglong save_bits= thd->variables.option_bits;
   int length;
   DBUG_ENTER("mysql_rename_table");
+  DBUG_ASSERT(base);
   DBUG_PRINT("enter", ("old: '%s'.'%s'  new: '%s'.'%s'",
                        old_db, old_name, new_db, new_name));
 
@@ -5156,8 +5157,7 @@ mysql_rename_table(handlerton *base, const char *old_db,
   if (flags & NO_FK_CHECKS) 
     thd->variables.option_bits|= OPTION_NO_FOREIGN_KEY_CHECKS;
 
-  file= (base == NULL ? 0 :
-         get_new_handler((TABLE_SHARE*) 0, thd->mem_root, base));
+  file= get_new_handler((TABLE_SHARE*) 0, thd->mem_root, base);
 
   build_table_filename(from, sizeof(from) - 1, old_db, old_name, "",
                        flags & FN_FROM_IS_TMP);