refs #5679 grab exclusive MDL for drop index

git-svn-id: file:///svn/mysql/tokudb-engine/tokudb-engine@49906 c7de825b-a66e-492c-adef-691d508d4ae1

storage/tokudb/ha_tokudb_alter_56.cc

@@ -209,6 +209,11 @@ ha_tokudb::check_if_supported_inplace_alter(TABLE *altered_table, Alter_inplace_
             !tables_have_same_keys(table, altered_table, false, false) &&
             is_disjoint_add_drop(ha_alter_info)) {
 
+            if (ctx->handler_flags & (Alter_inplace_info::DROP_INDEX + Alter_inplace_info::DROP_UNIQUE_INDEX)) {
+                // the fractal tree can not handle dropping an index concurrent with querying with the index.
+                // we grab an exclusive MDL for the drop index.
+                result = HA_ALTER_INPLACE_EXCLUSIVE_LOCK;
+            } else {
                 result = HA_ALTER_INPLACE_SHARED_LOCK;
 
                 // someday, allow multiple hot indexes via alter table add key. don't forget to change the store_lock function.
@@ -219,6 +224,7 @@ ha_tokudb::check_if_supported_inplace_alter(TABLE *altered_table, Alter_inplace_
                     result = HA_ALTER_INPLACE_NO_LOCK; 
                 }
             }
+        }
     } else
     // column default
     if (only_flags(ctx->handler_flags, Alter_inplace_info::ALTER_COLUMN_DEFAULT)) {