[mysql-5.6][PR] MyRocks clone: add missing donor mutex unlocks on error exits

Summary:


Pull Request resolved: #1317
GitHub Author: Laurynas Biveinis <laurynas.biveinis@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Reviewers: herman

Reviewed By: herman

Subscribers: webscalesql-eng@fb.com

Differential Revision: https://phabricator.intern.facebook.com/D46564079

Tags: accept2ship

Author: Chun Ni

storage/rocksdb/clone/donor.cc

@@ -917,7 +917,10 @@ int donor::copy(const THD *thd, uint task_id, Ha_clone_cbk &cbk) {
     }
 
     err = handle_any_error(thd);
-    if (err != 0) return err;
+    if (err != 0) {
+      mysql_mutex_unlock(&m_donor_mutex);
+      return err;
+    }
 
     if (should_roll_checkpoint()) {
       std::size_t checkpoint_size_delta;