1 files changed, 25 insertions, 2 deletions
diff --git a/storage/myisammrg/ha_myisammrg.cc b/storage/myisammrg/ha_myisammrg.cc
index 466c66ec769..26e54d61101 100644
--- a/storage/myisammrg/ha_myisammrg.cc
+++ b/storage/myisammrg/ha_myisammrg.cc
@@ -478,8 +478,31 @@ int ha_myisammrg::add_children_list(void)
     /* Set the expected table version, to not cause spurious re-prepare. */
     child_l->set_table_ref_id(mrg_child_def->get_child_table_ref_type(),
                               mrg_child_def->get_child_def_version());
-    /* Use the same metadata lock type for children. */
-    child_l->mdl_request.set_type(parent_l->mdl_request.type);
+    /*
+      For statements which acquire a SNW metadata lock on a parent table and
+      then later try to upgrade it to an X lock (e.g. ALTER TABLE), SNW
+      locks should be also taken on the children tables.
+
+      Otherwise we end up in a situation where the thread trying to upgrade SNW
+      to X lock on the parent also holds a SR metadata lock and a read
+      thr_lock.c lock on the child. As a result, another thread might be
+      blocked on the thr_lock.c lock for the child after successfully acquiring
+      a SR or SW metadata lock on it. If at the same time this second thread
+      has a shared metadata lock on the parent table or there is some other
+      thread which has a shared metadata lock on the parent and is waiting for
+      this second thread, we get a deadlock. This deadlock cannot be properly
+      detected by the MDL subsystem as part of the waiting happens within
+      thr_lock.c. By taking SNW locks on the child tables we ensure that any
+      thread which waits for a thread doing SNW -> X upgrade, does this within
+      the MDL subsystem and thus potential deadlocks are exposed to the deadlock
+      detector.
+
+      We don't do the same thing for SNRW locks as this would allow
+      DDL on implicitly locked underlying tables of a MERGE table.
+    */
+    if (! thd->locked_tables_mode &&
+        parent_l->mdl_request.type == MDL_SHARED_NO_WRITE)
+      child_l->mdl_request.set_type(MDL_SHARED_NO_WRITE);
     /* Link TABLE_LIST object into the children list. */
     if (this->children_last_l)
       child_l->prev_global= this->children_last_l;