Bug#58136: Crash in vio_close at concurrent disconnect and KILL

The problem is a race between a session closing its vio
(i.e. after a COM_QUIT) at the same time it is being killed by
another thread. This could trigger a assertion in vio_close()
as the two threads could end up closing the same vio, at the
same time. This could happen due to the implementation of
SIGNAL_WITH_VIO_CLOSE, which closes the vio of the thread
being killed.

The solution is to serialize the close of the Vio under
LOCK_thd_data, which protects THD data.

No regression test is added as this is essentially a debug
issue and the test case would be quite convoluted as we would
need to synchronize a session that is being killed -- which
is a bit difficult since debug sync points code does not
synchronize killed sessions.

sql/sql_class.cc

@@ -1283,6 +1283,40 @@ void THD::awake(THD::killed_state state_to_set)
   DBUG_VOID_RETURN;
 }
 
+
+/**
+  Close the Vio associated this session.
+
+  @remark LOCK_thd_data is taken due to the fact that
+          the Vio might be disassociated concurrently.
+*/
+
+void THD::disconnect()
+{
+  Vio *vio= NULL;
+
+  mysql_mutex_lock(&LOCK_thd_data);
+
+  killed= THD::KILL_CONNECTION;
+
+#ifdef SIGNAL_WITH_VIO_CLOSE
+  /*
+    Since a active vio might might have not been set yet, in
+    any case save a reference to avoid closing a inexistent
+    one or closing the vio twice if there is a active one.
+  */
+  vio= active_vio;
+  close_active_vio();
+#endif
+
+  /* Disconnect even if a active vio is not associated. */
+  if (net.vio != vio)
+    vio_close(net.vio);
+
+  mysql_mutex_unlock(&LOCK_thd_data);
+}
+
+
 /*
   Remember the location of thread info, the structure needed for
   sql_alloc() and the structure for the net buffer