refs codership/mysql-wsrep#201

- Fixes query cache so that it is aware of wsrep_sync_wait.
  Query cache would return (possibly stale) results to the
  client, regardless of the value of wsrep_sync_wait.
- Includes the test case that reproduced the issue.

sql/sql_cache.cc

@@ -1846,6 +1846,7 @@ Query_cache::send_result_to_client(THD *thd, char *org_sql, uint query_length)
       goto err;
     }
   }
+
   /*
     Try to obtain an exclusive lock on the query cache. If the cache is
     disabled or if a full cache flush is in progress, the attempt to
@@ -1957,6 +1958,25 @@ def_week_frmt: %lu, in_trans: %d, autocommit: %d",
   }
   DBUG_PRINT("qcache", ("Query in query hash 0x%lx", (ulong)query_block));
 
+#ifdef WITH_WSREP
+  if (WSREP_CLIENT(thd) && wsrep_must_sync_wait(thd)) {
+    unlock();
+    if (wsrep_sync_wait(thd))
+      goto err;
+    if (try_lock(TRUE))
+      goto err;
+    query_block = (Query_cache_block *) my_hash_search(&queries,
+                                                       (uchar*) sql,
+                                                       tot_length);
+    if (query_block == 0 ||
+        query_block->query()->result() == 0 ||
+        query_block->query()->result()->type != Query_cache_block::RESULT)
+    {
+      goto err_unlock;
+    }
+  }
+#endif /* WITH_WSREP */
+
   /* Now lock and test that nothing changed while blocks was unlocked */
   BLOCK_LOCK_RD(query_block);