Merge "Workaround is required to prevent deadlock in task serializers."

nailgun/nailgun/lcm/transaction_serializer.py

@@ -52,7 +52,13 @@ def _initialize_worker(serializers_factory, context):
     globals()['__factory'] = serializers_factory(context)
 
 
-def _serialize_task_for_node_in_worker(node_and_task):
+def _serialize_task_for_node_in_worker(node_and_task_and_err):
+    node_and_task, err = node_and_task_and_err
+
+    # NOTE(el): See comment for `wrap_task_iter` method.
+    if err:
+        raise err
+
     return _serialize_task_for_node(globals()['__factory'], node_and_task)
 
 
@@ -72,6 +78,23 @@ class SingleWorkerConcurrencyPolicy(object):
         )
 
 
+def wrap_task_iter(iterator):
+    """Workaround is required to prevent deadlock.
+
+    Due to a bug in core python library [1], if iterator passed
+    to imap or imap_unordered raises an exception, it hangs in
+    deadlock forever, the issue is fixed in python 2.7.10, but
+    upstream repositories have much older version.
+
+    [1] http://bugs.python.org/issue23051
+    """
+    try:
+        for i in iterator:
+            yield i, None
+    except Exception as e:
+        yield (None, None), e
+
+
 class MultiProcessingConcurrencyPolicy(object):
     def __init__(self, workers_num):
         self.workers_num = workers_num
@@ -92,7 +115,7 @@ class MultiProcessingConcurrencyPolicy(object):
 
         try:
             result = pool.imap_unordered(
-                _serialize_task_for_node_in_worker, tasks
+                _serialize_task_for_node_in_worker, wrap_task_iter(tasks)
             )
             for r in result:
                 yield r