Fix QueueFile to support read-to-end semantics and add some tests

util/registry/queueprocess.py

@@ -1,7 +1,8 @@
 from multiprocessing import Process, Queue
+from collections import namedtuple
+
 import logging
 import multiprocessing
-import os
 import time
 import sys
 import traceback
@@ -46,23 +47,27 @@ class QueueProcess(object):
     args = (self._get_producer, self._queues, self._chunk_size, self._args)
     QueueProcess.run_process(_run, args, finished=self._finished)
 
+
+QueueResult = namedtuple('QueueResult', ['data', 'exception'])
+
 def _run(get_producer, queues, chunk_size, args):
   producer = get_producer(*args)
   while True:
     try:
-      data = producer(chunk_size) or None
+      result = QueueResult(producer(chunk_size) or None, None)
     except Exception as ex:
       message = '%s\n%s' % (ex.message, "".join(traceback.format_exception(*sys.exc_info())))
-      data = Exception(message)
+      result = QueueResult(None, Exception(message))
 
     for queue in queues:
       try:
-        queue.put(data, block=True)
+        queue.put(result, block=True)
       except Exception as ex:
         logger.exception('Exception writing to queue.')
         return
 
-    if data is None or isinstance(data, Exception):
+    # Terminate the producer loop if the data produced is empty or an exception occurred.
+    if result.data is None or result.exception is not None:
       break
 
     # Important! This allows the thread that writes the queue data to the pipe