diff --git a/synapse/util/file_consumer.py b/synapse/util/file_consumer.py
new file mode 100644
index 0000000000..90a2608d6f
--- /dev/null
+++ b/synapse/util/file_consumer.py
@@ -0,0 +1,139 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from twisted.internet import threads, reactor
+
+from synapse.util.logcontext import make_deferred_yieldable, preserve_fn
+
+import Queue
+
+
+class BackgroundFileConsumer(object):
+ """A consumer that writes to a file like object. Supports both push
+ and pull producers
+
+ Args:
+ file_obj (file): The file like object to write to. Closed when
+ finished.
+ """
+
+ # For PushProducers pause if we have this many unwritten slices
+ _PAUSE_ON_QUEUE_SIZE = 5
+ # And resume once the size of the queue is less than this
+ _RESUME_ON_QUEUE_SIZE = 2
+
+ def __init__(self, file_obj):
+ self._file_obj = file_obj
+
+ # Producer we're registered with
+ self._producer = None
+
+ # True if PushProducer, false if PullProducer
+ self.streaming = False
+
+ # For PushProducers, indicates whether we've paused the producer and
+ # need to call resumeProducing before we get more data.
+ self._paused_producer = False
+
+ # Queue of slices of bytes to be written. When producer calls
+ # unregister a final None is sent.
+ self._bytes_queue = Queue.Queue()
+
+ # Deferred that is resolved when finished writing
+ self._finished_deferred = None
+
+ # If the _writer thread throws an exception it gets stored here.
+ self._write_exception = None
+
+ def registerProducer(self, producer, streaming):
+ """Part of IConsumer interface
+
+ Args:
+ producer (IProducer)
+ streaming (bool): True if push based producer, False if pull
+ based.
+ """
+ if self._producer:
+ raise Exception("registerProducer called twice")
+
+ self._producer = producer
+ self.streaming = streaming
+ self._finished_deferred = preserve_fn(threads.deferToThread)(self._writer)
+ if not streaming:
+ self._producer.resumeProducing()
+
+ def unregisterProducer(self):
+ """Part of IProducer interface
+ """
+ self._producer = None
+ if not self._finished_deferred.called:
+ self._bytes_queue.put_nowait(None)
+
+ def write(self, bytes):
+ """Part of IProducer interface
+ """
+ if self._write_exception:
+ raise self._write_exception
+
+ if self._finished_deferred.called:
+ raise Exception("consumer has closed")
+
+ self._bytes_queue.put_nowait(bytes)
+
+ # If this is a PushProducer and the queue is getting behind
+ # then we pause the producer.
+ if self.streaming and self._bytes_queue.qsize() >= self._PAUSE_ON_QUEUE_SIZE:
+ self._paused_producer = True
+ self._producer.pauseProducing()
+
+ def _writer(self):
+ """This is run in a background thread to write to the file.
+ """
+ try:
+ while self._producer or not self._bytes_queue.empty():
+ # If we've paused the producer check if we should resume the
+ # producer.
+ if self._producer and self._paused_producer:
+ if self._bytes_queue.qsize() <= self._RESUME_ON_QUEUE_SIZE:
+ reactor.callFromThread(self._resume_paused_producer)
+
+ bytes = self._bytes_queue.get()
+
+ # If we get a None (or empty list) then that's a signal used
+ # to indicate we should check if we should stop.
+ if bytes:
+ self._file_obj.write(bytes)
+
+ # If its a pull producer then we need to explicitly ask for
+ # more stuff.
+ if not self.streaming and self._producer:
+ reactor.callFromThread(self._producer.resumeProducing)
+ except Exception as e:
+ self._write_exception = e
+ raise
+ finally:
+ self._file_obj.close()
+
+ def wait(self):
+ """Returns a deferred that resolves when finished writing to file
+ """
+ return make_deferred_yieldable(self._finished_deferred)
+
+ def _resume_paused_producer(self):
+ """Gets called if we should resume producing after being paused
+ """
+ if self._paused_producer and self._producer:
+ self._paused_producer = False
+ self._producer.resumeProducing()
diff --git a/tests/util/test_file_consumer.py b/tests/util/test_file_consumer.py
new file mode 100644
index 0000000000..76e2234255
--- /dev/null
+++ b/tests/util/test_file_consumer.py
@@ -0,0 +1,176 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+from twisted.internet import defer, reactor
+from mock import NonCallableMock
+
+from synapse.util.file_consumer import BackgroundFileConsumer
+
+from tests import unittest
+from StringIO import StringIO
+
+import threading
+
+
+class FileConsumerTests(unittest.TestCase):
+
+ @defer.inlineCallbacks
+ def test_pull_consumer(self):
+ string_file = StringIO()
+ consumer = BackgroundFileConsumer(string_file)
+
+ try:
+ producer = DummyPullProducer()
+
+ yield producer.register_with_consumer(consumer)
+
+ yield producer.write_and_wait("Foo")
+
+ self.assertEqual(string_file.getvalue(), "Foo")
+
+ yield producer.write_and_wait("Bar")
+
+ self.assertEqual(string_file.getvalue(), "FooBar")
+ finally:
+ consumer.unregisterProducer()
+
+ yield consumer.wait()
+
+ self.assertTrue(string_file.closed)
+
+ @defer.inlineCallbacks
+ def test_push_consumer(self):
+ string_file = BlockingStringWrite()
+ consumer = BackgroundFileConsumer(string_file)
+
+ try:
+ producer = NonCallableMock(spec_set=[])
+
+ consumer.registerProducer(producer, True)
+
+ consumer.write("Foo")
+ yield string_file.wait_for_n_writes(1)
+
+ self.assertEqual(string_file.buffer, "Foo")
+
+ consumer.write("Bar")
+ yield string_file.wait_for_n_writes(2)
+
+ self.assertEqual(string_file.buffer, "FooBar")
+ finally:
+ consumer.unregisterProducer()
+
+ yield consumer.wait()
+
+ self.assertTrue(string_file.closed)
+
+ @defer.inlineCallbacks
+ def test_push_producer_feedback(self):
+ string_file = BlockingStringWrite()
+ consumer = BackgroundFileConsumer(string_file)
+
+ try:
+ producer = NonCallableMock(spec_set=["pauseProducing", "resumeProducing"])
+
+ resume_deferred = defer.Deferred()
+ producer.resumeProducing.side_effect = lambda: resume_deferred.callback(None)
+
+ consumer.registerProducer(producer, True)
+
+ number_writes = 0
+ with string_file.write_lock:
+ for _ in range(consumer._PAUSE_ON_QUEUE_SIZE):
+ consumer.write("Foo")
+ number_writes += 1
+
+ producer.pauseProducing.assert_called_once()
+
+ yield string_file.wait_for_n_writes(number_writes)
+
+ yield resume_deferred
+ producer.resumeProducing.assert_called_once()
+ finally:
+ consumer.unregisterProducer()
+
+ yield consumer.wait()
+
+ self.assertTrue(string_file.closed)
+
+
+class DummyPullProducer(object):
+ def __init__(self):
+ self.consumer = None
+ self.deferred = defer.Deferred()
+
+ def resumeProducing(self):
+ d = self.deferred
+ self.deferred = defer.Deferred()
+ d.callback(None)
+
+ def write_and_wait(self, bytes):
+ d = self.deferred
+ self.consumer.write(bytes)
+ return d
+
+ def register_with_consumer(self, consumer):
+ d = self.deferred
+ self.consumer = consumer
+ self.consumer.registerProducer(self, False)
+ return d
+
+
+class BlockingStringWrite(object):
+ def __init__(self):
+ self.buffer = ""
+ self.closed = False
+ self.write_lock = threading.Lock()
+
+ self._notify_write_deferred = None
+ self._number_of_writes = 0
+
+ def write(self, bytes):
+ with self.write_lock:
+ self.buffer += bytes
+ self._number_of_writes += 1
+
+ reactor.callFromThread(self._notify_write)
+
+ def close(self):
+ self.closed = True
+
+ def _notify_write(self):
+ "Called by write to indicate a write happened"
+ with self.write_lock:
+ if not self._notify_write_deferred:
+ return
+ d = self._notify_write_deferred
+ self._notify_write_deferred = None
+ d.callback(None)
+
+ @defer.inlineCallbacks
+ def wait_for_n_writes(self, n):
+ "Wait for n writes to have happened"
+ while True:
+ with self.write_lock:
+ if n <= self._number_of_writes:
+ return
+
+ if not self._notify_write_deferred:
+ self._notify_write_deferred = defer.Deferred()
+
+ d = self._notify_write_deferred
+
+ yield d
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