Add decent impl of a FileConsumer

Twisted core doesn't have a general purpose one, so we need to write one
ourselves.

Features:
- All writing happens in background thread
- Supports both push and pull producers
- Push producers get paused if the consumer falls behind

tests/util/test_file_consumer.py

@@ -0,0 +1,138 @@
+# -*- 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
+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 = StringIO()
+        consumer = BackgroundFileConsumer(string_file)
+
+        try:
+            producer = NonCallableMock(spec_set=[])
+
+            consumer.registerProducer(producer, True)
+
+            consumer.write("Foo")
+            yield consumer.wait_for_writes()
+
+            self.assertEqual(string_file.getvalue(), "Foo")
+
+            consumer.write("Bar")
+            yield consumer.wait_for_writes()
+
+            self.assertEqual(string_file.getvalue(), "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"])
+
+            consumer.registerProducer(producer, True)
+
+            with string_file.write_lock:
+                for _ in range(consumer._PAUSE_ON_QUEUE_SIZE):
+                    consumer.write("Foo")
+
+                producer.pauseProducing.assert_called_once()
+
+            yield consumer.wait_for_writes()
+            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()
+
+    def write(self, bytes):
+        self.buffer += bytes
+
+    def close(self):
+        self.closed = True