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

1 files changed, 158 insertions, 0 deletions

diff --git a/synapse/util/file_consumer.py b/synapse/util/file_consumer.py
new file mode 100644
index 0000000000..de478fcb3e
--- /dev/null
+++ b/synapse/util/file_consumer.py
@@ -0,0 +1,158 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vecotr 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, threads, reactor
+
+from synapse.util.logcontext import make_deferred_yieldable
+
+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
+
+        # 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
+
+        # A deferred that gets resolved when the bytes_queue gets empty.
+        # Mainly used for tests.
+        self._notify_empty_deferred = None
+
+    def registerProducer(self, producer, streaming):
+        """Part of IProducer interface
+
+        Args:
+            producer (IProducer)
+            streaming (bool): True if push based producer, False if pull
+                based.
+        """
+        self.producer = producer
+        self.streaming = streaming
+        self.finished_deferred = threads.deferToThread(self._writer)
+        if not streaming:
+            self.producer.resumeProducing()
+
+        self.paused_producer = False
+
+    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 pushed based consumer 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)
+
+                if self._notify_empty and self.bytes_queue.empty():
+                    reactor.callFromThread(self._notify_empty)
+
+                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()
+
+    def _notify_empty(self):
+        """Called when the _writer thread thinks the queue may be empty and
+        we should notify anything waiting on `wait_for_writes`
+        """
+        if self._notify_empty_deferred and self.bytes_queue.empty():
+            d = self._notify_empty_deferred
+            self._notify_empty_deferred = None
+            d.callback(None)
+
+    def wait_for_writes(self):
+        """Wait for the write queue to be empty and for writes to have
+        finished. This is mainly useful for tests.
+        """
+        if not self._notify_empty_deferred:
+            self._notify_empty_deferred = defer.Deferred()
+        return self._notify_empty_deferred