diff --git a/synapse/util/async.py b/synapse/util/async.py
index 40be7fe7e3..c84b23ff46 100644
--- a/synapse/util/async.py
+++ b/synapse/util/async.py
@@ -194,3 +194,85 @@ class Linearizer(object):
self.key_to_defer.pop(key, None)
defer.returnValue(_ctx_manager())
+
+
+class ReadWriteLock(object):
+ """A deferred style read write lock.
+
+ Example:
+
+ with (yield read_write_lock.read("test_key")):
+ # do some work
+ """
+
+ # IMPLEMENTATION NOTES
+ #
+ # We track the most recent queued reader and writer deferreds (which get
+ # resolved when they release the lock).
+ #
+ # Read: We know its safe to acquire a read lock when the latest writer has
+ # been resolved. The new reader is appeneded to the list of latest readers.
+ #
+ # Write: We know its safe to acquire the write lock when both the latest
+ # writers and readers have been resolved. The new writer replaces the latest
+ # writer.
+
+ def __init__(self):
+ # Latest readers queued
+ self.key_to_current_readers = {}
+
+ # Latest writer queued
+ self.key_to_current_writer = {}
+
+ @defer.inlineCallbacks
+ def read(self, key):
+ new_defer = defer.Deferred()
+
+ curr_readers = self.key_to_current_readers.setdefault(key, set())
+ curr_writer = self.key_to_current_writer.get(key, None)
+
+ curr_readers.add(new_defer)
+
+ # We wait for the latest writer to finish writing. We can safely ignore
+ # any existing readers... as they're readers.
+ yield curr_writer
+
+ @contextmanager
+ def _ctx_manager():
+ try:
+ yield
+ finally:
+ new_defer.callback(None)
+ self.key_to_current_readers.get(key, set()).discard(new_defer)
+
+ defer.returnValue(_ctx_manager())
+
+ @defer.inlineCallbacks
+ def write(self, key):
+ new_defer = defer.Deferred()
+
+ curr_readers = self.key_to_current_readers.get(key, set())
+ curr_writer = self.key_to_current_writer.get(key, None)
+
+ # We wait on all latest readers and writer.
+ to_wait_on = list(curr_readers)
+ if curr_writer:
+ to_wait_on.append(curr_writer)
+
+ # We can clear the list of current readers since the new writer waits
+ # for them to finish.
+ curr_readers.clear()
+ self.key_to_current_writer[key] = new_defer
+
+ yield defer.gatherResults(to_wait_on)
+
+ @contextmanager
+ def _ctx_manager():
+ try:
+ yield
+ finally:
+ new_defer.callback(None)
+ if self.key_to_current_writer[key] == new_defer:
+ self.key_to_current_writer.pop(key)
+
+ defer.returnValue(_ctx_manager())
diff --git a/tests/util/test_rwlock.py b/tests/util/test_rwlock.py
new file mode 100644
index 0000000000..1d745ae1a7
--- /dev/null
+++ b/tests/util/test_rwlock.py
@@ -0,0 +1,85 @@
+# -*- coding: utf-8 -*-
+# Copyright 2016 OpenMarket 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 tests import unittest
+
+from synapse.util.async import ReadWriteLock
+
+
+class ReadWriteLockTestCase(unittest.TestCase):
+
+ def _assert_called_before_not_after(self, lst, first_false):
+ for i, d in enumerate(lst[:first_false]):
+ self.assertTrue(d.called, msg="%d was unexpectedly false" % i)
+
+ for i, d in enumerate(lst[first_false:]):
+ self.assertFalse(
+ d.called, msg="%d was unexpectedly true" % (i + first_false)
+ )
+
+ def test_rwlock(self):
+ rwlock = ReadWriteLock()
+
+ key = object()
+
+ ds = [
+ rwlock.read(key), # 0
+ rwlock.read(key), # 1
+ rwlock.write(key), # 2
+ rwlock.write(key), # 3
+ rwlock.read(key), # 4
+ rwlock.read(key), # 5
+ rwlock.write(key), # 6
+ ]
+
+ self._assert_called_before_not_after(ds, 2)
+
+ with ds[0].result:
+ self._assert_called_before_not_after(ds, 2)
+ self._assert_called_before_not_after(ds, 2)
+
+ with ds[1].result:
+ self._assert_called_before_not_after(ds, 2)
+ self._assert_called_before_not_after(ds, 3)
+
+ with ds[2].result:
+ self._assert_called_before_not_after(ds, 3)
+ self._assert_called_before_not_after(ds, 4)
+
+ with ds[3].result:
+ self._assert_called_before_not_after(ds, 4)
+ self._assert_called_before_not_after(ds, 6)
+
+ with ds[5].result:
+ self._assert_called_before_not_after(ds, 6)
+ self._assert_called_before_not_after(ds, 6)
+
+ with ds[4].result:
+ self._assert_called_before_not_after(ds, 6)
+ self._assert_called_before_not_after(ds, 7)
+
+ with ds[6].result:
+ pass
+
+ d = rwlock.write(key)
+ self.assertTrue(d.called)
+ with d.result:
+ pass
+
+ d = rwlock.read(key)
+ self.assertTrue(d.called)
+ with d.result:
+ pass
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