From 2771447c29402e1f26bb45bdb730bdd0fe02682f Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Thu, 10 Nov 2016 14:49:26 +0000
Subject: [PATCH 1/9] Store Promise<Response> instead of Response for HTTP API
 transactions

This fixes a race whereby:
 - User hits an endpoint.
 - No cached transaction so executes main code.
 - User hits same endpoint.
 - No cache transaction so executes main code.
 - Main code finishes executing and caches response and returns.
 - Main code finishes executing and caches response and returns.

 This race is common in the wild when Synapse is struggling under load.
 This commit fixes the race by:
  - User hits an endpoint.
  - Caches the promise to execute the main code and executes main code.
  - User hits same endpoint.
  - Yields on the same promise as the first request.
  - Main code finishes executing and returns, unblocking both requests.
---
 synapse/rest/client/v1/base.py               |  4 +-
 synapse/rest/client/v1/room.py               | 82 ++++++++++----------
 synapse/rest/client/v1/transactions.py       | 52 ++++---------
 synapse/rest/client/v2_alpha/sendtodevice.py | 18 +++--
 4 files changed, 68 insertions(+), 88 deletions(-)

diff --git a/synapse/rest/client/v1/base.py b/synapse/rest/client/v1/base.py
index c2a8447860..22c740c30c 100644
--- a/synapse/rest/client/v1/base.py
+++ b/synapse/rest/client/v1/base.py
@@ -18,7 +18,7 @@
 
 from synapse.http.servlet import RestServlet
 from synapse.api.urls import CLIENT_PREFIX
-from .transactions import HttpTransactionStore
+from .transactions import HttpTransactionCache
 import re
 
 import logging
@@ -59,4 +59,4 @@ class ClientV1RestServlet(RestServlet):
         self.hs = hs
         self.builder_factory = hs.get_event_builder_factory()
         self.auth = hs.get_v1auth()
-        self.txns = HttpTransactionStore()
+        self.txns = HttpTransactionCache()
diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 010fbc7c32..2e919de9f3 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -56,15 +56,15 @@ class RoomCreateRestServlet(ClientV1RestServlet):
     @defer.inlineCallbacks
     def on_PUT(self, request, txn_id):
         try:
-            defer.returnValue(
-                self.txns.get_client_transaction(request, txn_id)
-            )
+            res_deferred = self.txns.get_client_transaction(request, txn_id)
+            res = yield res_deferred
+            defer.returnValue(res)
         except KeyError:
             pass
 
-        response = yield self.on_POST(request)
-
-        self.txns.store_client_transaction(request, txn_id, response)
+        res_deferred = self.on_POST(request)
+        self.txns.store_client_transaction(request, txn_id, res_deferred)
+        response = yield res_deferred
         defer.returnValue(response)
 
     @defer.inlineCallbacks
@@ -217,15 +217,15 @@ class RoomSendEventRestServlet(ClientV1RestServlet):
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, event_type, txn_id):
         try:
-            defer.returnValue(
-                self.txns.get_client_transaction(request, txn_id)
-            )
+            res_deferred = self.txns.get_client_transaction(request, txn_id)
+            res = yield res_deferred
+            defer.returnValue(res)
         except KeyError:
             pass
-
-        response = yield self.on_POST(request, room_id, event_type, txn_id)
-
-        self.txns.store_client_transaction(request, txn_id, response)
+        
+        res_deferred = self.on_POST(request, room_id, event_type, txn_id)
+        self.txns.store_client_transaction(request, txn_id, res_deferred)
+        response = yield res_deferred
         defer.returnValue(response)
 
 
@@ -286,15 +286,15 @@ class JoinRoomAliasServlet(ClientV1RestServlet):
     @defer.inlineCallbacks
     def on_PUT(self, request, room_identifier, txn_id):
         try:
-            defer.returnValue(
-                self.txns.get_client_transaction(request, txn_id)
-            )
+            res_deferred = self.txns.get_client_transaction(request, txn_id)
+            res = yield res_deferred
+            defer.returnValue(res)
         except KeyError:
             pass
-
-        response = yield self.on_POST(request, room_identifier, txn_id)
-
-        self.txns.store_client_transaction(request, txn_id, response)
+            
+        res_deferred = self.on_POST(request, room_identifier, txn_id)
+        self.txns.store_client_transaction(request, txn_id, res_deferred)
+        response = yield res_deferred
         defer.returnValue(response)
 
 
@@ -540,17 +540,15 @@ class RoomForgetRestServlet(ClientV1RestServlet):
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, txn_id):
         try:
-            defer.returnValue(
-                self.txns.get_client_transaction(request, txn_id)
-            )
+            res_deferred = self.txns.get_client_transaction(request, txn_id)
+            res = yield res_deferred
+            defer.returnValue(res)
         except KeyError:
             pass
-
-        response = yield self.on_POST(
-            request, room_id, txn_id
-        )
-
-        self.txns.store_client_transaction(request, txn_id, response)
+        
+        res_deferred = self.on_POST(request, room_id, txn_id)
+        self.txns.store_client_transaction(request, txn_id, res_deferred)
+        response = yield res_deferred
         defer.returnValue(response)
 
 
@@ -626,17 +624,15 @@ class RoomMembershipRestServlet(ClientV1RestServlet):
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, membership_action, txn_id):
         try:
-            defer.returnValue(
-                self.txns.get_client_transaction(request, txn_id)
-            )
+            res_deferred = self.txns.get_client_transaction(request, txn_id)
+            res = yield res_deferred
+            defer.returnValue(res)
         except KeyError:
             pass
 
-        response = yield self.on_POST(
-            request, room_id, membership_action, txn_id
-        )
-
-        self.txns.store_client_transaction(request, txn_id, response)
+        res_deferred = self.on_POST(request, room_id, membership_action, txn_id)
+        self.txns.store_client_transaction(request, txn_id, res_deferred)
+        response = yield res_deferred
         defer.returnValue(response)
 
 
@@ -672,15 +668,15 @@ class RoomRedactEventRestServlet(ClientV1RestServlet):
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, event_id, txn_id):
         try:
-            defer.returnValue(
-                self.txns.get_client_transaction(request, txn_id)
-            )
+            res_deferred = self.txns.get_client_transaction(request, txn_id)
+            res = yield res_deferred
+            defer.returnValue(res)
         except KeyError:
             pass
 
-        response = yield self.on_POST(request, room_id, event_id, txn_id)
-
-        self.txns.store_client_transaction(request, txn_id, response)
+        res_deferred = self.on_POST(request, room_id, event_id, txn_id)
+        self.txns.store_client_transaction(request, txn_id, res_deferred)
+        response = yield res_deferred
         defer.returnValue(response)
 
 
diff --git a/synapse/rest/client/v1/transactions.py b/synapse/rest/client/v1/transactions.py
index 2f2c9d0881..f5012c5f59 100644
--- a/synapse/rest/client/v1/transactions.py
+++ b/synapse/rest/client/v1/transactions.py
@@ -22,57 +22,35 @@ from synapse.api.auth import get_access_token_from_request
 logger = logging.getLogger(__name__)
 
 
-# FIXME: elsewhere we use FooStore to indicate something in the storage layer...
-class HttpTransactionStore(object):
+class HttpTransactionCache(object):
 
     def __init__(self):
-        # { key : (txn_id, response) }
+        # { key : (txn_id, response_deferred) }
         self.transactions = {}
 
-    def get_response(self, key, txn_id):
-        """Retrieve a response for this request.
-
-        Args:
-            key (str): A transaction-independent key for this request. Usually
-                this is a combination of the path (without the transaction id)
-                and the user's access token.
-            txn_id (str): The transaction ID for this request
-        Returns:
-            A tuple of (HTTP response code, response content) or None.
-        """
+    def _get_response(self, key, txn_id):
         try:
-            logger.debug("get_response TxnId: %s", txn_id)
-            (last_txn_id, response) = self.transactions[key]
+            (last_txn_id, response_deferred) = self.transactions[key]
             if txn_id == last_txn_id:
                 logger.info("get_response: Returning a response for %s", txn_id)
-                return response
+                return response_deferred
         except KeyError:
             pass
         return None
 
-    def store_response(self, key, txn_id, response):
-        """Stores an HTTP response tuple.
+    def _store_response(self, key, txn_id, response_deferred):
+        self.transactions[key] = (txn_id, response_deferred)
 
-        Args:
-            key (str): A transaction-independent key for this request. Usually
-                this is a combination of the path (without the transaction id)
-                and the user's access token.
-            txn_id (str): The transaction ID for this request.
-            response (tuple): A tuple of (HTTP response code, response content)
-        """
-        logger.debug("store_response TxnId: %s", txn_id)
-        self.transactions[key] = (txn_id, response)
-
-    def store_client_transaction(self, request, txn_id, response):
-        """Stores the request/response pair of an HTTP transaction.
+    def store_client_transaction(self, request, txn_id, response_deferred):
+        """Stores the request/Promise<response> pair of an HTTP transaction.
 
         Args:
             request (twisted.web.http.Request): The twisted HTTP request. This
             request must have the transaction ID as the last path segment.
-            response (tuple): A tuple of (response code, response dict)
+            response_deferred (Promise<tuple>): A tuple of (response code, response dict)
             txn_id (str): The transaction ID for this request.
         """
-        self.store_response(self._get_key(request), txn_id, response)
+        self._store_response(self._get_key(request), txn_id, response_deferred)
 
     def get_client_transaction(self, request, txn_id):
         """Retrieves a stored response if there was one.
@@ -82,14 +60,14 @@ class HttpTransactionStore(object):
             request must have the transaction ID as the last path segment.
             txn_id (str): The transaction ID for this request.
         Returns:
-            The response tuple.
+            Promise: Resolves to the response tuple.
         Raises:
             KeyError if the transaction was not found.
         """
-        response = self.get_response(self._get_key(request), txn_id)
-        if response is None:
+        response_deferred = self._get_response(self._get_key(request), txn_id)
+        if response_deferred is None:
             raise KeyError("Transaction not found.")
-        return response
+        return response_deferred
 
     def _get_key(self, request):
         token = get_access_token_from_request(request)
diff --git a/synapse/rest/client/v2_alpha/sendtodevice.py b/synapse/rest/client/v2_alpha/sendtodevice.py
index 5975164b37..7c800ca895 100644
--- a/synapse/rest/client/v2_alpha/sendtodevice.py
+++ b/synapse/rest/client/v2_alpha/sendtodevice.py
@@ -19,7 +19,7 @@ from twisted.internet import defer
 
 from synapse.http import servlet
 from synapse.http.servlet import parse_json_object_from_request
-from synapse.rest.client.v1.transactions import HttpTransactionStore
+from synapse.rest.client.v1.transactions import HttpTransactionCache
 
 from ._base import client_v2_patterns
 
@@ -40,18 +40,25 @@ class SendToDeviceRestServlet(servlet.RestServlet):
         super(SendToDeviceRestServlet, self).__init__()
         self.hs = hs
         self.auth = hs.get_auth()
-        self.txns = HttpTransactionStore()
+        self.txns = HttpTransactionCache()
         self.device_message_handler = hs.get_device_message_handler()
 
     @defer.inlineCallbacks
     def on_PUT(self, request, message_type, txn_id):
         try:
-            defer.returnValue(
-                self.txns.get_client_transaction(request, txn_id)
-            )
+            res_deferred = self.txns.get_client_transaction(request, txn_id)
+            res = yield res_deferred
+            defer.returnValue(res)
         except KeyError:
             pass
+        
+        res_deferred = self._put(request, message_type, txn_id)
+        self.txns.store_client_transaction(request, txn_id, res_deferred)
+        res = yield res_deferred
+        defer.returnValue(res)
 
+    @defer.inlineCallbacks
+    def _put(self, request, message_type, txn_id):
         requester = yield self.auth.get_user_by_req(request)
 
         content = parse_json_object_from_request(request)
@@ -63,7 +70,6 @@ class SendToDeviceRestServlet(servlet.RestServlet):
         )
 
         response = (200, {})
-        self.txns.store_client_transaction(request, txn_id, response)
         defer.returnValue(response)
 
 

From 8a8ad46f48525812c983dadcd9e2757cebaaa6cf Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Thu, 10 Nov 2016 15:22:11 +0000
Subject: [PATCH 2/9] Flake8

---
 synapse/rest/client/v1/room.py               | 6 +++---
 synapse/rest/client/v2_alpha/sendtodevice.py | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 2e919de9f3..caa779ffaf 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -222,7 +222,7 @@ class RoomSendEventRestServlet(ClientV1RestServlet):
             defer.returnValue(res)
         except KeyError:
             pass
-        
+
         res_deferred = self.on_POST(request, room_id, event_type, txn_id)
         self.txns.store_client_transaction(request, txn_id, res_deferred)
         response = yield res_deferred
@@ -291,7 +291,7 @@ class JoinRoomAliasServlet(ClientV1RestServlet):
             defer.returnValue(res)
         except KeyError:
             pass
-            
+
         res_deferred = self.on_POST(request, room_identifier, txn_id)
         self.txns.store_client_transaction(request, txn_id, res_deferred)
         response = yield res_deferred
@@ -545,7 +545,7 @@ class RoomForgetRestServlet(ClientV1RestServlet):
             defer.returnValue(res)
         except KeyError:
             pass
-        
+
         res_deferred = self.on_POST(request, room_id, txn_id)
         self.txns.store_client_transaction(request, txn_id, res_deferred)
         response = yield res_deferred
diff --git a/synapse/rest/client/v2_alpha/sendtodevice.py b/synapse/rest/client/v2_alpha/sendtodevice.py
index 7c800ca895..4d13e793ac 100644
--- a/synapse/rest/client/v2_alpha/sendtodevice.py
+++ b/synapse/rest/client/v2_alpha/sendtodevice.py
@@ -51,7 +51,7 @@ class SendToDeviceRestServlet(servlet.RestServlet):
             defer.returnValue(res)
         except KeyError:
             pass
-        
+
         res_deferred = self._put(request, message_type, txn_id)
         self.txns.store_client_transaction(request, txn_id, res_deferred)
         res = yield res_deferred

From c7daf3136c8ae024187cd50530d3ee90b1385b13 Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Fri, 11 Nov 2016 14:13:32 +0000
Subject: [PATCH 3/9] Use observable deferreds because they are sane

---
 synapse/rest/client/v2_alpha/sendtodevice.py | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/synapse/rest/client/v2_alpha/sendtodevice.py b/synapse/rest/client/v2_alpha/sendtodevice.py
index 4d13e793ac..31167ba535 100644
--- a/synapse/rest/client/v2_alpha/sendtodevice.py
+++ b/synapse/rest/client/v2_alpha/sendtodevice.py
@@ -20,6 +20,7 @@ from twisted.internet import defer
 from synapse.http import servlet
 from synapse.http.servlet import parse_json_object_from_request
 from synapse.rest.client.v1.transactions import HttpTransactionCache
+from synapse.util.async import ObservableDeferred
 
 from ._base import client_v2_patterns
 
@@ -47,14 +48,14 @@ class SendToDeviceRestServlet(servlet.RestServlet):
     def on_PUT(self, request, message_type, txn_id):
         try:
             res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred
+            res = yield res_deferred.observe()
             defer.returnValue(res)
         except KeyError:
             pass
 
-        res_deferred = self._put(request, message_type, txn_id)
+        res_deferred = ObservableDeferred(self._put(request, message_type, txn_id))
         self.txns.store_client_transaction(request, txn_id, res_deferred)
-        res = yield res_deferred
+        res = yield res_deferred.observe()
         defer.returnValue(res)
 
     @defer.inlineCallbacks

From 42c43cfafd3e0471e4e0f6fb05e951290783ba1f Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Fri, 11 Nov 2016 14:54:10 +0000
Subject: [PATCH 4/9] Use ObservableDeferreds instead of Deferreds as they
 behave as intended

---
 synapse/rest/client/v1/room.py         | 39 +++++++++++++-------------
 synapse/rest/client/v1/transactions.py | 22 +++++++--------
 2 files changed, 31 insertions(+), 30 deletions(-)

diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index caa779ffaf..47c4b84b73 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -22,6 +22,7 @@ from synapse.streams.config import PaginationConfig
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.filtering import Filter
 from synapse.types import UserID, RoomID, RoomAlias
+from synapse.util.async import ObservableDeferred
 from synapse.events.utils import serialize_event, format_event_for_client_v2
 from synapse.http.servlet import (
     parse_json_object_from_request, parse_string, parse_integer
@@ -57,14 +58,14 @@ class RoomCreateRestServlet(ClientV1RestServlet):
     def on_PUT(self, request, txn_id):
         try:
             res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred
+            res = yield res_deferred.observe()
             defer.returnValue(res)
         except KeyError:
             pass
 
-        res_deferred = self.on_POST(request)
+        res_deferred = ObservableDeferred(self.on_POST(request))
         self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred
+        response = yield res_deferred.observe()
         defer.returnValue(response)
 
     @defer.inlineCallbacks
@@ -218,14 +219,14 @@ class RoomSendEventRestServlet(ClientV1RestServlet):
     def on_PUT(self, request, room_id, event_type, txn_id):
         try:
             res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred
+            res = yield res_deferred.observe()
             defer.returnValue(res)
         except KeyError:
             pass
 
-        res_deferred = self.on_POST(request, room_id, event_type, txn_id)
+        res_deferred = ObservableDeferred(self.on_POST(request, room_id, event_type, txn_id))
         self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred
+        response = yield res_deferred.observe()
         defer.returnValue(response)
 
 
@@ -287,14 +288,14 @@ class JoinRoomAliasServlet(ClientV1RestServlet):
     def on_PUT(self, request, room_identifier, txn_id):
         try:
             res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred
+            res = yield res_deferred.observe()
             defer.returnValue(res)
         except KeyError:
             pass
 
-        res_deferred = self.on_POST(request, room_identifier, txn_id)
+        res_deferred = ObservableDeferred(self.on_POST(request, room_identifier, txn_id))
         self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred
+        response = yield res_deferred.observe()
         defer.returnValue(response)
 
 
@@ -541,14 +542,14 @@ class RoomForgetRestServlet(ClientV1RestServlet):
     def on_PUT(self, request, room_id, txn_id):
         try:
             res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred
+            res = yield res_deferred.observe()
             defer.returnValue(res)
         except KeyError:
             pass
 
-        res_deferred = self.on_POST(request, room_id, txn_id)
+        res_deferred = ObservableDeferred(self.on_POST(request, room_id, txn_id))
         self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred
+        response = yield res_deferred.observe()
         defer.returnValue(response)
 
 
@@ -624,15 +625,15 @@ class RoomMembershipRestServlet(ClientV1RestServlet):
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, membership_action, txn_id):
         try:
-            res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred
+            res_deferred = ObservableDeferred(self.txns.get_client_transaction(request, txn_id))
+            res = yield res_deferred.observe()
             defer.returnValue(res)
         except KeyError:
             pass
 
-        res_deferred = self.on_POST(request, room_id, membership_action, txn_id)
+        res_deferred = ObservableDeffself.on_POST(request, room_id, membership_action, txn_id)
         self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred
+        response = yield res_deferred.observe()
         defer.returnValue(response)
 
 
@@ -669,14 +670,14 @@ class RoomRedactEventRestServlet(ClientV1RestServlet):
     def on_PUT(self, request, room_id, event_id, txn_id):
         try:
             res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred
+            res = yield res_deferred.observe()
             defer.returnValue(res)
         except KeyError:
             pass
 
-        res_deferred = self.on_POST(request, room_id, event_id, txn_id)
+        res_deferred = ObservableDeferred(self.on_POST(request, room_id, event_id, txn_id))
         self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred
+        response = yield res_deferred.observe()
         defer.returnValue(response)
 
 
diff --git a/synapse/rest/client/v1/transactions.py b/synapse/rest/client/v1/transactions.py
index f5012c5f59..774430458a 100644
--- a/synapse/rest/client/v1/transactions.py
+++ b/synapse/rest/client/v1/transactions.py
@@ -25,32 +25,32 @@ logger = logging.getLogger(__name__)
 class HttpTransactionCache(object):
 
     def __init__(self):
-        # { key : (txn_id, response_deferred) }
+        # { key : (txn_id, res_observ_defer) }
         self.transactions = {}
 
     def _get_response(self, key, txn_id):
         try:
-            (last_txn_id, response_deferred) = self.transactions[key]
+            (last_txn_id, res_observ_defer) = self.transactions[key]
             if txn_id == last_txn_id:
                 logger.info("get_response: Returning a response for %s", txn_id)
-                return response_deferred
+                return res_observ_defer
         except KeyError:
             pass
         return None
 
-    def _store_response(self, key, txn_id, response_deferred):
-        self.transactions[key] = (txn_id, response_deferred)
+    def _store_response(self, key, txn_id, res_observ_defer):
+        self.transactions[key] = (txn_id, res_observ_defer)
 
-    def store_client_transaction(self, request, txn_id, response_deferred):
+    def store_client_transaction(self, request, txn_id, res_observ_defer):
         """Stores the request/Promise<response> pair of an HTTP transaction.
 
         Args:
             request (twisted.web.http.Request): The twisted HTTP request. This
             request must have the transaction ID as the last path segment.
-            response_deferred (Promise<tuple>): A tuple of (response code, response dict)
+            res_observ_defer (Promise<tuple>): A tuple of (response code, response dict)
             txn_id (str): The transaction ID for this request.
         """
-        self._store_response(self._get_key(request), txn_id, response_deferred)
+        self._store_response(self._get_key(request), txn_id, res_observ_defer)
 
     def get_client_transaction(self, request, txn_id):
         """Retrieves a stored response if there was one.
@@ -64,10 +64,10 @@ class HttpTransactionCache(object):
         Raises:
             KeyError if the transaction was not found.
         """
-        response_deferred = self._get_response(self._get_key(request), txn_id)
-        if response_deferred is None:
+        res_observ_defer = self._get_response(self._get_key(request), txn_id)
+        if res_observ_defer is None:
             raise KeyError("Transaction not found.")
-        return response_deferred
+        return res_observ_defer
 
     def _get_key(self, request):
         token = get_access_token_from_request(request)

From a88bc67f88bf5b932283372818c7e33bedfbaa0b Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Fri, 11 Nov 2016 15:02:29 +0000
Subject: [PATCH 5/9] Flake8 and fix whoopsie

---
 synapse/rest/client/v1/room.py | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 47c4b84b73..a866615f71 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -224,7 +224,9 @@ class RoomSendEventRestServlet(ClientV1RestServlet):
         except KeyError:
             pass
 
-        res_deferred = ObservableDeferred(self.on_POST(request, room_id, event_type, txn_id))
+        res_deferred = ObservableDeferred(
+            self.on_POST(request, room_id, event_type, txn_id)
+        )
         self.txns.store_client_transaction(request, txn_id, res_deferred)
         response = yield res_deferred.observe()
         defer.returnValue(response)
@@ -625,13 +627,13 @@ class RoomMembershipRestServlet(ClientV1RestServlet):
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, membership_action, txn_id):
         try:
-            res_deferred = ObservableDeferred(self.txns.get_client_transaction(request, txn_id))
+            res_deferred = self.txns.get_client_transaction(request, txn_id)
             res = yield res_deferred.observe()
             defer.returnValue(res)
         except KeyError:
             pass
 
-        res_deferred = ObservableDeffself.on_POST(request, room_id, membership_action, txn_id)
+        res_deferred = ObservableDeferred(self.on_POST(request, room_id, membership_action, txn_id))
         self.txns.store_client_transaction(request, txn_id, res_deferred)
         response = yield res_deferred.observe()
         defer.returnValue(response)
@@ -675,7 +677,9 @@ class RoomRedactEventRestServlet(ClientV1RestServlet):
         except KeyError:
             pass
 
-        res_deferred = ObservableDeferred(self.on_POST(request, room_id, event_id, txn_id))
+        res_deferred = ObservableDeferred(
+            self.on_POST(request, room_id, event_id, txn_id)
+        )
         self.txns.store_client_transaction(request, txn_id, res_deferred)
         response = yield res_deferred.observe()
         defer.returnValue(response)

From f6c48802f5d01cbbc5fbd9ac151b84a80a2e3ac3 Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Fri, 11 Nov 2016 15:08:24 +0000
Subject: [PATCH 6/9] More flake8

---
 synapse/rest/client/v1/room.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index a866615f71..7af4077721 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -633,7 +633,9 @@ class RoomMembershipRestServlet(ClientV1RestServlet):
         except KeyError:
             pass
 
-        res_deferred = ObservableDeferred(self.on_POST(request, room_id, membership_action, txn_id))
+        res_deferred = ObservableDeferred(
+            self.on_POST(request, room_id, membership_action, txn_id)
+        )
         self.txns.store_client_transaction(request, txn_id, res_deferred)
         response = yield res_deferred.observe()
         defer.returnValue(response)

From 8ecaff51a147948f977e745bace697ffcba6595b Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Fri, 11 Nov 2016 17:47:03 +0000
Subject: [PATCH 7/9] Review comments

---
 synapse/rest/client/transactions.py          | 85 +++++++++++++++++
 synapse/rest/client/v1/base.py               |  3 +-
 synapse/rest/client/v1/room.py               | 97 ++++++--------------
 synapse/rest/client/v1/transactions.py       | 75 ---------------
 synapse/rest/client/v2_alpha/sendtodevice.py | 17 +---
 5 files changed, 119 insertions(+), 158 deletions(-)
 create mode 100644 synapse/rest/client/transactions.py
 delete mode 100644 synapse/rest/client/v1/transactions.py

diff --git a/synapse/rest/client/transactions.py b/synapse/rest/client/transactions.py
new file mode 100644
index 0000000000..1db972a378
--- /dev/null
+++ b/synapse/rest/client/transactions.py
@@ -0,0 +1,85 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014-2016 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.
+
+"""This module contains logic for storing HTTP PUT transactions. This is used
+to ensure idempotency when performing PUTs using the REST API."""
+import logging
+
+from synapse.api.auth import get_access_token_from_request
+from synapse.util.async import ObservableDeferred
+
+logger = logging.getLogger(__name__)
+
+
+def get_transaction_key(request):
+    """A helper function which returns a transaction key that can be used
+    with TransactionCache for idempotent requests.
+
+    Idempotency is based on the returned key being the same for separate
+    requests to the same endpoint. The key is formed from the HTTP request
+    path and the access_token for the requesting user.
+
+    Args:
+        request (twisted.web.http.Request): The incoming request. Must
+        contain an access_token.
+    Returns:
+        str: A transaction key
+    """
+    token = get_access_token_from_request(request)
+    return request.path + "/" + token
+
+
+class HttpTransactionCache(object):
+
+    def __init__(self):
+        self.transactions = {
+            # $txn_key: ObservableDeferred<(res_code, res_json_body)>
+        }
+
+    def fetch_or_execute_request(self, request, fn, *args, **kwargs):
+        """A helper function for fetch_or_execute which extracts
+        a transaction key from the given request.
+
+        See:
+            fetch_or_execute
+        """
+        return self.fetch_or_execute(
+            get_transaction_key(request), fn, *args, **kwargs
+        )
+
+    def fetch_or_execute(self, txn_key, fn, *args, **kwargs):
+        """Fetches the response for this transaction, or executes the given function
+        to produce a response for this transaction.
+
+        Args:
+            txn_key (str): A key to ensure idempotency should fetch_or_execute be
+            called again at a later point in time.
+            fn (function): A function which returns a tuple of
+            (response_code, response_dict)d
+            *args: Arguments to pass to fn.
+            **kwargs: Keyword arguments to pass to fn.
+        Returns:
+            synapse.util.async.ObservableDeferred which resolves to a tuple
+            of (response_code, response_dict).
+        """
+        try:
+            return self.transactions[txn_key]
+        except KeyError:
+            pass  # execute the function instead.
+
+        deferred = fn(*args, **kwargs)
+        observable = ObservableDeferred(deferred)
+        self.transactions[txn_key] = observable
+        return observable
diff --git a/synapse/rest/client/v1/base.py b/synapse/rest/client/v1/base.py
index 22c740c30c..07ff5b218c 100644
--- a/synapse/rest/client/v1/base.py
+++ b/synapse/rest/client/v1/base.py
@@ -18,7 +18,8 @@
 
 from synapse.http.servlet import RestServlet
 from synapse.api.urls import CLIENT_PREFIX
-from .transactions import HttpTransactionCache
+from synapse.rest.client.transactions import HttpTransactionCache
+
 import re
 
 import logging
diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 7af4077721..0622e64380 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -22,7 +22,6 @@ from synapse.streams.config import PaginationConfig
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.filtering import Filter
 from synapse.types import UserID, RoomID, RoomAlias
-from synapse.util.async import ObservableDeferred
 from synapse.events.utils import serialize_event, format_event_for_client_v2
 from synapse.http.servlet import (
     parse_json_object_from_request, parse_string, parse_integer
@@ -56,17 +55,11 @@ class RoomCreateRestServlet(ClientV1RestServlet):
 
     @defer.inlineCallbacks
     def on_PUT(self, request, txn_id):
-        try:
-            res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred.observe()
-            defer.returnValue(res)
-        except KeyError:
-            pass
-
-        res_deferred = ObservableDeferred(self.on_POST(request))
-        self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred.observe()
-        defer.returnValue(response)
+        observable = self.txns.fetch_or_execute_request(
+            request, self.on_POST, request
+        )
+        res = yield observable.observe()
+        defer.returnValue(res)
 
     @defer.inlineCallbacks
     def on_POST(self, request):
@@ -217,19 +210,11 @@ class RoomSendEventRestServlet(ClientV1RestServlet):
 
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, event_type, txn_id):
-        try:
-            res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred.observe()
-            defer.returnValue(res)
-        except KeyError:
-            pass
-
-        res_deferred = ObservableDeferred(
-            self.on_POST(request, room_id, event_type, txn_id)
+        observable = self.txns.fetch_or_execute_request(
+            request, self.on_POST, request, room_id, event_type, txn_id
         )
-        self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred.observe()
-        defer.returnValue(response)
+        res = yield observable.observe()
+        defer.returnValue(res)
 
 
 # TODO: Needs unit testing for room ID + alias joins
@@ -288,17 +273,11 @@ class JoinRoomAliasServlet(ClientV1RestServlet):
 
     @defer.inlineCallbacks
     def on_PUT(self, request, room_identifier, txn_id):
-        try:
-            res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred.observe()
-            defer.returnValue(res)
-        except KeyError:
-            pass
-
-        res_deferred = ObservableDeferred(self.on_POST(request, room_identifier, txn_id))
-        self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred.observe()
-        defer.returnValue(response)
+        observable = self.txns.fetch_or_execute_request(
+            request, self.on_POST, request, room_identifier, txn_id
+        )
+        res = yield observable.observe()
+        defer.returnValue(res)
 
 
 # TODO: Needs unit testing
@@ -542,17 +521,11 @@ class RoomForgetRestServlet(ClientV1RestServlet):
 
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, txn_id):
-        try:
-            res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred.observe()
-            defer.returnValue(res)
-        except KeyError:
-            pass
-
-        res_deferred = ObservableDeferred(self.on_POST(request, room_id, txn_id))
-        self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred.observe()
-        defer.returnValue(response)
+        observable = self.txns.fetch_or_execute_request(
+            request, self.on_POST, request, room_id, txn_id
+        )
+        res = yield observable.observe()
+        defer.returnValue(res)
 
 
 # TODO: Needs unit testing
@@ -626,19 +599,11 @@ class RoomMembershipRestServlet(ClientV1RestServlet):
 
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, membership_action, txn_id):
-        try:
-            res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred.observe()
-            defer.returnValue(res)
-        except KeyError:
-            pass
-
-        res_deferred = ObservableDeferred(
-            self.on_POST(request, room_id, membership_action, txn_id)
+        observable = self.txns.fetch_or_execute_request(
+            request, self.on_POST, request, room_id, membership_action, txn_id
         )
-        self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred.observe()
-        defer.returnValue(response)
+        res = yield observable.observe()
+        defer.returnValue(res)
 
 
 class RoomRedactEventRestServlet(ClientV1RestServlet):
@@ -672,19 +637,11 @@ class RoomRedactEventRestServlet(ClientV1RestServlet):
 
     @defer.inlineCallbacks
     def on_PUT(self, request, room_id, event_id, txn_id):
-        try:
-            res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred.observe()
-            defer.returnValue(res)
-        except KeyError:
-            pass
-
-        res_deferred = ObservableDeferred(
-            self.on_POST(request, room_id, event_id, txn_id)
+        observable = self.txns.fetch_or_execute_request(
+            request, self.on_POST, request, room_id, event_id, txn_id
         )
-        self.txns.store_client_transaction(request, txn_id, res_deferred)
-        response = yield res_deferred.observe()
-        defer.returnValue(response)
+        res = yield observable.observe()
+        defer.returnValue(res)
 
 
 class RoomTypingRestServlet(ClientV1RestServlet):
diff --git a/synapse/rest/client/v1/transactions.py b/synapse/rest/client/v1/transactions.py
deleted file mode 100644
index 774430458a..0000000000
--- a/synapse/rest/client/v1/transactions.py
+++ /dev/null
@@ -1,75 +0,0 @@
-# -*- coding: utf-8 -*-
-# Copyright 2014-2016 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.
-
-"""This module contains logic for storing HTTP PUT transactions. This is used
-to ensure idempotency when performing PUTs using the REST API."""
-import logging
-
-from synapse.api.auth import get_access_token_from_request
-
-logger = logging.getLogger(__name__)
-
-
-class HttpTransactionCache(object):
-
-    def __init__(self):
-        # { key : (txn_id, res_observ_defer) }
-        self.transactions = {}
-
-    def _get_response(self, key, txn_id):
-        try:
-            (last_txn_id, res_observ_defer) = self.transactions[key]
-            if txn_id == last_txn_id:
-                logger.info("get_response: Returning a response for %s", txn_id)
-                return res_observ_defer
-        except KeyError:
-            pass
-        return None
-
-    def _store_response(self, key, txn_id, res_observ_defer):
-        self.transactions[key] = (txn_id, res_observ_defer)
-
-    def store_client_transaction(self, request, txn_id, res_observ_defer):
-        """Stores the request/Promise<response> pair of an HTTP transaction.
-
-        Args:
-            request (twisted.web.http.Request): The twisted HTTP request. This
-            request must have the transaction ID as the last path segment.
-            res_observ_defer (Promise<tuple>): A tuple of (response code, response dict)
-            txn_id (str): The transaction ID for this request.
-        """
-        self._store_response(self._get_key(request), txn_id, res_observ_defer)
-
-    def get_client_transaction(self, request, txn_id):
-        """Retrieves a stored response if there was one.
-
-        Args:
-            request (twisted.web.http.Request): The twisted HTTP request. This
-            request must have the transaction ID as the last path segment.
-            txn_id (str): The transaction ID for this request.
-        Returns:
-            Promise: Resolves to the response tuple.
-        Raises:
-            KeyError if the transaction was not found.
-        """
-        res_observ_defer = self._get_response(self._get_key(request), txn_id)
-        if res_observ_defer is None:
-            raise KeyError("Transaction not found.")
-        return res_observ_defer
-
-    def _get_key(self, request):
-        token = get_access_token_from_request(request)
-        path_without_txn_id = request.path.rsplit("/", 1)[0]
-        return path_without_txn_id + "/" + token
diff --git a/synapse/rest/client/v2_alpha/sendtodevice.py b/synapse/rest/client/v2_alpha/sendtodevice.py
index 31167ba535..2ce038c6cd 100644
--- a/synapse/rest/client/v2_alpha/sendtodevice.py
+++ b/synapse/rest/client/v2_alpha/sendtodevice.py
@@ -19,8 +19,7 @@ from twisted.internet import defer
 
 from synapse.http import servlet
 from synapse.http.servlet import parse_json_object_from_request
-from synapse.rest.client.v1.transactions import HttpTransactionCache
-from synapse.util.async import ObservableDeferred
+from synapse.rest.client.transactions import HttpTransactionCache
 
 from ._base import client_v2_patterns
 
@@ -46,16 +45,10 @@ class SendToDeviceRestServlet(servlet.RestServlet):
 
     @defer.inlineCallbacks
     def on_PUT(self, request, message_type, txn_id):
-        try:
-            res_deferred = self.txns.get_client_transaction(request, txn_id)
-            res = yield res_deferred.observe()
-            defer.returnValue(res)
-        except KeyError:
-            pass
-
-        res_deferred = ObservableDeferred(self._put(request, message_type, txn_id))
-        self.txns.store_client_transaction(request, txn_id, res_deferred)
-        res = yield res_deferred.observe()
+        observable = self.txns.fetch_or_execute_request(
+            request, self._put, request, message_type, txn_id
+        )
+        res = yield observable.observe()
         defer.returnValue(res)
 
     @defer.inlineCallbacks

From af4a1bac5088e8083f55eea05f6fad44208a3a51 Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Mon, 14 Nov 2016 09:52:41 +0000
Subject: [PATCH 8/9] Move .observe() up to the cache to make things neater

---
 synapse/rest/client/transactions.py          |  9 +++---
 synapse/rest/client/v1/room.py               | 30 ++++----------------
 synapse/rest/client/v2_alpha/sendtodevice.py |  5 +---
 3 files changed, 11 insertions(+), 33 deletions(-)

diff --git a/synapse/rest/client/transactions.py b/synapse/rest/client/transactions.py
index 1db972a378..8d69e12d36 100644
--- a/synapse/rest/client/transactions.py
+++ b/synapse/rest/client/transactions.py
@@ -67,19 +67,18 @@ class HttpTransactionCache(object):
             txn_key (str): A key to ensure idempotency should fetch_or_execute be
             called again at a later point in time.
             fn (function): A function which returns a tuple of
-            (response_code, response_dict)d
+            (response_code, response_dict).
             *args: Arguments to pass to fn.
             **kwargs: Keyword arguments to pass to fn.
         Returns:
-            synapse.util.async.ObservableDeferred which resolves to a tuple
-            of (response_code, response_dict).
+            Deferred which resolves to a tuple of (response_code, response_dict).
         """
         try:
-            return self.transactions[txn_key]
+            return self.transactions[txn_key].observe()
         except KeyError:
             pass  # execute the function instead.
 
         deferred = fn(*args, **kwargs)
         observable = ObservableDeferred(deferred)
         self.transactions[txn_key] = observable
-        return observable
+        return observable.observe()
diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 0622e64380..3fb1f2deb3 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -53,13 +53,10 @@ class RoomCreateRestServlet(ClientV1RestServlet):
                                    client_path_patterns("/createRoom(?:/.*)?$"),
                                    self.on_OPTIONS)
 
-    @defer.inlineCallbacks
     def on_PUT(self, request, txn_id):
-        observable = self.txns.fetch_or_execute_request(
+        return self.txns.fetch_or_execute_request(
             request, self.on_POST, request
         )
-        res = yield observable.observe()
-        defer.returnValue(res)
 
     @defer.inlineCallbacks
     def on_POST(self, request):
@@ -208,13 +205,10 @@ class RoomSendEventRestServlet(ClientV1RestServlet):
     def on_GET(self, request, room_id, event_type, txn_id):
         return (200, "Not implemented")
 
-    @defer.inlineCallbacks
     def on_PUT(self, request, room_id, event_type, txn_id):
-        observable = self.txns.fetch_or_execute_request(
+        return self.txns.fetch_or_execute_request(
             request, self.on_POST, request, room_id, event_type, txn_id
         )
-        res = yield observable.observe()
-        defer.returnValue(res)
 
 
 # TODO: Needs unit testing for room ID + alias joins
@@ -271,13 +265,10 @@ class JoinRoomAliasServlet(ClientV1RestServlet):
 
         defer.returnValue((200, {"room_id": room_id}))
 
-    @defer.inlineCallbacks
     def on_PUT(self, request, room_identifier, txn_id):
-        observable = self.txns.fetch_or_execute_request(
+        return self.txns.fetch_or_execute_request(
             request, self.on_POST, request, room_identifier, txn_id
         )
-        res = yield observable.observe()
-        defer.returnValue(res)
 
 
 # TODO: Needs unit testing
@@ -519,13 +510,10 @@ class RoomForgetRestServlet(ClientV1RestServlet):
 
         defer.returnValue((200, {}))
 
-    @defer.inlineCallbacks
     def on_PUT(self, request, room_id, txn_id):
-        observable = self.txns.fetch_or_execute_request(
+        return self.txns.fetch_or_execute_request(
             request, self.on_POST, request, room_id, txn_id
         )
-        res = yield observable.observe()
-        defer.returnValue(res)
 
 
 # TODO: Needs unit testing
@@ -597,13 +585,10 @@ class RoomMembershipRestServlet(ClientV1RestServlet):
                 return False
         return True
 
-    @defer.inlineCallbacks
     def on_PUT(self, request, room_id, membership_action, txn_id):
-        observable = self.txns.fetch_or_execute_request(
+        return self.txns.fetch_or_execute_request(
             request, self.on_POST, request, room_id, membership_action, txn_id
         )
-        res = yield observable.observe()
-        defer.returnValue(res)
 
 
 class RoomRedactEventRestServlet(ClientV1RestServlet):
@@ -635,13 +620,10 @@ class RoomRedactEventRestServlet(ClientV1RestServlet):
 
         defer.returnValue((200, {"event_id": event.event_id}))
 
-    @defer.inlineCallbacks
     def on_PUT(self, request, room_id, event_id, txn_id):
-        observable = self.txns.fetch_or_execute_request(
+        return self.txns.fetch_or_execute_request(
             request, self.on_POST, request, room_id, event_id, txn_id
         )
-        res = yield observable.observe()
-        defer.returnValue(res)
 
 
 class RoomTypingRestServlet(ClientV1RestServlet):
diff --git a/synapse/rest/client/v2_alpha/sendtodevice.py b/synapse/rest/client/v2_alpha/sendtodevice.py
index 2ce038c6cd..2187350d42 100644
--- a/synapse/rest/client/v2_alpha/sendtodevice.py
+++ b/synapse/rest/client/v2_alpha/sendtodevice.py
@@ -43,13 +43,10 @@ class SendToDeviceRestServlet(servlet.RestServlet):
         self.txns = HttpTransactionCache()
         self.device_message_handler = hs.get_device_message_handler()
 
-    @defer.inlineCallbacks
     def on_PUT(self, request, message_type, txn_id):
-        observable = self.txns.fetch_or_execute_request(
+        return self.txns.fetch_or_execute_request(
             request, self._put, request, message_type, txn_id
         )
-        res = yield observable.observe()
-        defer.returnValue(res)
 
     @defer.inlineCallbacks
     def _put(self, request, message_type, txn_id):

From 3991b4cbdb5f5fbdf61ad6efa879b3881143c214 Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Mon, 14 Nov 2016 11:19:24 +0000
Subject: [PATCH 9/9] Clean transactions based on time. Add
 HttpTransactionCache tests.

---
 synapse/rest/client/transactions.py          | 24 +++++--
 synapse/rest/client/v1/base.py               |  2 +-
 synapse/rest/client/v2_alpha/sendtodevice.py |  2 +-
 synapse/util/__init__.py                     | 10 ++-
 tests/rest/client/test_transactions.py       | 69 ++++++++++++++++++++
 5 files changed, 99 insertions(+), 8 deletions(-)
 create mode 100644 tests/rest/client/test_transactions.py

diff --git a/synapse/rest/client/transactions.py b/synapse/rest/client/transactions.py
index 8d69e12d36..351170edbc 100644
--- a/synapse/rest/client/transactions.py
+++ b/synapse/rest/client/transactions.py
@@ -41,12 +41,19 @@ def get_transaction_key(request):
     return request.path + "/" + token
 
 
+CLEANUP_PERIOD_MS = 1000 * 60 * 30  # 30 mins
+
+
 class HttpTransactionCache(object):
 
-    def __init__(self):
+    def __init__(self, clock):
+        self.clock = clock
         self.transactions = {
-            # $txn_key: ObservableDeferred<(res_code, res_json_body)>
+            # $txn_key: (ObservableDeferred<(res_code, res_json_body)>, timestamp)
         }
+        # Try to clean entries every 30 mins. This means entries will exist
+        # for at *LEAST* 30 mins, and at *MOST* 60 mins.
+        self.cleaner = self.clock.looping_call(self._cleanup, CLEANUP_PERIOD_MS)
 
     def fetch_or_execute_request(self, request, fn, *args, **kwargs):
         """A helper function for fetch_or_execute which extracts
@@ -74,11 +81,18 @@ class HttpTransactionCache(object):
             Deferred which resolves to a tuple of (response_code, response_dict).
         """
         try:
-            return self.transactions[txn_key].observe()
-        except KeyError:
+            return self.transactions[txn_key][0].observe()
+        except (KeyError, IndexError):
             pass  # execute the function instead.
 
         deferred = fn(*args, **kwargs)
         observable = ObservableDeferred(deferred)
-        self.transactions[txn_key] = observable
+        self.transactions[txn_key] = (observable, self.clock.time_msec())
         return observable.observe()
+
+    def _cleanup(self):
+        now = self.clock.time_msec()
+        for key in self.transactions.keys():
+            ts = self.transactions[key][1]
+            if now > (ts + CLEANUP_PERIOD_MS):  # after cleanup period
+                del self.transactions[key]
diff --git a/synapse/rest/client/v1/base.py b/synapse/rest/client/v1/base.py
index 07ff5b218c..c7aa0bbf59 100644
--- a/synapse/rest/client/v1/base.py
+++ b/synapse/rest/client/v1/base.py
@@ -60,4 +60,4 @@ class ClientV1RestServlet(RestServlet):
         self.hs = hs
         self.builder_factory = hs.get_event_builder_factory()
         self.auth = hs.get_v1auth()
-        self.txns = HttpTransactionCache()
+        self.txns = HttpTransactionCache(hs.get_clock())
diff --git a/synapse/rest/client/v2_alpha/sendtodevice.py b/synapse/rest/client/v2_alpha/sendtodevice.py
index 2187350d42..ac660669f3 100644
--- a/synapse/rest/client/v2_alpha/sendtodevice.py
+++ b/synapse/rest/client/v2_alpha/sendtodevice.py
@@ -40,7 +40,7 @@ class SendToDeviceRestServlet(servlet.RestServlet):
         super(SendToDeviceRestServlet, self).__init__()
         self.hs = hs
         self.auth = hs.get_auth()
-        self.txns = HttpTransactionCache()
+        self.txns = HttpTransactionCache(hs.get_clock())
         self.device_message_handler = hs.get_device_message_handler()
 
     def on_PUT(self, request, message_type, txn_id):
diff --git a/synapse/util/__init__.py b/synapse/util/__init__.py
index 2b3f0bef3c..c05b9450be 100644
--- a/synapse/util/__init__.py
+++ b/synapse/util/__init__.py
@@ -34,7 +34,7 @@ class Clock(object):
     """A small utility that obtains current time-of-day so that time may be
     mocked during unit-tests.
 
-    TODO(paul): Also move the sleep() functionallity into it
+    TODO(paul): Also move the sleep() functionality into it
     """
 
     def time(self):
@@ -46,6 +46,14 @@ class Clock(object):
         return int(self.time() * 1000)
 
     def looping_call(self, f, msec):
+        """Call a function repeatedly.
+
+         Waits `msec` initially before calling `f` for the first time.
+
+        Args:
+            f(function): The function to call repeatedly.
+            msec(float): How long to wait between calls in milliseconds.
+        """
         l = task.LoopingCall(f)
         l.start(msec / 1000.0, now=False)
         return l
diff --git a/tests/rest/client/test_transactions.py b/tests/rest/client/test_transactions.py
new file mode 100644
index 0000000000..d7cea30260
--- /dev/null
+++ b/tests/rest/client/test_transactions.py
@@ -0,0 +1,69 @@
+from synapse.rest.client.transactions import HttpTransactionCache
+from synapse.rest.client.transactions import CLEANUP_PERIOD_MS
+from twisted.internet import defer
+from mock import Mock, call
+from tests import unittest
+from tests.utils import MockClock
+
+
+class HttpTransactionCacheTestCase(unittest.TestCase):
+
+    def setUp(self):
+        self.clock = MockClock()
+        self.cache = HttpTransactionCache(self.clock)
+
+        self.mock_http_response = (200, "GOOD JOB!")
+        self.mock_key = "foo"
+
+    @defer.inlineCallbacks
+    def test_executes_given_function(self):
+        cb = Mock(
+            return_value=defer.succeed(self.mock_http_response)
+        )
+        res = yield self.cache.fetch_or_execute(
+            self.mock_key, cb, "some_arg", keyword="arg"
+        )
+        cb.assert_called_once_with("some_arg", keyword="arg")
+        self.assertEqual(res, self.mock_http_response)
+
+    @defer.inlineCallbacks
+    def test_deduplicates_based_on_key(self):
+        cb = Mock(
+            return_value=defer.succeed(self.mock_http_response)
+        )
+        for i in range(3):  # invoke multiple times
+            res = yield self.cache.fetch_or_execute(
+                self.mock_key, cb, "some_arg", keyword="arg", changing_args=i
+            )
+            self.assertEqual(res, self.mock_http_response)
+        # expect only a single call to do the work
+        cb.assert_called_once_with("some_arg", keyword="arg", changing_args=0)
+
+    @defer.inlineCallbacks
+    def test_cleans_up(self):
+        cb = Mock(
+            return_value=defer.succeed(self.mock_http_response)
+        )
+        yield self.cache.fetch_or_execute(
+            self.mock_key, cb, "an arg"
+        )
+        # should NOT have cleaned up yet
+        self.clock.advance_time_msec(CLEANUP_PERIOD_MS / 2)
+
+        yield self.cache.fetch_or_execute(
+            self.mock_key, cb, "an arg"
+        )
+        # still using cache
+        cb.assert_called_once_with("an arg")
+
+        self.clock.advance_time_msec(CLEANUP_PERIOD_MS)
+
+        yield self.cache.fetch_or_execute(
+            self.mock_key, cb, "an arg"
+        )
+        # no longer using cache
+        self.assertEqual(cb.call_count, 2)
+        self.assertEqual(
+            cb.call_args_list,
+            [call("an arg",), call("an arg",)]
+        )