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.

1 files changed, 12 insertions, 6 deletions

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)