From fd2eef49c8a63e12f3a1f724a4543ae96aa55611 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Tue, 14 Mar 2017 15:03:46 +0000
Subject: [PATCH] Reduce spurious calls to generate sync

---
 synapse/notifier.py | 44 +++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/synapse/notifier.py b/synapse/notifier.py
index 8051a7a842..6abb33bb3f 100644
--- a/synapse/notifier.py
+++ b/synapse/notifier.py
@@ -73,6 +73,13 @@ class _NotifierUserStream(object):
         self.user_id = user_id
         self.rooms = set(rooms)
         self.current_token = current_token
+
+        # The last token for which we should wake up any streams that have a
+        # token that comes before it. This gets updated everytime we get poked.
+        # We start it at the current token since if we get any streams
+        # that have a token from before we have no idea whether they should be
+        # woken up or not, so lets just wake them up.
+        self.last_notified_token = current_token
         self.last_notified_ms = time_now_ms
 
         with PreserveLoggingContext():
@@ -89,6 +96,7 @@ class _NotifierUserStream(object):
         self.current_token = self.current_token.copy_and_advance(
             stream_key, stream_id
         )
+        self.last_notified_token = self.current_token
         self.last_notified_ms = time_now_ms
         noify_deferred = self.notify_deferred
 
@@ -113,8 +121,14 @@ class _NotifierUserStream(object):
     def new_listener(self, token):
         """Returns a deferred that is resolved when there is a new token
         greater than the given token.
+
+        Args:
+            token: The token from which we are streaming from, i.e. we shouldn't
+                notify for things that happened before this.
         """
-        if self.current_token.is_after(token):
+        # Immediately wake up stream if something has already since happened
+        # since their last token.
+        if self.last_notified_token.is_after(token):
             return _NotificationListener(defer.succeed(self.current_token))
         else:
             return _NotificationListener(self.notify_deferred.observe())
@@ -294,40 +308,44 @@ class Notifier(object):
             self._register_with_keys(user_stream)
 
         result = None
+        prev_token = from_token
         if timeout:
             end_time = self.clock.time_msec() + timeout
 
-            prev_token = from_token
             while not result:
                 try:
-                    current_token = user_stream.current_token
-
-                    result = yield callback(prev_token, current_token)
-                    if result:
-                        break
-
                     now = self.clock.time_msec()
                     if end_time <= now:
                         break
 
                     # Now we wait for the _NotifierUserStream to be told there
                     # is a new token.
-                    # We need to supply the token we supplied to callback so
-                    # that we don't miss any current_token updates.
-                    prev_token = current_token
                     listener = user_stream.new_listener(prev_token)
                     with PreserveLoggingContext():
                         yield self.clock.time_bound_deferred(
                             listener.deferred,
                             time_out=(end_time - now) / 1000.
                         )
+
+                    current_token = user_stream.current_token
+
+                    result = yield callback(prev_token, current_token)
+                    if result:
+                        break
+
+                    # Update the prev_token to the current_token since nothing
+                    # has happened between the old prev_token and the current_token
+                    prev_token = current_token
                 except DeferredTimedOutError:
                     break
                 except defer.CancelledError:
                     break
-        else:
+
+        if result is None:
+            # This happened if there was no timeout or if the timeout had
+            # already expired.
             current_token = user_stream.current_token
-            result = yield callback(from_token, current_token)
+            result = yield callback(prev_token, current_token)
 
         defer.returnValue(result)