From b5cb6347a4d9b9fb5c21a2a6e3a2852c14b837fb Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 5 Apr 2017 13:25:40 +0100
Subject: [PATCH] Don't immediately notify the master about users whose syncs
 have gone away

---
 synapse/app/synchrotron.py | 40 ++++++++++++++++++++++++++++++++++----
 1 file changed, 36 insertions(+), 4 deletions(-)

diff --git a/synapse/app/synchrotron.py b/synapse/app/synchrotron.py
index a1ef5dfa7..a5dfbef8d 100644
--- a/synapse/app/synchrotron.py
+++ b/synapse/app/synchrotron.py
@@ -120,12 +120,46 @@ class SynchrotronPresence(object):
             for state in active_presence
         }
 
+        # user_id -> last_sync_ms. Lists the users that have stopped syncing
+        # but we haven't notified the master of that yet
+        self.users_going_offline = {}
+
+        self._send_stop_syncing_loop = self.clock.looping_call(
+            self.send_stop_syncing, 10 * 1000
+        )
+
         self.process_id = random_string(16)
         logger.info("Presence process_id is %r", self.process_id)
 
     def send_user_sync(self, user_id, is_syncing, last_sync_ms):
         self.hs.get_tcp_replication().send_user_sync(user_id, is_syncing, last_sync_ms)
 
+    def mark_as_coming_online(self, user_id):
+        """A user has started syncing. Send a UserSync to the master, unless they
+        had recently stopped syncing.
+        """
+        going_offline = self.users_going_offline.pop(user_id, None)
+        if not going_offline:
+            self.send_user_sync(user_id, True, self.clock.time_msec())
+
+    def mark_as_going_offline(self, user_id):
+        """A user has stopped syncing. We wait before notifying the master as
+        its likely they'll come back soon. This allows us to avoid sending
+        a stopped syncing immediately followed by a started syncing notification
+        to the master
+        """
+        self.users_going_offline[user_id] = self.clock.time_msec()
+
+    def send_stop_syncing(self):
+        """Check if there are any users who have stopped syncing a while ago
+        and haven't come back yet. If there are poke the master about them.
+        """
+        now = self.clock.time_msec()
+        for user_id, last_sync_ms in self.users_going_offline.items():
+            if now - last_sync_ms > 10 * 1000:
+                self.users_going_offline.pop(user_id, None)
+                self.send_user_sync(user_id, False, last_sync_ms)
+
     def set_state(self, user, state, ignore_status_msg=False):
         # TODO Hows this supposed to work?
         pass
@@ -142,8 +176,7 @@ class SynchrotronPresence(object):
 
             # If we went from no in flight sync to some, notify replication
             if self.user_to_num_current_syncs[user_id] == 1:
-                now = self.clock.time_msec()
-                self.send_user_sync(user_id, True, now)
+                self.mark_as_coming_online(user_id)
 
         def _end():
             # We check that the user_id is in user_to_num_current_syncs because
@@ -154,8 +187,7 @@ class SynchrotronPresence(object):
 
                 # If we went from one in flight sync to non, notify replication
                 if self.user_to_num_current_syncs[user_id] == 0:
-                    now = self.clock.time_msec()
-                    self.send_user_sync(user_id, False, now)
+                    self.mark_as_going_offline(user_id)
 
         @contextlib.contextmanager
         def _user_syncing():