Fix race in federation sender that delayed device updates. (#6799)

We were sending device updates down both the federation stream and
device streams. This mean there was a race if the federation sender
worker processed the federation stream first, as when the sender checked
if there were new device updates the slaved ID generator hadn't been
updated with the new stream IDs and so returned nothing.

This situation is correctly handled by events/receipts/etc by not
sending updates down the federation stream and instead having the
federation sender worker listen on the other streams and poke the
transaction queues as appropriate.

changelog.d/6799.bugfix

@@ -0,0 +1 @@
+Fix race in federation sender worker that delayed sending of device updates.

synapse/app/federation_sender.py

@@ -38,7 +38,11 @@
 from synapse.replication.slave.storage.registration import SlavedRegistrationStore
 from synapse.replication.slave.storage.transactions import SlavedTransactionStore
 from synapse.replication.tcp.client import ReplicationClientHandler
-from synapse.replication.tcp.streams._base import ReceiptsStream
+from synapse.replication.tcp.streams._base import (
+    DeviceListsStream,
+    ReceiptsStream,
+    ToDeviceStream,
+)
 from synapse.server import HomeServer
 from synapse.storage.database import Database
 from synapse.types import ReadReceipt
@@ -256,6 +260,20 @@ def process_replication_rows(self, stream_name, token, rows):
                 "process_receipts_for_federation", self._on_new_receipts, rows
             )
 
+        # ... as well as device updates and messages
+        elif stream_name == DeviceListsStream.NAME:
+            hosts = set(row.destination for row in rows)
+            for host in hosts:
+                self.federation_sender.send_device_messages(host)
+
+        elif stream_name == ToDeviceStream.NAME:
+            # The to_device stream includes stuff to be pushed to both local
+            # clients and remote servers, so we ignore entities that start with
+            # '@' (since they'll be local users rather than destinations).
+            hosts = set(row.entity for row in rows if not row.entity.startswith("@"))
+            for host in hosts:
+                self.federation_sender.send_device_messages(host)
+
     @defer.inlineCallbacks
     def _on_new_receipts(self, rows):
         """

synapse/federation/send_queue.py

@@ -69,8 +69,6 @@ def __init__(self, hs):
 
         self.edus = SortedDict()  # stream position -> Edu
 
-        self.device_messages = SortedDict()  # stream position -> destination
-
         self.pos = 1
         self.pos_time = SortedDict()
 
@@ -92,7 +90,6 @@ def register(name, queue):
             "keyed_edu",
             "keyed_edu_changed",
             "edus",
-            "device_messages",
             "pos_time",
             "presence_destinations",
         ]:
@@ -171,12 +168,6 @@ def _clear_queue_before_pos(self, position_to_delete):
             for key in keys[:i]:
                 del self.edus[key]
 
-            # Delete things out of device map
-            keys = self.device_messages.keys()
-            i = self.device_messages.bisect_left(position_to_delete)
-            for key in keys[:i]:
-                del self.device_messages[key]
-
     def notify_new_events(self, current_id):
         """As per FederationSender"""
         # We don't need to replicate this as it gets sent down a different
@@ -249,9 +240,8 @@ def send_presence_to_destinations(self, states, destinations):
 
     def send_device_messages(self, destination):
         """As per FederationSender"""
-        pos = self._next_pos()
-        self.device_messages[pos] = destination
-        self.notifier.on_new_replication_data()
+        # We don't need to replicate this as it gets sent down a different
+        # stream.
 
     def get_current_token(self):
         return self.pos - 1
@@ -339,14 +329,6 @@ async def get_replication_rows(
         for (pos, edu) in edus:
             rows.append((pos, EduRow(edu)))
 
-        # Fetch changed device messages
-        i = self.device_messages.bisect_right(from_token)
-        j = self.device_messages.bisect_right(to_token) + 1
-        device_messages = {v: k for k, v in self.device_messages.items()[i:j]}
-
-        for (destination, pos) in iteritems(device_messages):
-            rows.append((pos, DeviceRow(destination=destination)))
-
         # Sort rows based on pos
         rows.sort()
 
@@ -504,7 +486,6 @@ def add_to_buffer(self, buff):
         "presence_destinations",  # list of tuples of UserPresenceState and destinations
         "keyed_edus",  # dict of destination -> { key -> Edu }
         "edus",  # dict of destination -> [Edu]
-        "device_destinations",  # set of destinations
     ),
 )
 
@@ -523,11 +504,7 @@ def process_rows_for_federation(transaction_queue, rows):
     # them into the appropriate collection and then send them off.
 
     buff = ParsedFederationStreamData(
-        presence=[],
-        presence_destinations=[],
-        keyed_edus={},
-        edus={},
-        device_destinations=set(),
+        presence=[], presence_destinations=[], keyed_edus={}, edus={},
     )
 
     # Parse the rows in the stream and add to the buffer
@@ -555,6 +532,3 @@ def process_rows_for_federation(transaction_queue, rows):
     for destination, edu_list in iteritems(buff.edus):
         for edu in edu_list:
             transaction_queue.send_edu(edu, None)
-
-    for destination in buff.device_destinations:
-        transaction_queue.send_device_messages(destination)