2 files changed, 35 insertions, 20 deletions
diff --git a/synapse/replication/tcp/handler.py b/synapse/replication/tcp/handler.py
index 6f7054d5af..0e8a38fd80 100644
--- a/synapse/replication/tcp/handler.py
+++ b/synapse/replication/tcp/handler.py
@@ -99,7 +99,8 @@ class ReplicationCommandHandler:
         # The factory used to create connections.
         self._factory = None  # type: Optional[ReconnectingClientFactory]
 
-        # The currently connected connections.
+        # The currently connected connections. (The list of places we need to send
+        # outgoing replication commands to.)
         self._connections = []  # type: List[AbstractConnection]
 
         LaterGauge(
diff --git a/synapse/replication/tcp/redis.py b/synapse/replication/tcp/redis.py
index 617e860f95..61dbf4ddbe 100644
--- a/synapse/replication/tcp/redis.py
+++ b/synapse/replication/tcp/redis.py
@@ -18,7 +18,7 @@ from typing import TYPE_CHECKING
 
 import txredisapi
 
-from synapse.logging.context import PreserveLoggingContext
+from synapse.logging.context import make_deferred_yieldable
 from synapse.metrics.background_process_metrics import run_as_background_process
 from synapse.replication.tcp.commands import (
     Command,
@@ -41,8 +41,14 @@ logger = logging.getLogger(__name__)
 class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
     """Connection to redis subscribed to replication stream.
 
-    Parses incoming messages from redis into replication commands, and passes
-    them to `ReplicationCommandHandler`
+    This class fulfils two functions:
+
+    (a) it implements the twisted Protocol API, where it handles the SUBSCRIBEd redis
+    connection, parsing *incoming* messages into replication commands, and passing them
+    to `ReplicationCommandHandler`
+
+    (b) it implements the AbstractConnection API, where it sends *outgoing* commands
+    onto outbound_redis_connection.
 
     Due to the vagaries of `txredisapi` we don't want to have a custom
     constructor, so instead we expect the defined attributes below to be set
@@ -50,8 +56,8 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
 
     Attributes:
         handler: The command handler to handle incoming commands.
-        stream_name: The *redis* stream name to subscribe to (not anything to
-            do with Synapse replication streams).
+        stream_name: The *redis* stream name to subscribe to and publish from
+            (not anything to do with Synapse replication streams).
         outbound_redis_connection: The connection to redis to use to send
             commands.
     """
@@ -61,12 +67,22 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
     outbound_redis_connection = None  # type: txredisapi.RedisProtocol
 
     def connectionMade(self):
-        logger.info("Connected to redis instance")
-        self.subscribe(self.stream_name)
-        self.send_command(ReplicateCommand())
-
+        logger.info("Connected to redis")
+        run_as_background_process("subscribe-replication", self._send_subscribe)
         self.handler.new_connection(self)
 
+    async def _send_subscribe(self):
+        # it's important to make sure that we only send the REPLICATE command once we
+        # have successfully subscribed to the stream - otherwise we might miss the
+        # POSITION response sent back by the other end.
+        logger.info("Sending redis SUBSCRIBE for %s", self.stream_name)
+        await make_deferred_yieldable(self.subscribe(self.stream_name))
+        logger.info(
+            "Successfully subscribed to redis stream, sending REPLICATE command"
+        )
+        await self._async_send_command(ReplicateCommand())
+        logger.info("REPLICATE successfully sent")
+
     def messageReceived(self, pattern: str, channel: str, message: str):
         """Received a message from redis.
         """
@@ -119,7 +135,7 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
             logger.warning("Unhandled command: %r", cmd)
 
     def connectionLost(self, reason):
-        logger.info("Lost connection to redis instance")
+        logger.info("Lost connection to redis")
         self.handler.lost_connection(self)
 
     def send_command(self, cmd: Command):
@@ -128,6 +144,10 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
         Args:
             cmd (Command)
         """
+        run_as_background_process("send-cmd", self._send_command, cmd)
+
+    async def _async_send_command(self, cmd: Command):
+        """Encode a replication command and send it over our outbound connection"""
         string = "%s %s" % (cmd.NAME, cmd.to_line())
         if "\n" in string:
             raise Exception("Unexpected newline in command: %r", string)
@@ -138,15 +158,9 @@ class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
         # remote instances.
         tcp_outbound_commands_counter.labels(cmd.NAME, "redis").inc()
 
-        async def _send():
-            with PreserveLoggingContext():
-                # Note that we use the other connection as we can't send
-                # commands using the subscription connection.
-                await self.outbound_redis_connection.publish(
-                    self.stream_name, encoded_string
-                )
-
-        run_as_background_process("send-cmd", _send)
+        await make_deferred_yieldable(
+            self.outbound_redis_connection.publish(self.stream_name, encoded_string)
+        )
 
 
 class RedisDirectTcpReplicationClientFactory(txredisapi.SubscriberFactory):