Remove usage of "conn_id" for presence. (#7128)

* Remove `conn_id` usage for UserSyncCommand.

Each tcp replication connection is assigned a "conn_id", which is used
to give an ID to a remotely connected worker. In a redis world, there
will no longer be a one to one mapping between connection and instance,
so instead we need to replace such usages with an ID generated by the
remote instances and included in the replicaiton commands.

This really only effects UserSyncCommand.

* Add CLEAR_USER_SYNCS command that is sent on shutdown.

This should help with the case where a synchrotron gets restarted
gracefully, rather than rely on 5 minute timeout.

synapse/replication/tcp/commands.py

@@ -207,30 +207,32 @@ class UserSyncCommand(Command):
 
     Format::
 
-        USER_SYNC <user_id> <state> <last_sync_ms>
+        USER_SYNC <instance_id> <user_id> <state> <last_sync_ms>
 
     Where <state> is either "start" or "stop"
     """
 
     NAME = "USER_SYNC"
 
-    def __init__(self, user_id, is_syncing, last_sync_ms):
+    def __init__(self, instance_id, user_id, is_syncing, last_sync_ms):
+        self.instance_id = instance_id
         self.user_id = user_id
         self.is_syncing = is_syncing
         self.last_sync_ms = last_sync_ms
 
     @classmethod
     def from_line(cls, line):
-        user_id, state, last_sync_ms = line.split(" ", 2)
+        instance_id, user_id, state, last_sync_ms = line.split(" ", 3)
 
         if state not in ("start", "end"):
             raise Exception("Invalid USER_SYNC state %r" % (state,))
 
-        return cls(user_id, state == "start", int(last_sync_ms))
+        return cls(instance_id, user_id, state == "start", int(last_sync_ms))
 
     def to_line(self):
         return " ".join(
             (
+                self.instance_id,
                 self.user_id,
                 "start" if self.is_syncing else "end",
                 str(self.last_sync_ms),
@@ -238,6 +240,30 @@ class UserSyncCommand(Command):
         )
 
 
+class ClearUserSyncsCommand(Command):
+    """Sent by the client to inform the server that it should drop all
+    information about syncing users sent by the client.
+
+    Mainly used when client is about to shut down.
+
+    Format::
+
+        CLEAR_USER_SYNC <instance_id>
+    """
+
+    NAME = "CLEAR_USER_SYNC"
+
+    def __init__(self, instance_id):
+        self.instance_id = instance_id
+
+    @classmethod
+    def from_line(cls, line):
+        return cls(line)
+
+    def to_line(self):
+        return self.instance_id
+
+
 class FederationAckCommand(Command):
     """Sent by the client when it has processed up to a given point in the
     federation stream. This allows the master to drop in-memory caches of the
@@ -398,6 +424,7 @@ _COMMANDS = (
     InvalidateCacheCommand,
     UserIpCommand,
     RemoteServerUpCommand,
+    ClearUserSyncsCommand,
 )  # type: Tuple[Type[Command], ...]
 
 # Map of command name to command type.
@@ -420,6 +447,7 @@ VALID_CLIENT_COMMANDS = (
     ReplicateCommand.NAME,
     PingCommand.NAME,
     UserSyncCommand.NAME,
+    ClearUserSyncsCommand.NAME,
     FederationAckCommand.NAME,
     RemovePusherCommand.NAME,
     InvalidateCacheCommand.NAME,