Add a Synapse Module for configuring presence update routing (#9491)

At the moment, if you'd like to share presence between local or remote users, those users must be sharing a room together. This isn't always the most convenient or useful situation though.

This PR adds a module to Synapse that will allow deployments to set up extra logic on where presence updates should be routed. The module must implement two methods, `get_users_for_states` and `get_interested_users`. These methods are given presence updates or user IDs and must return information that Synapse will use to grant passing presence updates around.

A method is additionally added to `ModuleApi` which allows triggering a set of users to receive the current, online presence information for all users they are considered interested in. This is the equivalent of that user receiving presence information during an initial sync. 

The goal of this module is to be fairly generic and useful for a variety of applications, with hard requirements being:

* Sending state for a specific set or all known users to a defined set of local and remote users.
* The ability to trigger an initial sync for specific users, so they receive all current state.

1 files changed, 5 insertions, 0 deletions

diff --git a/synapse/server.py b/synapse/server.py
index e42f7b1a18..cfb55c230d 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -51,6 +51,7 @@ from synapse.crypto import context_factory
 from synapse.crypto.context_factory import RegularPolicyForHTTPS
 from synapse.crypto.keyring import Keyring
 from synapse.events.builder import EventBuilderFactory
+from synapse.events.presence_router import PresenceRouter
 from synapse.events.spamcheck import SpamChecker
 from synapse.events.third_party_rules import ThirdPartyEventRules
 from synapse.events.utils import EventClientSerializer
@@ -426,6 +427,10 @@ class HomeServer(metaclass=abc.ABCMeta):
             raise Exception("Workers cannot write typing")
 
     @cache_in_self
+    def get_presence_router(self) -> PresenceRouter:
+        return PresenceRouter(self)
+
+    @cache_in_self
     def get_typing_handler(self) -> FollowerTypingHandler:
         if self.config.worker.writers.typing == self.get_instance_name():
             # Use get_typing_writer_handler to ensure that we use the same