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, 50 insertions, 0 deletions

diff --git a/synapse/module_api/__init__.py b/synapse/module_api/__init__.py
index 781e02fbbb..3ecd46c038 100644
--- a/synapse/module_api/__init__.py
+++ b/synapse/module_api/__init__.py
@@ -50,11 +50,20 @@ class ModuleApi:
         self._auth = hs.get_auth()
         self._auth_handler = auth_handler
         self._server_name = hs.hostname
+        self._presence_stream = hs.get_event_sources().sources["presence"]
 
         # We expose these as properties below in order to attach a helpful docstring.
         self._http_client = hs.get_simple_http_client()  # type: SimpleHttpClient
         self._public_room_list_manager = PublicRoomListManager(hs)
 
+        # The next time these users sync, they will receive the current presence
+        # state of all local users. Users are added by send_local_online_presence_to,
+        # and removed after a successful sync.
+        #
+        # We make this a private variable to deter modules from accessing it directly,
+        # though other classes in Synapse will still do so.
+        self._send_full_presence_to_local_users = set()
+
     @property
     def http_client(self):
         """Allows making outbound HTTP requests to remote resources.
@@ -385,6 +394,47 @@ class ModuleApi:
 
         return event
 
+    async def send_local_online_presence_to(self, users: Iterable[str]) -> None:
+        """
+        Forces the equivalent of a presence initial_sync for a set of local or remote
+        users. The users will receive presence for all currently online users that they
+        are considered interested in.
+
+        Updates to remote users will be sent immediately, whereas local users will receive
+        them on their next sync attempt.
+
+        Note that this method can only be run on the main or federation_sender worker
+        processes.
+        """
+        if not self._hs.should_send_federation():
+            raise Exception(
+                "send_local_online_presence_to can only be run "
+                "on processes that send federation",
+            )
+
+        for user in users:
+            if self._hs.is_mine_id(user):
+                # Modify SyncHandler._generate_sync_entry_for_presence to call
+                # presence_source.get_new_events with an empty `from_key` if
+                # that user's ID were in a list modified by ModuleApi somewhere.
+                # That user would then get all presence state on next incremental sync.
+
+                # Force a presence initial_sync for this user next time
+                self._send_full_presence_to_local_users.add(user)
+            else:
+                # Retrieve presence state for currently online users that this user
+                # is considered interested in
+                presence_events, _ = await self._presence_stream.get_new_events(
+                    UserID.from_string(user), from_key=None, include_offline=False
+                )
+
+                # Send to remote destinations
+                await make_deferred_yieldable(
+                    # We pull the federation sender here as we can only do so on workers
+                    # that support sending presence
+                    self._hs.get_federation_sender().send_presence(presence_events)
+                )
+
 
 class PublicRoomListManager:
     """Contains methods for adding to, removing from and querying whether a room