Add a Synapse Module for configuring presence update routing

README.rst

@@ -384,7 +384,12 @@ massive excess of outgoing federation requests (see `discussion
 indicate that your server is also issuing far more outgoing federation
 requests than can be accounted for by your users' activity, this is a
 likely cause. The misbehavior can be worked around by setting
-``use_presence: false`` in the Synapse config file.
+the following in the Synapse config file:
+
+.. code-block:: yaml
+
+   presence:
+       enabled: false
 
 People can't accept room invitations from me
 --------------------------------------------

changelog.d/9491.feature

@@ -0,0 +1 @@
+Add a Synapse module for routing presence updates between users.

docs/presence_router_module.md

@@ -0,0 +1,231 @@
+# Presence Router Module
+
+Synapse supports configuring a module that can specify additional users
+(local or remote) to should receive certain presence updates from local
+users.
+
+Note that routing presence via an Application Service transactions is not
+currently supported.
+
+The presence routing module is implemented as a Python class, which will be imported by
+the running Synapse.
+
+## Python Presence Router Class
+
+The Python class is instantiated with two objects:
+
+* A configuration object of some type (see below).
+* An instance of `synapse.module_api.ModuleApi`.
+
+It then implements methods related to presence routing.
+
+Note that one method of `ModuleApi` that may be useful is:
+
+```python
+async def ModuleApi.send_local_online_presence_to(users: Iterable[str]) -> None
+```
+
+which can be given a list of local or remote MXIDs to broadcast known, online user
+presence to (for those users that the receiving user is considered interested in). 
+It does not include state for users who are currently offline, and it can only be
+called on workers that support sending federation.
+
+### Module structure
+
+Below is a list of possible methods that can be implemented, and whether they are
+required.
+
+#### `parse_config`
+
+```python
+def parse_config(config_dict: dict) -> Any
+```
+**Required.** A static method that is passed a dictionary of config options, and
+  should return a validated config object. This method is described further in
+  [Configuration](#configuration).
+
+#### `get_users_for_states`
+
+```python
+async def get_users_for_states(
+    self,
+    state_updates: Iterable[UserPresenceState],
+) -> Dict[str, Set[UserPresenceState]]:
+```
+
+**Required.** An asynchronous method that is passed an iterable of user presence
+state. This method can determine whether a given presence update should be sent to certain
+users. It does this by returning a dictionary with keys representing local or remote
+Matrix User IDs, and values being a python set
+of `synapse.handlers.presence.UserPresenceState` instances.
+
+Synapse will then attempt to send the specified presence updates to each user when
+possible.
+
+#### `get_interested_users`
+
+```python
+async def get_interested_users(self, user_id: str) -> Union[Set[str], str]
+```
+
+**Required.** An asynchronous method that is passed a single Matrix User ID. This
+method is expected to return the users that the passed in user may be interested in the
+presence of, in addition to users they share a room with. This may be local or remote
+users. It does so by returning a python set of Matrix User IDs, or the object
+`synapse.events.presence_router.PresenceRouter.ALL_USERS` to indicate that the passed
+user should receive presence information for *all* known users.
+
+For clarity, if the user `@alice:example.org` is passed to this method, and the Set
+`{"@bob:example.com", "@charlie:somewhere.org"}` is returned, this signifies that Alice
+should receive presence updates sent by Bob and Charlie, regardless of whether these
+users share a room.
+
+### Example
+
+Below is an example implementation of a presence router class.
+
+```python
+from typing import Dict, Iterable, Set, Union
+from synapse.events.presence_router import PresenceRouter
+from synapse.handlers.presence import UserPresenceState
+from synapse.module_api import ModuleApi
+
+class PresenceRouterConfig:
+    def __init__(self):
+        # Config options with their defaults
+        # A list of users to always send all user presence updates to
+        self.always_send_to_users = []  # type: List[str]
+
+        # A list of users to ignore presence updates for. Does not affect
+        # shared-room presence relationships
+        self.blacklisted_users = []  # type: List[str]
+
+class ExamplePresenceRouter:
+    """An example implementation of synapse.presence_router.PresenceRouter.
+    Supports routing all presence to a configured set of users, or a subset
+    of presence from certain users to members of certain rooms.
+
+    Args:
+        config: A configuration object.
+        module_api: An instance of Synapse's ModuleApi.
+    """
+    def __init__(self, config: PresenceRouterConfig, module_api: ModuleApi):
+        self._config = config
+        self._module_api = module_api
+
+    @staticmethod
+    def parse_config(config_dict: dict) -> PresenceRouterConfig:
+        """Parse a configuration dictionary from the homeserver config, do
+        some validation and return a typed PresenceRouterConfig.
+
+        Args:
+            config_dict: The configuration dictionary.
+
+        Returns:
+            A validated config object.
+        """
+        # Initialise a typed config object
+        config = PresenceRouterConfig()
+        always_send_to_users = config_dict.get("always_send_to_users")
+        blacklisted_users = config_dict.get("blacklisted_users")
+
+        # Do some validation of config options... otherwise raise a
+        # synapse.config.ConfigError.
+        config.always_send_to_users = always_send_to_users
+        config.blacklisted_users = blacklisted_users
+
+        return config
+
+    async def get_users_for_states(
+        self,
+        state_updates: Iterable[UserPresenceState],
+    ) -> Dict[str, Set[UserPresenceState]]:
+        """Given an iterable of user presence updates, determine where each one
+        needs to go. Returned results will not affect presence updates that are
+        sent between users who share a room.
+
+        Args:
+            state_updates: An iterable of user presence state updates.
+
+        Returns:
+          A dictionary of user_id -> set of UserPresenceState that the user should 
+          receive.
+        """
+        destination_users = {}  # type: Dict[str, Set[UserPresenceState]
+
+        # Ignore any updates for blacklisted users
+        desired_updates = set()
+        for update in state_updates:
+            if update.state_key not in self._config.blacklisted_users:
+                desired_updates.add(update)
+
+        # Send all presence updates to specific users
+        for user_id in self._config.always_send_to_users:
+            destination_users[user_id] = desired_updates
+
+        return destination_users
+
+    async def get_interested_users(
+        self,
+        user_id: str,
+    ) -> Union[Set[str], PresenceRouter.ALL_USERS]:
+        """
+        Retrieve a list of users that `user_id` is interested in receiving the
+        presence of. This will be in addition to those they share a room with.
+        Optionally, the object PresenceRouter.ALL_USERS can be returned to indicate
+        that this user should receive all incoming local and remote presence updates.
+
+        Note that this method will only be called for local users.
+
+        Args:
+          user_id: A user requesting presence updates.
+
+        Returns:
+          A set of user IDs to return additional presence updates for, or
+          PresenceRouter.ALL_USERS to return presence updates for all other users.
+        """
+        if user_id in self._config.always_send_to_users:
+            return PresenceRouter.ALL_USERS
+
+        return set()
+```
+
+#### A note on `get_users_for_states` and `get_interested_users`
+
+Both of these methods are effectively two different sides of the same coin. The logic
+regarding which users should receive updates for other users should be the same 
+between them.
+
+`get_users_for_states` is called when presence updates come in from either federation 
+or local users, and is used to either direct local presence to remote users, or to
+wake up the sync streams of local users to collect remote presence.
+
+In contrast, `get_interested_users` is used to determine the users that presence should
+be fetched for when a local user is syncing. This presence is then retrieved, before
+being fed through `get_users_for_states` once again, with only the syncing user's
+routing information pulled from the resulting dictionary.
+
+Their routing logic should thus line up, else you may run into unintended behaviour.
+
+## Configuration
+
+Once you've crafted your module and installed it into the same Python environment as
+Synapse, amend your homeserver config file with the following.
+
+```yaml
+presence:
+  routing_module:
+    module: my_module.ExamplePresenceRouter
+    config:
+      # Any configuration options for your module. The below is an example.
+      # of setting options for ExamplePresenceRouter.
+      always_send_to_users: ["@presence_gobbler:example.org"]
+      blacklisted_users:
+        - "@alice:example.com"
+        - "@bob:example.com"
+      ...
+```
+
+The contents of `config` will be passed as a Python dictionary to the static
+`parse_config` method of your class. The object returned by this method will
+then be passed to the `__init__` method of your module as `config`.

docs/sample_config.yaml

@@ -82,9 +82,28 @@ pid_file: DATADIR/homeserver.pid
 #
 #soft_file_limit: 0
 
-# Set to false to disable presence tracking on this homeserver.
+# Presence tracking allows users to see the state (e.g online/offline)
+# of other local and remote users.
 #
-#use_presence: false
+presence:
+  # Uncomment to disable presence tracking on this homeserver. This option
+  # replaces the previous top-level 'use_presence' option.
+  #
+  #enabled: false
+
+  # Presence routers are third-party modules that can specify additional logic
+  # to where presence updates from users are routed.
+  #
+  presence_router:
+    # The custom module's class. Uncomment to use a custom presence router module.
+    #
+    #module: "my_custom_router.PresenceRouter"
+
+    # Configuration options of the custom module. Refer to your module's
+    # documentation for available options.
+    #
+    #config:
+    #  example_option: 'something'
 
 # Whether to require authentication to retrieve profile data (avatars,
 # display names) of other users through the client API. Defaults to

synapse/app/generic_worker.py

@@ -281,6 +281,7 @@ def __init__(self, hs):
         self.hs = hs
         self.is_mine_id = hs.is_mine_id
 
+        self.presence_router = hs.get_presence_router()
         self._presence_enabled = hs.config.use_presence
 
         # The number of ongoing syncs on this process, by user id.
@@ -395,7 +396,7 @@ def _user_syncing():
         return _user_syncing()
 
     async def notify_from_replication(self, states, stream_id):
-        parties = await get_interested_parties(self.store, states)
+        parties = await get_interested_parties(self.store, self.presence_router, states)
         room_ids_to_states, users_to_states = parties
 
         self.notifier.on_new_event(

synapse/config/server.py

@@ -27,6 +27,7 @@
 from netaddr import AddrFormatError, IPNetwork, IPSet
 
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
+from synapse.util.module_loader import load_module
 from synapse.util.stringutils import parse_and_validate_server_name
 
 from ._base import Config, ConfigError
@@ -238,7 +239,20 @@ def read_config(self, config, **kwargs):
         self.public_baseurl = config.get("public_baseurl")
 
         # Whether to enable user presence.
-        self.use_presence = config.get("use_presence", True)
+        presence_config = config.get("presence") or {}
+        self.use_presence = presence_config.get("enabled")
+        if self.use_presence is None:
+            self.use_presence = config.get("use_presence", True)
+
+        # Custom presence router module
+        self.presence_router_module_class = None
+        self.presence_router_config = None
+        presence_router_config = presence_config.get("presence_router")
+        if presence_router_config:
+            (
+                self.presence_router_module_class,
+                self.presence_router_config,
+            ) = load_module(presence_router_config, ("presence", "presence_router"))
 
         # Whether to update the user directory or not. This should be set to
         # false only if we are updating the user directory in a worker
@@ -834,9 +848,28 @@ def generate_config_section(
         #
         #soft_file_limit: 0
 
-        # Set to false to disable presence tracking on this homeserver.
+        # Presence tracking allows users to see the state (e.g online/offline)
+        # of other local and remote users.
         #
-        #use_presence: false
+        presence:
+          # Uncomment to disable presence tracking on this homeserver. This option
+          # replaces the previous top-level 'use_presence' option.
+          #
+          #enabled: false
+
+          # Presence routers are third-party modules that can specify additional logic
+          # to where presence updates from users are routed.
+          #
+          presence_router:
+            # The custom module's class. Uncomment to use a custom presence router module.
+            #
+            #module: "my_custom_router.PresenceRouter"
+
+            # Configuration options of the custom module. Refer to your module's
+            # documentation for available options.
+            #
+            #config:
+            #  example_option: 'something'
 
         # Whether to require authentication to retrieve profile data (avatars,
         # display names) of other users through the client API. Defaults to