Standardise the module interface (#10062)

This PR adds a common configuration section for all modules (see docs). These modules are then loaded at startup by the homeserver. Modules register their hooks and web resources using the new `register_[...]_callbacks` and `register_web_resource` methods of the module API.

synapse/server.py

@@ -1,6 +1,4 @@
-# Copyright 2014-2016 OpenMarket Ltd
-# Copyright 2017-2018 New Vector Ltd
-# Copyright 2019 The Matrix.org Foundation C.I.C.
+# Copyright 2021 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -39,6 +37,7 @@ import twisted.internet.tcp
 from twisted.internet import defer
 from twisted.mail.smtp import sendmail
 from twisted.web.iweb import IPolicyForHTTPS
+from twisted.web.resource import IResource
 
 from synapse.api.auth import Auth
 from synapse.api.filtering import Filtering
@@ -258,6 +257,38 @@ class HomeServer(metaclass=abc.ABCMeta):
 
         self.datastores = None  # type: Optional[Databases]
 
+        self._module_web_resources: Dict[str, IResource] = {}
+        self._module_web_resources_consumed = False
+
+    def register_module_web_resource(self, path: str, resource: IResource):
+        """Allows a module to register a web resource to be served at the given path.
+
+        If multiple modules register a resource for the same path, the module that
+        appears the highest in the configuration file takes priority.
+
+        Args:
+            path: The path to register the resource for.
+            resource: The resource to attach to this path.
+
+        Raises:
+            SynapseError(500): A module tried to register a web resource after the HTTP
+                listeners have been started.
+        """
+        if self._module_web_resources_consumed:
+            raise RuntimeError(
+                "Tried to register a web resource from a module after startup",
+            )
+
+        # Don't register a resource that's already been registered.
+        if path not in self._module_web_resources.keys():
+            self._module_web_resources[path] = resource
+        else:
+            logger.warning(
+                "Module tried to register a web resource for path %s but another module"
+                " has already registered a resource for this path.",
+                path,
+            )
+
     def get_instance_id(self) -> str:
         """A unique ID for this synapse process instance.
 
@@ -646,7 +677,7 @@ class HomeServer(metaclass=abc.ABCMeta):
 
     @cache_in_self
     def get_spam_checker(self) -> SpamChecker:
-        return SpamChecker(self)
+        return SpamChecker()
 
     @cache_in_self
     def get_third_party_event_rules(self) -> ThirdPartyEventRules: