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.

13 files changed, 375 insertions, 126 deletions

diff --git a/synapse/app/_base.py b/synapse/app/_base.py
index 1dde9d7173..00ab67e7e4 100644
--- a/synapse/app/_base.py
+++ b/synapse/app/_base.py
@@ -35,6 +35,7 @@ from synapse.app import check_bind_error
 from synapse.app.phone_stats_home import start_phone_stats_home
 from synapse.config.homeserver import HomeServerConfig
 from synapse.crypto import context_factory
+from synapse.events.spamcheck import load_legacy_spam_checkers
 from synapse.logging.context import PreserveLoggingContext
 from synapse.metrics.background_process_metrics import wrap_as_background_process
 from synapse.metrics.jemalloc import setup_jemalloc_stats
@@ -330,6 +331,14 @@ async def start(hs: "synapse.server.HomeServer"):
     # Start the tracer
     synapse.logging.opentracing.init_tracer(hs)  # type: ignore[attr-defined] # noqa
 
+    # Instantiate the modules so they can register their web resources to the module API
+    # before we start the listeners.
+    module_api = hs.get_module_api()
+    for module, config in hs.config.modules.loaded_modules:
+        module(config=config, api=module_api)
+
+    load_legacy_spam_checkers(hs)
+
     # It is now safe to start your Synapse.
     hs.start_listening()
     hs.get_datastore().db_pool.start_profiling()
diff --git a/synapse/app/generic_worker.py b/synapse/app/generic_worker.py
index 57c2fc2e88..8e648c6ee0 100644
--- a/synapse/app/generic_worker.py
+++ b/synapse/app/generic_worker.py
@@ -354,6 +354,10 @@ class GenericWorkerServer(HomeServer):
                 if name == "replication":
                     resources[REPLICATION_PREFIX] = ReplicationRestResource(self)
 
+        # Attach additional resources registered by modules.
+        resources.update(self._module_web_resources)
+        self._module_web_resources_consumed = True
+
         root_resource = create_resource_tree(resources, OptionsResource())
 
         _base.listen_tcp(
diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index fb16bceff8..f31467bde7 100644
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -124,6 +124,10 @@ class SynapseHomeServer(HomeServer):
                 )
             resources[path] = resource
 
+        # Attach additional resources registered by modules.
+        resources.update(self._module_web_resources)
+        self._module_web_resources_consumed = True
+
         # try to find something useful to redirect '/' to
         if WEB_CLIENT_PREFIX in resources:
             root_resource = RootOptionsRedirectResource(WEB_CLIENT_PREFIX)
diff --git a/synapse/config/_base.pyi b/synapse/config/_base.pyi
index 4e7bfa8b3b..844ecd4708 100644
--- a/synapse/config/_base.pyi
+++ b/synapse/config/_base.pyi
@@ -16,6 +16,7 @@ from synapse.config import (
     key,
     logger,
     metrics,
+    modules,
     oidc,
     password_auth_providers,
     push,
@@ -85,6 +86,7 @@ class RootConfig:
     thirdpartyrules: third_party_event_rules.ThirdPartyRulesConfig
     tracer: tracer.TracerConfig
     redis: redis.RedisConfig
+    modules: modules.ModulesConfig
 
     config_classes: List = ...
     def __init__(self) -> None: ...
diff --git a/synapse/config/homeserver.py b/synapse/config/homeserver.py
index 5ae0f55bcc..1f42a51857 100644
--- a/synapse/config/homeserver.py
+++ b/synapse/config/homeserver.py
@@ -1,5 +1,4 @@
-# Copyright 2014-2016 OpenMarket Ltd
-# Copyright 2018 New Vector Ltd
+# 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.
@@ -30,6 +29,7 @@ from .jwt import JWTConfig
 from .key import KeyConfig
 from .logger import LoggingConfig
 from .metrics import MetricsConfig
+from .modules import ModulesConfig
 from .oidc import OIDCConfig
 from .password_auth_providers import PasswordAuthProviderConfig
 from .push import PushConfig
@@ -56,6 +56,7 @@ from .workers import WorkerConfig
 class HomeServerConfig(RootConfig):
 
     config_classes = [
+        ModulesConfig,
         ServerConfig,
         TlsConfig,
         FederationConfig,
diff --git a/synapse/config/modules.py b/synapse/config/modules.py
new file mode 100644
index 0000000000..3209e1c492
--- /dev/null
+++ b/synapse/config/modules.py
@@ -0,0 +1,49 @@
+# 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.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from typing import Any, Dict, List, Tuple
+
+from synapse.config._base import Config, ConfigError
+from synapse.util.module_loader import load_module
+
+
+class ModulesConfig(Config):
+    section = "modules"
+
+    def read_config(self, config: dict, **kwargs):
+        self.loaded_modules: List[Tuple[Any, Dict]] = []
+
+        configured_modules = config.get("modules") or []
+        for i, module in enumerate(configured_modules):
+            config_path = ("modules", "<item %i>" % i)
+            if not isinstance(module, dict):
+                raise ConfigError("expected a mapping", config_path)
+
+            self.loaded_modules.append(load_module(module, config_path))
+
+    def generate_config_section(self, **kwargs):
+        return """
+            ## Modules ##
+
+            # Server admins can expand Synapse's functionality with external modules.
+            #
+            # See https://matrix-org.github.io/synapse/develop/modules.html for more
+            # documentation on how to configure or create custom modules for Synapse.
+            #
+            modules:
+                # - module: my_super_module.MySuperClass
+                #   config:
+                #       do_thing: true
+                # - module: my_other_super_module.SomeClass
+                #   config: {}
+            """
diff --git a/synapse/config/spam_checker.py b/synapse/config/spam_checker.py
index 447ba3303b..c24165eb8a 100644
--- a/synapse/config/spam_checker.py
+++ b/synapse/config/spam_checker.py
@@ -42,18 +42,3 @@ class SpamCheckerConfig(Config):
                 self.spam_checkers.append(load_module(spam_checker, config_path))
         else:
             raise ConfigError("spam_checker syntax is incorrect")
-
-    def generate_config_section(self, **kwargs):
-        return """\
-        # Spam checkers are third-party modules that can block specific actions
-        # of local users, such as creating rooms and registering undesirable
-        # usernames, as well as remote users by redacting incoming events.
-        #
-        spam_checker:
-           #- module: "my_custom_project.SuperSpamChecker"
-           #  config:
-           #    example_option: 'things'
-           #- module: "some_other_project.BadEventStopper"
-           #  config:
-           #    example_stop_events_from: ['@bad:example.com']
-        """
diff --git a/synapse/events/spamcheck.py b/synapse/events/spamcheck.py
index d5fa195094..45ec96dfc1 100644
--- a/synapse/events/spamcheck.py
+++ b/synapse/events/spamcheck.py
@@ -15,7 +15,18 @@
 
 import inspect
 import logging
-from typing import TYPE_CHECKING, Any, Collection, Dict, List, Optional, Tuple, Union
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Awaitable,
+    Callable,
+    Collection,
+    Dict,
+    List,
+    Optional,
+    Tuple,
+    Union,
+)
 
 from synapse.rest.media.v1._base import FileInfo
 from synapse.rest.media.v1.media_storage import ReadableFileWrapper
@@ -29,20 +40,186 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
+CHECK_EVENT_FOR_SPAM_CALLBACK = Callable[
+    ["synapse.events.EventBase"],
+    Awaitable[Union[bool, str]],
+]
+USER_MAY_INVITE_CALLBACK = Callable[[str, str, str], Awaitable[bool]]
+USER_MAY_CREATE_ROOM_CALLBACK = Callable[[str], Awaitable[bool]]
+USER_MAY_CREATE_ROOM_ALIAS_CALLBACK = Callable[[str, RoomAlias], Awaitable[bool]]
+USER_MAY_PUBLISH_ROOM_CALLBACK = Callable[[str, str], Awaitable[bool]]
+CHECK_USERNAME_FOR_SPAM_CALLBACK = Callable[[Dict[str, str]], Awaitable[bool]]
+LEGACY_CHECK_REGISTRATION_FOR_SPAM_CALLBACK = Callable[
+    [
+        Optional[dict],
+        Optional[str],
+        Collection[Tuple[str, str]],
+    ],
+    Awaitable[RegistrationBehaviour],
+]
+CHECK_REGISTRATION_FOR_SPAM_CALLBACK = Callable[
+    [
+        Optional[dict],
+        Optional[str],
+        Collection[Tuple[str, str]],
+        Optional[str],
+    ],
+    Awaitable[RegistrationBehaviour],
+]
+CHECK_MEDIA_FILE_FOR_SPAM_CALLBACK = Callable[
+    [ReadableFileWrapper, FileInfo],
+    Awaitable[bool],
+]
+
+
+def load_legacy_spam_checkers(hs: "synapse.server.HomeServer"):
+    """Wrapper that loads spam checkers configured using the old configuration, and
+    registers the spam checker hooks they implement.
+    """
+    spam_checkers = []  # type: List[Any]
+    api = hs.get_module_api()
+    for module, config in hs.config.spam_checkers:
+        # Older spam checkers don't accept the `api` argument, so we
+        # try and detect support.
+        spam_args = inspect.getfullargspec(module)
+        if "api" in spam_args.args:
+            spam_checkers.append(module(config=config, api=api))
+        else:
+            spam_checkers.append(module(config=config))
+
+    # The known spam checker hooks. If a spam checker module implements a method
+    # which name appears in this set, we'll want to register it.
+    spam_checker_methods = {
+        "check_event_for_spam",
+        "user_may_invite",
+        "user_may_create_room",
+        "user_may_create_room_alias",
+        "user_may_publish_room",
+        "check_username_for_spam",
+        "check_registration_for_spam",
+        "check_media_file_for_spam",
+    }
+
+    for spam_checker in spam_checkers:
+        # Methods on legacy spam checkers might not be async, so we wrap them around a
+        # wrapper that will call maybe_awaitable on the result.
+        def async_wrapper(f: Optional[Callable]) -> Optional[Callable[..., Awaitable]]:
+            # f might be None if the callback isn't implemented by the module. In this
+            # case we don't want to register a callback at all so we return None.
+            if f is None:
+                return None
+
+            if f.__name__ == "check_registration_for_spam":
+                checker_args = inspect.signature(f)
+                if len(checker_args.parameters) == 3:
+                    # Backwards compatibility; some modules might implement a hook that
+                    # doesn't expect a 4th argument. In this case, wrap it in a function
+                    # that gives it only 3 arguments and drops the auth_provider_id on
+                    # the floor.
+                    def wrapper(
+                        email_threepid: Optional[dict],
+                        username: Optional[str],
+                        request_info: Collection[Tuple[str, str]],
+                        auth_provider_id: Optional[str],
+                    ) -> Union[Awaitable[RegistrationBehaviour], RegistrationBehaviour]:
+                        # We've already made sure f is not None above, but mypy doesn't
+                        # do well across function boundaries so we need to tell it f is
+                        # definitely not None.
+                        assert f is not None
+
+                        return f(
+                            email_threepid,
+                            username,
+                            request_info,
+                        )
+
+                    f = wrapper
+                elif len(checker_args.parameters) != 4:
+                    raise RuntimeError(
+                        "Bad signature for callback check_registration_for_spam",
+                    )
+
+            def run(*args, **kwargs):
+                # We've already made sure f is not None above, but mypy doesn't do well
+                # across function boundaries so we need to tell it f is definitely not
+                # None.
+                assert f is not None
+
+                return maybe_awaitable(f(*args, **kwargs))
+
+            return run
+
+        # Register the hooks through the module API.
+        hooks = {
+            hook: async_wrapper(getattr(spam_checker, hook, None))
+            for hook in spam_checker_methods
+        }
+
+        api.register_spam_checker_callbacks(**hooks)
+
 
 class SpamChecker:
-    def __init__(self, hs: "synapse.server.HomeServer"):
-        self.spam_checkers = []  # type: List[Any]
-        api = hs.get_module_api()
-
-        for module, config in hs.config.spam_checkers:
-            # Older spam checkers don't accept the `api` argument, so we
-            # try and detect support.
-            spam_args = inspect.getfullargspec(module)
-            if "api" in spam_args.args:
-                self.spam_checkers.append(module(config=config, api=api))
-            else:
-                self.spam_checkers.append(module(config=config))
+    def __init__(self):
+        self._check_event_for_spam_callbacks: List[CHECK_EVENT_FOR_SPAM_CALLBACK] = []
+        self._user_may_invite_callbacks: List[USER_MAY_INVITE_CALLBACK] = []
+        self._user_may_create_room_callbacks: List[USER_MAY_CREATE_ROOM_CALLBACK] = []
+        self._user_may_create_room_alias_callbacks: List[
+            USER_MAY_CREATE_ROOM_ALIAS_CALLBACK
+        ] = []
+        self._user_may_publish_room_callbacks: List[USER_MAY_PUBLISH_ROOM_CALLBACK] = []
+        self._check_username_for_spam_callbacks: List[
+            CHECK_USERNAME_FOR_SPAM_CALLBACK
+        ] = []
+        self._check_registration_for_spam_callbacks: List[
+            CHECK_REGISTRATION_FOR_SPAM_CALLBACK
+        ] = []
+        self._check_media_file_for_spam_callbacks: List[
+            CHECK_MEDIA_FILE_FOR_SPAM_CALLBACK
+        ] = []
+
+    def register_callbacks(
+        self,
+        check_event_for_spam: Optional[CHECK_EVENT_FOR_SPAM_CALLBACK] = None,
+        user_may_invite: Optional[USER_MAY_INVITE_CALLBACK] = None,
+        user_may_create_room: Optional[USER_MAY_CREATE_ROOM_CALLBACK] = None,
+        user_may_create_room_alias: Optional[
+            USER_MAY_CREATE_ROOM_ALIAS_CALLBACK
+        ] = None,
+        user_may_publish_room: Optional[USER_MAY_PUBLISH_ROOM_CALLBACK] = None,
+        check_username_for_spam: Optional[CHECK_USERNAME_FOR_SPAM_CALLBACK] = None,
+        check_registration_for_spam: Optional[
+            CHECK_REGISTRATION_FOR_SPAM_CALLBACK
+        ] = None,
+        check_media_file_for_spam: Optional[CHECK_MEDIA_FILE_FOR_SPAM_CALLBACK] = None,
+    ):
+        """Register callbacks from module for each hook."""
+        if check_event_for_spam is not None:
+            self._check_event_for_spam_callbacks.append(check_event_for_spam)
+
+        if user_may_invite is not None:
+            self._user_may_invite_callbacks.append(user_may_invite)
+
+        if user_may_create_room is not None:
+            self._user_may_create_room_callbacks.append(user_may_create_room)
+
+        if user_may_create_room_alias is not None:
+            self._user_may_create_room_alias_callbacks.append(
+                user_may_create_room_alias,
+            )
+
+        if user_may_publish_room is not None:
+            self._user_may_publish_room_callbacks.append(user_may_publish_room)
+
+        if check_username_for_spam is not None:
+            self._check_username_for_spam_callbacks.append(check_username_for_spam)
+
+        if check_registration_for_spam is not None:
+            self._check_registration_for_spam_callbacks.append(
+                check_registration_for_spam,
+            )
+
+        if check_media_file_for_spam is not None:
+            self._check_media_file_for_spam_callbacks.append(check_media_file_for_spam)
 
     async def check_event_for_spam(
         self, event: "synapse.events.EventBase"
@@ -60,9 +237,10 @@ class SpamChecker:
             True or a string if the event is spammy. If a string is returned it
             will be used as the error message returned to the user.
         """
-        for spam_checker in self.spam_checkers:
-            if await maybe_awaitable(spam_checker.check_event_for_spam(event)):
-                return True
+        for callback in self._check_event_for_spam_callbacks:
+            res = await callback(event)  # type: Union[bool, str]
+            if res:
+                return res
 
         return False
 
@@ -81,15 +259,8 @@ class SpamChecker:
         Returns:
             True if the user may send an invite, otherwise False
         """
-        for spam_checker in self.spam_checkers:
-            if (
-                await maybe_awaitable(
-                    spam_checker.user_may_invite(
-                        inviter_userid, invitee_userid, room_id
-                    )
-                )
-                is False
-            ):
+        for callback in self._user_may_invite_callbacks:
+            if await callback(inviter_userid, invitee_userid, room_id) is False:
                 return False
 
         return True
@@ -105,11 +276,8 @@ class SpamChecker:
         Returns:
             True if the user may create a room, otherwise False
         """
-        for spam_checker in self.spam_checkers:
-            if (
-                await maybe_awaitable(spam_checker.user_may_create_room(userid))
-                is False
-            ):
+        for callback in self._user_may_create_room_callbacks:
+            if await callback(userid) is False:
                 return False
 
         return True
@@ -128,13 +296,8 @@ class SpamChecker:
         Returns:
             True if the user may create a room alias, otherwise False
         """
-        for spam_checker in self.spam_checkers:
-            if (
-                await maybe_awaitable(
-                    spam_checker.user_may_create_room_alias(userid, room_alias)
-                )
-                is False
-            ):
+        for callback in self._user_may_create_room_alias_callbacks:
+            if await callback(userid, room_alias) is False:
                 return False
 
         return True
@@ -151,13 +314,8 @@ class SpamChecker:
         Returns:
             True if the user may publish the room, otherwise False
         """
-        for spam_checker in self.spam_checkers:
-            if (
-                await maybe_awaitable(
-                    spam_checker.user_may_publish_room(userid, room_id)
-                )
-                is False
-            ):
+        for callback in self._user_may_publish_room_callbacks:
+            if await callback(userid, room_id) is False:
                 return False
 
         return True
@@ -177,15 +335,11 @@ class SpamChecker:
         Returns:
             True if the user is spammy.
         """
-        for spam_checker in self.spam_checkers:
-            # For backwards compatibility, only run if the method exists on the
-            # spam checker
-            checker = getattr(spam_checker, "check_username_for_spam", None)
-            if checker:
-                # Make a copy of the user profile object to ensure the spam checker
-                # cannot modify it.
-                if await maybe_awaitable(checker(user_profile.copy())):
-                    return True
+        for callback in self._check_username_for_spam_callbacks:
+            # Make a copy of the user profile object to ensure the spam checker cannot
+            # modify it.
+            if await callback(user_profile.copy()):
+                return True
 
         return False
 
@@ -211,33 +365,13 @@ class SpamChecker:
             Enum for how the request should be handled
         """
 
-        for spam_checker in self.spam_checkers:
-            # For backwards compatibility, only run if the method exists on the
-            # spam checker
-            checker = getattr(spam_checker, "check_registration_for_spam", None)
-            if checker:
-                # Provide auth_provider_id if the function supports it
-                checker_args = inspect.signature(checker)
-                if len(checker_args.parameters) == 4:
-                    d = checker(
-                        email_threepid,
-                        username,
-                        request_info,
-                        auth_provider_id,
-                    )
-                elif len(checker_args.parameters) == 3:
-                    d = checker(email_threepid, username, request_info)
-                else:
-                    logger.error(
-                        "Invalid signature for %s.check_registration_for_spam. Denying registration",
-                        spam_checker.__module__,
-                    )
-                    return RegistrationBehaviour.DENY
-
-                behaviour = await maybe_awaitable(d)
-                assert isinstance(behaviour, RegistrationBehaviour)
-                if behaviour != RegistrationBehaviour.ALLOW:
-                    return behaviour
+        for callback in self._check_registration_for_spam_callbacks:
+            behaviour = await (
+                callback(email_threepid, username, request_info, auth_provider_id)
+            )
+            assert isinstance(behaviour, RegistrationBehaviour)
+            if behaviour != RegistrationBehaviour.ALLOW:
+                return behaviour
 
         return RegistrationBehaviour.ALLOW
 
@@ -275,13 +409,9 @@ class SpamChecker:
             allowed.
         """
 
-        for spam_checker in self.spam_checkers:
-            # For backwards compatibility, only run if the method exists on the
-            # spam checker
-            checker = getattr(spam_checker, "check_media_file_for_spam", None)
-            if checker:
-                spam = await maybe_awaitable(checker(file_wrapper, file_info))
-                if spam:
-                    return True
+        for callback in self._check_media_file_for_spam_callbacks:
+            spam = await callback(file_wrapper, file_info)
+            if spam:
+                return True
 
         return False
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index 4ceef3fab3..ca1ed6a5c0 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -195,7 +195,7 @@ class RegistrationHandler(BaseHandler):
             bind_emails: list of emails to bind to this account.
             by_admin: True if this registration is being made via the
               admin api, otherwise False.
-            user_agent_ips: Tuples of IP addresses and user-agents used
+            user_agent_ips: Tuples of user-agents and IP addresses used
                 during the registration process.
             auth_provider_id: The SSO IdP the user used, if any.
         Returns:
diff --git a/synapse/module_api/__init__.py b/synapse/module_api/__init__.py
index cecdc96bf5..58b255eb1b 100644
--- a/synapse/module_api/__init__.py
+++ b/synapse/module_api/__init__.py
@@ -16,6 +16,7 @@ import logging
 from typing import TYPE_CHECKING, Any, Generator, Iterable, List, Optional, Tuple
 
 from twisted.internet import defer
+from twisted.web.resource import IResource
 
 from synapse.events import EventBase
 from synapse.http.client import SimpleHttpClient
@@ -42,7 +43,7 @@ class ModuleApi:
     can register new users etc if necessary.
     """
 
-    def __init__(self, hs, auth_handler):
+    def __init__(self, hs: "HomeServer", auth_handler):
         self._hs = hs
 
         self._store = hs.get_datastore()
@@ -56,6 +57,33 @@ class ModuleApi:
         self._http_client = hs.get_simple_http_client()  # type: SimpleHttpClient
         self._public_room_list_manager = PublicRoomListManager(hs)
 
+        self._spam_checker = hs.get_spam_checker()
+
+    #################################################################################
+    # The following methods should only be called during the module's initialisation.
+
+    @property
+    def register_spam_checker_callbacks(self):
+        """Registers callbacks for spam checking capabilities."""
+        return self._spam_checker.register_callbacks
+
+    def register_web_resource(self, path: str, resource: IResource):
+        """Registers a web resource to be served at the given path.
+
+        This function should be called during initialisation of the module.
+
+        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.
+        """
+        self._hs.register_module_web_resource(path, resource)
+
+    #########################################################################
+    # The following methods can be called by the module at any point in time.
+
     @property
     def http_client(self):
         """Allows making outbound HTTP requests to remote resources.
diff --git a/synapse/module_api/errors.py b/synapse/module_api/errors.py
index d24864c549..02bbb0be39 100644
--- a/synapse/module_api/errors.py
+++ b/synapse/module_api/errors.py
@@ -15,3 +15,4 @@
 """Exception types which are exposed as part of the stable module API"""
 
 from synapse.api.errors import RedirectException, SynapseError  # noqa: F401
+from synapse.config._base import ConfigError  # noqa: F401
diff --git a/synapse/server.py b/synapse/server.py
index e8dd2fa9f2..2c27d2a7e8 100644
--- a/synapse/server.py
+++ b/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:
diff --git a/synapse/util/module_loader.py b/synapse/util/module_loader.py
index cbfbd097f9..5a638c6e9a 100644
--- a/synapse/util/module_loader.py
+++ b/synapse/util/module_loader.py
@@ -51,21 +51,26 @@ def load_module(provider: dict, config_path: Iterable[str]) -> Tuple[Type, Any]:
 
     # Load the module config. If None, pass an empty dictionary instead
     module_config = provider.get("config") or {}
-    try:
-        provider_config = provider_class.parse_config(module_config)
-    except jsonschema.ValidationError as e:
-        raise json_error_to_config_error(e, itertools.chain(config_path, ("config",)))
-    except ConfigError as e:
-        raise _wrap_config_error(
-            "Failed to parse config for module %r" % (modulename,),
-            prefix=itertools.chain(config_path, ("config",)),
-            e=e,
-        )
-    except Exception as e:
-        raise ConfigError(
-            "Failed to parse config for module %r" % (modulename,),
-            path=itertools.chain(config_path, ("config",)),
-        ) from e
+    if hasattr(provider_class, "parse_config"):
+        try:
+            provider_config = provider_class.parse_config(module_config)
+        except jsonschema.ValidationError as e:
+            raise json_error_to_config_error(
+                e, itertools.chain(config_path, ("config",))
+            )
+        except ConfigError as e:
+            raise _wrap_config_error(
+                "Failed to parse config for module %r" % (modulename,),
+                prefix=itertools.chain(config_path, ("config",)),
+                e=e,
+            )
+        except Exception as e:
+            raise ConfigError(
+                "Failed to parse config for module %r" % (modulename,),
+                path=itertools.chain(config_path, ("config",)),
+            ) from e
+    else:
+        provider_config = module_config
 
     return provider_class, provider_config