Merge pull request #8060 from matrix-org/erikj/type_server

Change HomeServer definition to work with typing.

8 files changed, 269 insertions, 403 deletions

diff --git a/changelog.d/8060.misc b/changelog.d/8060.misc
new file mode 100644
index 0000000000..a0caf008d7
--- /dev/null
+++ b/changelog.d/8060.misc
@@ -0,0 +1 @@
+Improve typing information on `HomeServer` object.
diff --git a/synapse/app/generic_worker.py b/synapse/app/generic_worker.py
index 7957586d69..739b013d4c 100644
--- a/synapse/app/generic_worker.py
+++ b/synapse/app/generic_worker.py
@@ -125,7 +125,7 @@ from synapse.rest.client.v2_alpha.register import RegisterRestServlet
 from synapse.rest.client.versions import VersionsRestServlet
 from synapse.rest.health import HealthResource
 from synapse.rest.key.v2 import KeyApiV2Resource
-from synapse.server import HomeServer
+from synapse.server import HomeServer, cache_in_self
 from synapse.storage.databases.main.censor_events import CensorEventsStore
 from synapse.storage.databases.main.media_repository import MediaRepositoryStore
 from synapse.storage.databases.main.monthly_active_users import (
@@ -635,10 +635,12 @@ class GenericWorkerServer(HomeServer):
     async def remove_pusher(self, app_id, push_key, user_id):
         self.get_tcp_replication().send_remove_pusher(app_id, push_key, user_id)
 
-    def build_replication_data_handler(self):
+    @cache_in_self
+    def get_replication_data_handler(self):
         return GenericWorkerReplicationHandler(self)
 
-    def build_presence_handler(self):
+    @cache_in_self
+    def get_presence_handler(self):
         return GenericWorkerPresence(self)
 
 
diff --git a/synapse/handlers/oidc_handler.py b/synapse/handlers/oidc_handler.py
index 87f0c5e197..fa5ee5de8f 100644
--- a/synapse/handlers/oidc_handler.py
+++ b/synapse/handlers/oidc_handler.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 import json
 import logging
-from typing import Dict, Generic, List, Optional, Tuple, TypeVar
+from typing import TYPE_CHECKING, Dict, Generic, List, Optional, Tuple, TypeVar
 from urllib.parse import urlencode
 
 import attr
@@ -39,9 +39,11 @@ from synapse.http.server import respond_with_html
 from synapse.http.site import SynapseRequest
 from synapse.logging.context import make_deferred_yieldable
 from synapse.push.mailer import load_jinja2_templates
-from synapse.server import HomeServer
 from synapse.types import UserID, map_username_to_mxid_localpart
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 SESSION_COOKIE_NAME = b"oidc_session"
@@ -91,7 +93,7 @@ class OidcHandler:
     """Handles requests related to the OpenID Connect login flow.
     """
 
-    def __init__(self, hs: HomeServer):
+    def __init__(self, hs: "HomeServer"):
         self._callback_url = hs.config.oidc_callback_url  # type: str
         self._scopes = hs.config.oidc_scopes  # type: List[str]
         self._client_auth = ClientAuth(
diff --git a/synapse/secrets.py b/synapse/secrets.py
index 5f43f81eb0..ff86950a54 100644
--- a/synapse/secrets.py
+++ b/synapse/secrets.py
@@ -25,8 +25,12 @@ import sys
 if sys.version_info[0:2] >= (3, 6):
     import secrets
 
-    def Secrets():
-        return secrets
+    class Secrets:
+        def token_bytes(self, nbytes=32):
+            return secrets.token_bytes(nbytes)
+
+        def token_hex(self, nbytes=32):
+            return secrets.token_hex(nbytes)
 
 
 else:
diff --git a/synapse/server.py b/synapse/server.py
index 81d7f26f9c..9055b97ac3 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -22,10 +22,14 @@
 
 # Imports required for the default HomeServer() implementation
 import abc
+import functools
 import logging
 import os
+from typing import TYPE_CHECKING, Any, Callable, Dict, List, Optional, TypeVar, cast
 
+import twisted
 from twisted.mail.smtp import sendmail
+from twisted.web.iweb import IPolicyForHTTPS
 
 from synapse.api.auth import Auth
 from synapse.api.filtering import Filtering
@@ -93,7 +97,7 @@ from synapse.push.pusherpool import PusherPool
 from synapse.replication.tcp.client import ReplicationDataHandler
 from synapse.replication.tcp.handler import ReplicationCommandHandler
 from synapse.replication.tcp.resource import ReplicationStreamer
-from synapse.replication.tcp.streams import STREAMS_MAP
+from synapse.replication.tcp.streams import STREAMS_MAP, Stream
 from synapse.rest.media.v1.media_repository import (
     MediaRepository,
     MediaRepositoryResource,
@@ -107,30 +111,72 @@ from synapse.server_notices.worker_server_notices_sender import (
 from synapse.state import StateHandler, StateResolutionHandler
 from synapse.storage import Databases, DataStore, Storage
 from synapse.streams.events import EventSources
+from synapse.types import DomainSpecificString
 from synapse.util import Clock
 from synapse.util.distributor import Distributor
 from synapse.util.stringutils import random_string
 
 logger = logging.getLogger(__name__)
 
+if TYPE_CHECKING:
+    from synapse.handlers.oidc_handler import OidcHandler
+    from synapse.handlers.saml_handler import SamlHandler
 
-class HomeServer(object):
+
+T = TypeVar("T", bound=Callable[..., Any])
+
+
+def cache_in_self(builder: T) -> T:
+    """Wraps a function called e.g. `get_foo`, checking if `self.foo` exists and
+    returning if so. If not, calls the given function and sets `self.foo` to it.
+
+    Also ensures that dependency cycles throw an exception correctly, rather
+    than overflowing the stack.
+    """
+
+    if not builder.__name__.startswith("get_"):
+        raise Exception(
+            "@cache_in_self can only be used on functions starting with `get_`"
+        )
+
+    depname = builder.__name__[len("get_") :]
+
+    building = [False]
+
+    @functools.wraps(builder)
+    def _get(self):
+        try:
+            return getattr(self, depname)
+        except AttributeError:
+            pass
+
+        # Prevent cyclic dependencies from deadlocking
+        if building[0]:
+            raise ValueError("Cyclic dependency while building %s" % (depname,))
+
+        building[0] = True
+        try:
+            dep = builder(self)
+            setattr(self, depname, dep)
+        finally:
+            building[0] = False
+
+        return dep
+
+    # We cast here as we need to tell mypy that `_get` has the same signature as
+    # `builder`.
+    return cast(T, _get)
+
+
+class HomeServer(metaclass=abc.ABCMeta):
     """A basic homeserver object without lazy component builders.
 
     This will need all of the components it requires to either be passed as
     constructor arguments, or the relevant methods overriding to create them.
     Typically this would only be used for unit tests.
 
-    For every dependency in the DEPENDENCIES list below, this class creates one
-    method,
-        def get_DEPENDENCY(self)
-    which returns the value of that dependency. If no value has yet been set
-    nor was provided to the constructor, it will attempt to call a lazy builder
-    method called
-        def build_DEPENDENCY(self)
-    which must be implemented by the subclass. This code may call any of the
-    required "get" methods on the instance to obtain the sub-dependencies that
-    one requires.
+    Dependencies should be added by creating a `def get_<depname>(self)`
+    function, wrapping it in `@cache_in_self`.
 
     Attributes:
         config (synapse.config.homeserver.HomeserverConfig):
@@ -138,86 +184,6 @@ class HomeServer(object):
             we are listening on to provide HTTP services.
     """
 
-    __metaclass__ = abc.ABCMeta
-
-    DEPENDENCIES = [
-        "http_client",
-        "federation_client",
-        "federation_server",
-        "handlers",
-        "auth",
-        "room_creation_handler",
-        "room_shutdown_handler",
-        "state_handler",
-        "state_resolution_handler",
-        "presence_handler",
-        "sync_handler",
-        "typing_handler",
-        "room_list_handler",
-        "acme_handler",
-        "auth_handler",
-        "device_handler",
-        "stats_handler",
-        "e2e_keys_handler",
-        "e2e_room_keys_handler",
-        "event_handler",
-        "event_stream_handler",
-        "initial_sync_handler",
-        "application_service_api",
-        "application_service_scheduler",
-        "application_service_handler",
-        "device_message_handler",
-        "profile_handler",
-        "event_creation_handler",
-        "deactivate_account_handler",
-        "set_password_handler",
-        "notifier",
-        "event_sources",
-        "keyring",
-        "pusherpool",
-        "event_builder_factory",
-        "filtering",
-        "http_client_context_factory",
-        "simple_http_client",
-        "proxied_http_client",
-        "media_repository",
-        "media_repository_resource",
-        "federation_transport_client",
-        "federation_sender",
-        "receipts_handler",
-        "macaroon_generator",
-        "tcp_replication",
-        "read_marker_handler",
-        "action_generator",
-        "user_directory_handler",
-        "groups_local_handler",
-        "groups_server_handler",
-        "groups_attestation_signing",
-        "groups_attestation_renewer",
-        "secrets",
-        "spam_checker",
-        "third_party_event_rules",
-        "room_member_handler",
-        "federation_registry",
-        "server_notices_manager",
-        "server_notices_sender",
-        "message_handler",
-        "pagination_handler",
-        "room_context_handler",
-        "sendmail",
-        "registration_handler",
-        "account_validity_handler",
-        "cas_handler",
-        "saml_handler",
-        "oidc_handler",
-        "event_client_serializer",
-        "password_policy_handler",
-        "storage",
-        "replication_streamer",
-        "replication_data_handler",
-        "replication_streams",
-    ]
-
     REQUIRED_ON_MASTER_STARTUP = ["user_directory_handler", "stats_handler"]
 
     # This is overridden in derived application classes
@@ -232,16 +198,17 @@ class HomeServer(object):
             config: The full config for the homeserver.
         """
         if not reactor:
-            from twisted.internet import reactor
+            from twisted.internet import reactor as _reactor
+
+            reactor = _reactor
 
         self._reactor = reactor
         self.hostname = hostname
         # the key we use to sign events and requests
         self.signing_key = config.key.signing_key[0]
         self.config = config
-        self._building = {}
-        self._listening_services = []
-        self.start_time = None
+        self._listening_services = []  # type: List[twisted.internet.tcp.Port]
+        self.start_time = None  # type: Optional[int]
 
         self._instance_id = random_string(5)
         self._instance_name = config.worker_name or "master"
@@ -255,13 +222,13 @@ class HomeServer(object):
             burst_count=config.rc_registration.burst_count,
         )
 
-        self.datastores = None
+        self.datastores = None  # type: Optional[Databases]
 
         # Other kwargs are explicit dependencies
         for depname in kwargs:
             setattr(self, depname, kwargs[depname])
 
-    def get_instance_id(self):
+    def get_instance_id(self) -> str:
         """A unique ID for this synapse process instance.
 
         This is used to distinguish running instances in worker-based
@@ -277,13 +244,13 @@ class HomeServer(object):
         """
         return self._instance_name
 
-    def setup(self):
+    def setup(self) -> None:
         logger.info("Setting up.")
         self.start_time = int(self.get_clock().time())
         self.datastores = Databases(self.DATASTORE_CLASS, self)
         logger.info("Finished setting up.")
 
-    def setup_master(self):
+    def setup_master(self) -> None:
         """
         Some handlers have side effects on instantiation (like registering
         background updates). This function causes them to be fetched, and
@@ -292,192 +259,242 @@ class HomeServer(object):
         for i in self.REQUIRED_ON_MASTER_STARTUP:
             getattr(self, "get_" + i)()
 
-    def get_reactor(self):
+    def get_reactor(self) -> twisted.internet.base.ReactorBase:
         """
         Fetch the Twisted reactor in use by this HomeServer.
         """
         return self._reactor
 
-    def get_ip_from_request(self, request):
+    def get_ip_from_request(self, request) -> str:
         # X-Forwarded-For is handled by our custom request type.
         return request.getClientIP()
 
-    def is_mine(self, domain_specific_string):
+    def is_mine(self, domain_specific_string: DomainSpecificString) -> bool:
         return domain_specific_string.domain == self.hostname
 
-    def is_mine_id(self, string):
+    def is_mine_id(self, string: str) -> bool:
         return string.split(":", 1)[1] == self.hostname
 
-    def get_clock(self):
+    def get_clock(self) -> Clock:
         return self.clock
 
     def get_datastore(self) -> DataStore:
+        if not self.datastores:
+            raise Exception("HomeServer.setup must be called before getting datastores")
+
         return self.datastores.main
 
-    def get_datastores(self):
+    def get_datastores(self) -> Databases:
+        if not self.datastores:
+            raise Exception("HomeServer.setup must be called before getting datastores")
+
         return self.datastores
 
-    def get_config(self):
+    def get_config(self) -> HomeServerConfig:
         return self.config
 
-    def get_distributor(self):
+    def get_distributor(self) -> Distributor:
         return self.distributor
 
     def get_registration_ratelimiter(self) -> Ratelimiter:
         return self.registration_ratelimiter
 
-    def build_federation_client(self):
+    @cache_in_self
+    def get_federation_client(self) -> FederationClient:
         return FederationClient(self)
 
-    def build_federation_server(self):
+    @cache_in_self
+    def get_federation_server(self) -> FederationServer:
         return FederationServer(self)
 
-    def build_handlers(self):
+    @cache_in_self
+    def get_handlers(self) -> Handlers:
         return Handlers(self)
 
-    def build_notifier(self):
+    @cache_in_self
+    def get_notifier(self) -> Notifier:
         return Notifier(self)
 
-    def build_auth(self):
+    @cache_in_self
+    def get_auth(self) -> Auth:
         return Auth(self)
 
-    def build_http_client_context_factory(self):
+    @cache_in_self
+    def get_http_client_context_factory(self) -> IPolicyForHTTPS:
         return (
             InsecureInterceptableContextFactory()
             if self.config.use_insecure_ssl_client_just_for_testing_do_not_use
             else RegularPolicyForHTTPS()
         )
 
-    def build_simple_http_client(self):
+    @cache_in_self
+    def get_simple_http_client(self) -> SimpleHttpClient:
         return SimpleHttpClient(self)
 
-    def build_proxied_http_client(self):
+    @cache_in_self
+    def get_proxied_http_client(self) -> SimpleHttpClient:
         return SimpleHttpClient(
             self,
             http_proxy=os.getenvb(b"http_proxy"),
             https_proxy=os.getenvb(b"HTTPS_PROXY"),
         )
 
-    def build_room_creation_handler(self):
+    @cache_in_self
+    def get_room_creation_handler(self) -> RoomCreationHandler:
         return RoomCreationHandler(self)
 
-    def build_room_shutdown_handler(self):
+    @cache_in_self
+    def get_room_shutdown_handler(self) -> RoomShutdownHandler:
         return RoomShutdownHandler(self)
 
-    def build_sendmail(self):
+    @cache_in_self
+    def get_sendmail(self) -> sendmail:
         return sendmail
 
-    def build_state_handler(self):
+    @cache_in_self
+    def get_state_handler(self) -> StateHandler:
         return StateHandler(self)
 
-    def build_state_resolution_handler(self):
+    @cache_in_self
+    def get_state_resolution_handler(self) -> StateResolutionHandler:
         return StateResolutionHandler(self)
 
-    def build_presence_handler(self):
+    @cache_in_self
+    def get_presence_handler(self) -> PresenceHandler:
         return PresenceHandler(self)
 
-    def build_typing_handler(self):
+    @cache_in_self
+    def get_typing_handler(self):
         if self.config.worker.writers.typing == self.get_instance_name():
             return TypingWriterHandler(self)
         else:
             return FollowerTypingHandler(self)
 
-    def build_sync_handler(self):
+    @cache_in_self
+    def get_sync_handler(self) -> SyncHandler:
         return SyncHandler(self)
 
-    def build_room_list_handler(self):
+    @cache_in_self
+    def get_room_list_handler(self) -> RoomListHandler:
         return RoomListHandler(self)
 
-    def build_auth_handler(self):
+    @cache_in_self
+    def get_auth_handler(self) -> AuthHandler:
         return AuthHandler(self)
 
-    def build_macaroon_generator(self):
+    @cache_in_self
+    def get_macaroon_generator(self) -> MacaroonGenerator:
         return MacaroonGenerator(self)
 
-    def build_device_handler(self):
+    @cache_in_self
+    def get_device_handler(self):
         if self.config.worker_app:
             return DeviceWorkerHandler(self)
         else:
             return DeviceHandler(self)
 
-    def build_device_message_handler(self):
+    @cache_in_self
+    def get_device_message_handler(self) -> DeviceMessageHandler:
         return DeviceMessageHandler(self)
 
-    def build_e2e_keys_handler(self):
+    @cache_in_self
+    def get_e2e_keys_handler(self) -> E2eKeysHandler:
         return E2eKeysHandler(self)
 
-    def build_e2e_room_keys_handler(self):
+    @cache_in_self
+    def get_e2e_room_keys_handler(self) -> E2eRoomKeysHandler:
         return E2eRoomKeysHandler(self)
 
-    def build_acme_handler(self):
+    @cache_in_self
+    def get_acme_handler(self) -> AcmeHandler:
         return AcmeHandler(self)
 
-    def build_application_service_api(self):
+    @cache_in_self
+    def get_application_service_api(self) -> ApplicationServiceApi:
         return ApplicationServiceApi(self)
 
-    def build_application_service_scheduler(self):
+    @cache_in_self
+    def get_application_service_scheduler(self) -> ApplicationServiceScheduler:
         return ApplicationServiceScheduler(self)
 
-    def build_application_service_handler(self):
+    @cache_in_self
+    def get_application_service_handler(self) -> ApplicationServicesHandler:
         return ApplicationServicesHandler(self)
 
-    def build_event_handler(self):
+    @cache_in_self
+    def get_event_handler(self) -> EventHandler:
         return EventHandler(self)
 
-    def build_event_stream_handler(self):
+    @cache_in_self
+    def get_event_stream_handler(self) -> EventStreamHandler:
         return EventStreamHandler(self)
 
-    def build_initial_sync_handler(self):
+    @cache_in_self
+    def get_initial_sync_handler(self) -> InitialSyncHandler:
         return InitialSyncHandler(self)
 
-    def build_profile_handler(self):
+    @cache_in_self
+    def get_profile_handler(self):
         if self.config.worker_app:
             return BaseProfileHandler(self)
         else:
             return MasterProfileHandler(self)
 
-    def build_event_creation_handler(self):
+    @cache_in_self
+    def get_event_creation_handler(self) -> EventCreationHandler:
         return EventCreationHandler(self)
 
-    def build_deactivate_account_handler(self):
+    @cache_in_self
+    def get_deactivate_account_handler(self) -> DeactivateAccountHandler:
         return DeactivateAccountHandler(self)
 
-    def build_set_password_handler(self):
+    @cache_in_self
+    def get_set_password_handler(self) -> SetPasswordHandler:
         return SetPasswordHandler(self)
 
-    def build_event_sources(self):
+    @cache_in_self
+    def get_event_sources(self) -> EventSources:
         return EventSources(self)
 
-    def build_keyring(self):
+    @cache_in_self
+    def get_keyring(self) -> Keyring:
         return Keyring(self)
 
-    def build_event_builder_factory(self):
+    @cache_in_self
+    def get_event_builder_factory(self) -> EventBuilderFactory:
         return EventBuilderFactory(self)
 
-    def build_filtering(self):
+    @cache_in_self
+    def get_filtering(self) -> Filtering:
         return Filtering(self)
 
-    def build_pusherpool(self):
+    @cache_in_self
+    def get_pusherpool(self) -> PusherPool:
         return PusherPool(self)
 
-    def build_http_client(self):
+    @cache_in_self
+    def get_http_client(self) -> MatrixFederationHttpClient:
         tls_client_options_factory = context_factory.FederationPolicyForHTTPS(
             self.config
         )
         return MatrixFederationHttpClient(self, tls_client_options_factory)
 
-    def build_media_repository_resource(self):
+    @cache_in_self
+    def get_media_repository_resource(self) -> MediaRepositoryResource:
         # build the media repo resource. This indirects through the HomeServer
         # to ensure that we only have a single instance of
         return MediaRepositoryResource(self)
 
-    def build_media_repository(self):
+    @cache_in_self
+    def get_media_repository(self) -> MediaRepository:
         return MediaRepository(self)
 
-    def build_federation_transport_client(self):
+    @cache_in_self
+    def get_federation_transport_client(self) -> TransportLayerClient:
         return TransportLayerClient(self)
 
-    def build_federation_sender(self):
+    @cache_in_self
+    def get_federation_sender(self):
         if self.should_send_federation():
             return FederationSender(self)
         elif not self.config.worker_app:
@@ -485,156 +502,152 @@ class HomeServer(object):
         else:
             raise Exception("Workers cannot send federation traffic")
 
-    def build_receipts_handler(self):
+    @cache_in_self
+    def get_receipts_handler(self) -> ReceiptsHandler:
         return ReceiptsHandler(self)
 
-    def build_read_marker_handler(self):
+    @cache_in_self
+    def get_read_marker_handler(self) -> ReadMarkerHandler:
         return ReadMarkerHandler(self)
 
-    def build_tcp_replication(self):
+    @cache_in_self
+    def get_tcp_replication(self) -> ReplicationCommandHandler:
         return ReplicationCommandHandler(self)
 
-    def build_action_generator(self):
+    @cache_in_self
+    def get_action_generator(self) -> ActionGenerator:
         return ActionGenerator(self)
 
-    def build_user_directory_handler(self):
+    @cache_in_self
+    def get_user_directory_handler(self) -> UserDirectoryHandler:
         return UserDirectoryHandler(self)
 
-    def build_groups_local_handler(self):
+    @cache_in_self
+    def get_groups_local_handler(self):
         if self.config.worker_app:
             return GroupsLocalWorkerHandler(self)
         else:
             return GroupsLocalHandler(self)
 
-    def build_groups_server_handler(self):
+    @cache_in_self
+    def get_groups_server_handler(self):
         if self.config.worker_app:
             return GroupsServerWorkerHandler(self)
         else:
             return GroupsServerHandler(self)
 
-    def build_groups_attestation_signing(self):
+    @cache_in_self
+    def get_groups_attestation_signing(self) -> GroupAttestationSigning:
         return GroupAttestationSigning(self)
 
-    def build_groups_attestation_renewer(self):
+    @cache_in_self
+    def get_groups_attestation_renewer(self) -> GroupAttestionRenewer:
         return GroupAttestionRenewer(self)
 
-    def build_secrets(self):
+    @cache_in_self
+    def get_secrets(self) -> Secrets:
         return Secrets()
 
-    def build_stats_handler(self):
+    @cache_in_self
+    def get_stats_handler(self) -> StatsHandler:
         return StatsHandler(self)
 
-    def build_spam_checker(self):
+    @cache_in_self
+    def get_spam_checker(self):
         return SpamChecker(self)
 
-    def build_third_party_event_rules(self):
+    @cache_in_self
+    def get_third_party_event_rules(self) -> ThirdPartyEventRules:
         return ThirdPartyEventRules(self)
 
-    def build_room_member_handler(self):
+    @cache_in_self
+    def get_room_member_handler(self):
         if self.config.worker_app:
             return RoomMemberWorkerHandler(self)
         return RoomMemberMasterHandler(self)
 
-    def build_federation_registry(self):
+    @cache_in_self
+    def get_federation_registry(self) -> FederationHandlerRegistry:
         return FederationHandlerRegistry(self)
 
-    def build_server_notices_manager(self):
+    @cache_in_self
+    def get_server_notices_manager(self):
         if self.config.worker_app:
             raise Exception("Workers cannot send server notices")
         return ServerNoticesManager(self)
 
-    def build_server_notices_sender(self):
+    @cache_in_self
+    def get_server_notices_sender(self):
         if self.config.worker_app:
             return WorkerServerNoticesSender(self)
         return ServerNoticesSender(self)
 
-    def build_message_handler(self):
+    @cache_in_self
+    def get_message_handler(self) -> MessageHandler:
         return MessageHandler(self)
 
-    def build_pagination_handler(self):
+    @cache_in_self
+    def get_pagination_handler(self) -> PaginationHandler:
         return PaginationHandler(self)
 
-    def build_room_context_handler(self):
+    @cache_in_self
+    def get_room_context_handler(self) -> RoomContextHandler:
         return RoomContextHandler(self)
 
-    def build_registration_handler(self):
+    @cache_in_self
+    def get_registration_handler(self) -> RegistrationHandler:
         return RegistrationHandler(self)
 
-    def build_account_validity_handler(self):
+    @cache_in_self
+    def get_account_validity_handler(self) -> AccountValidityHandler:
         return AccountValidityHandler(self)
 
-    def build_cas_handler(self):
+    @cache_in_self
+    def get_cas_handler(self) -> CasHandler:
         return CasHandler(self)
 
-    def build_saml_handler(self):
+    @cache_in_self
+    def get_saml_handler(self) -> "SamlHandler":
         from synapse.handlers.saml_handler import SamlHandler
 
         return SamlHandler(self)
 
-    def build_oidc_handler(self):
+    @cache_in_self
+    def get_oidc_handler(self) -> "OidcHandler":
         from synapse.handlers.oidc_handler import OidcHandler
 
         return OidcHandler(self)
 
-    def build_event_client_serializer(self):
+    @cache_in_self
+    def get_event_client_serializer(self) -> EventClientSerializer:
         return EventClientSerializer(self)
 
-    def build_password_policy_handler(self):
+    @cache_in_self
+    def get_password_policy_handler(self) -> PasswordPolicyHandler:
         return PasswordPolicyHandler(self)
 
-    def build_storage(self) -> Storage:
-        return Storage(self, self.datastores)
+    @cache_in_self
+    def get_storage(self) -> Storage:
+        return Storage(self, self.get_datastores())
 
-    def build_replication_streamer(self) -> ReplicationStreamer:
+    @cache_in_self
+    def get_replication_streamer(self) -> ReplicationStreamer:
         return ReplicationStreamer(self)
 
-    def build_replication_data_handler(self):
+    @cache_in_self
+    def get_replication_data_handler(self) -> ReplicationDataHandler:
         return ReplicationDataHandler(self)
 
-    def build_replication_streams(self):
+    @cache_in_self
+    def get_replication_streams(self) -> Dict[str, Stream]:
         return {stream.NAME: stream(self) for stream in STREAMS_MAP.values()}
 
-    def remove_pusher(self, app_id, push_key, user_id):
-        return self.get_pusherpool().remove_pusher(app_id, push_key, user_id)
+    async def remove_pusher(self, app_id: str, push_key: str, user_id: str):
+        return await self.get_pusherpool().remove_pusher(app_id, push_key, user_id)
 
-    def should_send_federation(self):
+    def should_send_federation(self) -> bool:
         "Should this server be sending federation traffic directly?"
         return self.config.send_federation and (
             not self.config.worker_app
             or self.config.worker_app == "synapse.app.federation_sender"
         )
-
-
-def _make_dependency_method(depname):
-    def _get(hs):
-        try:
-            return getattr(hs, depname)
-        except AttributeError:
-            pass
-
-        try:
-            builder = getattr(hs, "build_%s" % (depname))
-        except AttributeError:
-            raise NotImplementedError(
-                "%s has no %s nor a builder for it" % (type(hs).__name__, depname)
-            )
-
-        # Prevent cyclic dependencies from deadlocking
-        if depname in hs._building:
-            raise ValueError("Cyclic dependency while building %s" % (depname,))
-
-        hs._building[depname] = 1
-        try:
-            dep = builder()
-            setattr(hs, depname, dep)
-        finally:
-            del hs._building[depname]
-
-        return dep
-
-    setattr(HomeServer, "get_%s" % (depname), _get)
-
-
-# Build magic accessors for every dependency
-for depname in HomeServer.DEPENDENCIES:
-    _make_dependency_method(depname)
diff --git a/synapse/server.pyi b/synapse/server.pyi
deleted file mode 100644
index 6a9bbb4713..0000000000
--- a/synapse/server.pyi
+++ /dev/null
@@ -1,161 +0,0 @@
-from typing import Dict
-
-import twisted.internet
-
-import synapse.api.auth
-import synapse.config.homeserver
-import synapse.crypto.keyring
-import synapse.federation.federation_server
-import synapse.federation.sender
-import synapse.federation.transport.client
-import synapse.handlers
-import synapse.handlers.auth
-import synapse.handlers.deactivate_account
-import synapse.handlers.device
-import synapse.handlers.e2e_keys
-import synapse.handlers.message
-import synapse.handlers.presence
-import synapse.handlers.register
-import synapse.handlers.room
-import synapse.handlers.room_member
-import synapse.handlers.set_password
-import synapse.http.client
-import synapse.http.matrixfederationclient
-import synapse.notifier
-import synapse.push.pusherpool
-import synapse.replication.tcp.client
-import synapse.replication.tcp.handler
-import synapse.rest.media.v1.media_repository
-import synapse.server_notices.server_notices_manager
-import synapse.server_notices.server_notices_sender
-import synapse.state
-import synapse.storage
-from synapse.events.builder import EventBuilderFactory
-from synapse.handlers.appservice import ApplicationServicesHandler
-from synapse.handlers.typing import FollowerTypingHandler
-from synapse.replication.tcp.streams import Stream
-from synapse.streams.events import EventSources
-
-class HomeServer(object):
-    @property
-    def config(self) -> synapse.config.homeserver.HomeServerConfig:
-        pass
-    @property
-    def hostname(self) -> str:
-        pass
-    def get_auth(self) -> synapse.api.auth.Auth:
-        pass
-    def get_auth_handler(self) -> synapse.handlers.auth.AuthHandler:
-        pass
-    def get_datastore(self) -> synapse.storage.DataStore:
-        pass
-    def get_device_handler(self) -> synapse.handlers.device.DeviceHandler:
-        pass
-    def get_e2e_keys_handler(self) -> synapse.handlers.e2e_keys.E2eKeysHandler:
-        pass
-    def get_handlers(self) -> synapse.handlers.Handlers:
-        pass
-    def get_state_handler(self) -> synapse.state.StateHandler:
-        pass
-    def get_state_resolution_handler(self) -> synapse.state.StateResolutionHandler:
-        pass
-    def get_simple_http_client(self) -> synapse.http.client.SimpleHttpClient:
-        """Fetch an HTTP client implementation which doesn't do any blacklisting
-        or support any HTTP_PROXY settings"""
-        pass
-    def get_proxied_http_client(self) -> synapse.http.client.SimpleHttpClient:
-        """Fetch an HTTP client implementation which doesn't do any blacklisting
-        but does support HTTP_PROXY settings"""
-        pass
-    def get_deactivate_account_handler(
-        self,
-    ) -> synapse.handlers.deactivate_account.DeactivateAccountHandler:
-        pass
-    def get_room_creation_handler(self) -> synapse.handlers.room.RoomCreationHandler:
-        pass
-    def get_room_member_handler(self) -> synapse.handlers.room_member.RoomMemberHandler:
-        pass
-    def get_room_shutdown_handler(self) -> synapse.handlers.room.RoomShutdownHandler:
-        pass
-    def get_event_creation_handler(
-        self,
-    ) -> synapse.handlers.message.EventCreationHandler:
-        pass
-    def get_set_password_handler(
-        self,
-    ) -> synapse.handlers.set_password.SetPasswordHandler:
-        pass
-    def get_federation_sender(self) -> synapse.federation.sender.FederationSender:
-        pass
-    def get_federation_transport_client(
-        self,
-    ) -> synapse.federation.transport.client.TransportLayerClient:
-        pass
-    def get_media_repository_resource(
-        self,
-    ) -> synapse.rest.media.v1.media_repository.MediaRepositoryResource:
-        pass
-    def get_media_repository(
-        self,
-    ) -> synapse.rest.media.v1.media_repository.MediaRepository:
-        pass
-    def get_server_notices_manager(
-        self,
-    ) -> synapse.server_notices.server_notices_manager.ServerNoticesManager:
-        pass
-    def get_server_notices_sender(
-        self,
-    ) -> synapse.server_notices.server_notices_sender.ServerNoticesSender:
-        pass
-    def get_notifier(self) -> synapse.notifier.Notifier:
-        pass
-    def get_presence_handler(self) -> synapse.handlers.presence.BasePresenceHandler:
-        pass
-    def get_clock(self) -> synapse.util.Clock:
-        pass
-    def get_reactor(self) -> twisted.internet.base.ReactorBase:
-        pass
-    def get_keyring(self) -> synapse.crypto.keyring.Keyring:
-        pass
-    def get_tcp_replication(
-        self,
-    ) -> synapse.replication.tcp.handler.ReplicationCommandHandler:
-        pass
-    def get_replication_data_handler(
-        self,
-    ) -> synapse.replication.tcp.client.ReplicationDataHandler:
-        pass
-    def get_federation_registry(
-        self,
-    ) -> synapse.federation.federation_server.FederationHandlerRegistry:
-        pass
-    def is_mine_id(self, domain_id: str) -> bool:
-        pass
-    def get_instance_id(self) -> str:
-        pass
-    def get_instance_name(self) -> str:
-        pass
-    def get_event_builder_factory(self) -> EventBuilderFactory:
-        pass
-    def get_storage(self) -> synapse.storage.Storage:
-        pass
-    def get_registration_handler(self) -> synapse.handlers.register.RegistrationHandler:
-        pass
-    def get_macaroon_generator(self) -> synapse.handlers.auth.MacaroonGenerator:
-        pass
-    def get_pusherpool(self) -> synapse.push.pusherpool.PusherPool:
-        pass
-    def get_replication_streams(self) -> Dict[str, Stream]:
-        pass
-    def get_http_client(
-        self,
-    ) -> synapse.http.matrixfederationclient.MatrixFederationHttpClient:
-        pass
-    def should_send_federation(self) -> bool:
-        pass
-    def get_typing_handler(self) -> FollowerTypingHandler:
-        pass
-    def get_event_sources(self) -> EventSources:
-        pass
-    def get_application_service_handler(self):
-        return ApplicationServicesHandler(self)
diff --git a/synapse/storage/databases/__init__.py b/synapse/storage/databases/__init__.py
index b163eebf39..4406e58273 100644
--- a/synapse/storage/databases/__init__.py
+++ b/synapse/storage/databases/__init__.py
@@ -38,9 +38,9 @@ class Databases(object):
         # store.
 
         self.databases = []
-        self.main = None
-        self.state = None
-        self.persist_events = None
+        main = None
+        state = None
+        persist_events = None
 
         for database_config in hs.config.database.databases:
             db_name = database_config.name
@@ -61,27 +61,25 @@ class Databases(object):
 
                     # Sanity check we don't try and configure the main store on
                     # multiple databases.
-                    if self.main:
+                    if main:
                         raise Exception("'main' data store already configured")
 
-                    self.main = main_store_class(database, db_conn, hs)
+                    main = main_store_class(database, db_conn, hs)
 
                     # If we're on a process that can persist events also
                     # instantiate a `PersistEventsStore`
                     if hs.config.worker.writers.events == hs.get_instance_name():
-                        self.persist_events = PersistEventsStore(
-                            hs, database, self.main
-                        )
+                        persist_events = PersistEventsStore(hs, database, main)
 
                 if "state" in database_config.databases:
                     logger.info("Starting 'state' data store")
 
                     # Sanity check we don't try and configure the state store on
                     # multiple databases.
-                    if self.state:
+                    if state:
                         raise Exception("'state' data store already configured")
 
-                    self.state = StateGroupDataStore(database, db_conn, hs)
+                    state = StateGroupDataStore(database, db_conn, hs)
 
                 db_conn.commit()
 
@@ -90,8 +88,14 @@ class Databases(object):
                 logger.info("Database %r prepared", db_name)
 
         # Sanity check that we have actually configured all the required stores.
-        if not self.main:
+        if not main:
             raise Exception("No 'main' data store configured")
 
-        if not self.state:
+        if not state:
             raise Exception("No 'main' data store configured")
+
+        # We use local variables here to ensure that the databases do not have
+        # optional types.
+        self.main = main
+        self.state = state
+        self.persist_events = persist_events
diff --git a/tox.ini b/tox.ini
index 54f28990ce..217590edef 100644
--- a/tox.ini
+++ b/tox.ini
@@ -203,6 +203,7 @@ commands = mypy \
             synapse/push/push_rule_evaluator.py \
             synapse/replication \
             synapse/rest \
+            synapse/server.py \
             synapse/server_notices \
             synapse/spam_checker_api \
             synapse/storage/databases/main/ui_auth.py \