Support MSC3916 by adding a federation /download endpoint

synapse/federation/transport/server/__init__.py

@@ -19,6 +19,7 @@
 # [This file includes modifications made by New Vector Limited]
 #
 #
+import inspect
 import logging
 from typing import TYPE_CHECKING, Dict, Iterable, List, Optional, Tuple, Type
 
@@ -323,6 +324,21 @@ def register_servlets(
                 ):
                     continue
 
+                # don't load the endpoint if the storage provider is incompatible
+                media_repo = hs.get_media_repository()
+                load_download_endpoint = True
+                for provider in media_repo.media_storage.storage_providers:
+                    signature = inspect.signature(provider.backend.fetch)
+                    if "federation" not in signature.parameters:
+                        logger.warning(
+                            "Federation `/download` enpoint will not be enabled as your storage "
+                            "provider is not compatible with this endpoint."
+                        )
+                        load_download_endpoint = False
+
+                if not load_download_endpoint:
+                    continue
+
             servletclass(
                 hs=hs,
                 authenticator=authenticator,