Support MSC3916 by adding a federation /download endpoint

synapse/config/experimental.py

@@ -436,3 +436,7 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         self.msc4115_membership_on_events = experimental.get(
             "msc4115_membership_on_events", False
         )
+
+        self.msc3916_authenticated_media_enabled = experimental.get(
+            "msc3916_authenticated_media_enabled", False
+        )

synapse/federation/transport/server/__init__.py

@@ -316,11 +316,12 @@ def register_servlets(
             ):
                 continue
 
-            if (
-                servletclass == FederationUnstableMediaDownloadServlet
-                and not hs.config.server.enable_media_repo
-            ):
-                continue
+            if servletclass == FederationUnstableMediaDownloadServlet:
+                if (
+                    not hs.config.server.enable_media_repo
+                    or not hs.config.experimental.msc3916_authenticated_media_enabled
+                ):
+                    continue
 
             servletclass(
                 hs=hs,

synapse/media/media_storage.py

@@ -516,27 +516,37 @@ def beginFileTransfer(
         return deferred
 
     def resumeProducing(self) -> None:
+        # write the first field, which will always be a json field
         if not self.json_field_written:
-            self.consumer.write(CRLF + b"--" + self.boundary + b"" + CRLF)
+            self.consumer.write(CRLF + b"--" + self.boundary + CRLF)
+
             content_type = Header(b"Content-Type", b"application/json")
             self.consumer.write(bytes(content_type) + CRLF)
+
             json_field = json.dumps(self.json_field)
             json_bytes = json_field.encode("utf-8")
             self.consumer.write(json_bytes)
-            self.consumer.write(CRLF + b"--" + self.boundary + b"" + CRLF)
+            self.consumer.write(CRLF + b"--" + self.boundary + CRLF)
+
             self.json_field_written = True
+
         chunk: Any = ""
         if self.file:
+            # if we haven't written the content type yet, do so
             if not self.content_type_written:
                 type = self.file_content_type.encode("utf-8")
                 content_type = Header(b"Content-Type", type)
                 self.consumer.write(bytes(content_type) + CRLF)
                 self.content_type_written = True
+
             chunk = self.file.read(self.CHUNK_SIZE)
+
         if not chunk:
+            # we've reached the end of the file
             self.consumer.write(CRLF + b"--" + self.boundary + b"--" + CRLF)
             self.file = None
             self.consumer.unregisterProducer()
+
             if self.deferred:
                 self.deferred.callback(self.lastSent)
                 self.deferred = None

tests/federation/test_federation_media.py

@@ -33,6 +33,7 @@
 
 from tests import unittest
 from tests.test_utils import SMALL_PNG
+from tests.unittest import override_config
 
 
 class FederationUnstableMediaDownloads(unittest.FederatingHomeserverTestCase):
@@ -54,6 +55,9 @@ def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
         )
         self.media_repo = hs.get_media_repository()
 
+    @override_config(
+        {"experimental_features": {"msc3916_authenticated_media_enabled": True}}
+    )
     def test_file_download(self) -> None:
         content = io.BytesIO(b"file_to_stream")
         content_uri = self.get_success(