Support MSC3916 by adding a federation /thumbnail endpoint and authenticated `_matrix/client/v1/media/thumbnail` endpoint (#17388)

[MSC3916](https://github.com/matrix-org/matrix-spec-proposals/pull/3916)
added the endpoints `_matrix/federation/v1/media/thumbnail` and the
authenticated `_matrix/client/v1/media/thumbnail`.

This PR implements those endpoints, along with stabilizing
`_matrix/client/v1/media/config` and
`_matrix/client/v1/media/preview_url`.

Complement tests are at
https://github.com/matrix-org/complement/pull/728

2 files changed, 70 insertions, 23 deletions

diff --git a/synapse/media/media_repository.py b/synapse/media/media_repository.py
index 542642b900..87c929eb20 100644
--- a/synapse/media/media_repository.py
+++ b/synapse/media/media_repository.py
@@ -542,7 +542,12 @@ class MediaRepository:
             respond_404(request)
 
     async def get_remote_media_info(
-        self, server_name: str, media_id: str, max_timeout_ms: int, ip_address: str
+        self,
+        server_name: str,
+        media_id: str,
+        max_timeout_ms: int,
+        ip_address: str,
+        use_federation: bool,
     ) -> RemoteMedia:
         """Gets the media info associated with the remote file, downloading
         if necessary.
@@ -553,6 +558,8 @@ class MediaRepository:
             max_timeout_ms: the maximum number of milliseconds to wait for the
                 media to be uploaded.
             ip_address: IP address of the requester
+            use_federation: if a download is necessary, whether to request the remote file
+                over the federation `/download` endpoint
 
         Returns:
             The media info of the file
@@ -573,7 +580,7 @@ class MediaRepository:
                 max_timeout_ms,
                 self.download_ratelimiter,
                 ip_address,
-                False,
+                use_federation,
             )
 
         # Ensure we actually use the responder so that it releases resources
diff --git a/synapse/media/thumbnailer.py b/synapse/media/thumbnailer.py
index f8a9560784..413a720e40 100644
--- a/synapse/media/thumbnailer.py
+++ b/synapse/media/thumbnailer.py
@@ -36,9 +36,11 @@ from synapse.media._base import (
     ThumbnailInfo,
     respond_404,
     respond_with_file,
+    respond_with_multipart_responder,
     respond_with_responder,
 )
-from synapse.media.media_storage import MediaStorage
+from synapse.media.media_storage import FileResponder, MediaStorage
+from synapse.storage.databases.main.media_repository import LocalMedia
 
 if TYPE_CHECKING:
     from synapse.media.media_repository import MediaRepository
@@ -271,6 +273,7 @@ class ThumbnailProvider:
         method: str,
         m_type: str,
         max_timeout_ms: int,
+        for_federation: bool,
     ) -> None:
         media_info = await self.media_repo.get_local_media_info(
             request, media_id, max_timeout_ms
@@ -290,6 +293,8 @@ class ThumbnailProvider:
             media_id,
             url_cache=bool(media_info.url_cache),
             server_name=None,
+            for_federation=for_federation,
+            media_info=media_info,
         )
 
     async def select_or_generate_local_thumbnail(
@@ -301,6 +306,7 @@ class ThumbnailProvider:
         desired_method: str,
         desired_type: str,
         max_timeout_ms: int,
+        for_federation: bool,
     ) -> None:
         media_info = await self.media_repo.get_local_media_info(
             request, media_id, max_timeout_ms
@@ -326,10 +332,16 @@ class ThumbnailProvider:
 
                 responder = await self.media_storage.fetch_media(file_info)
                 if responder:
-                    await respond_with_responder(
-                        request, responder, info.type, info.length
-                    )
-                    return
+                    if for_federation:
+                        await respond_with_multipart_responder(
+                            self.hs.get_clock(), request, responder, media_info
+                        )
+                        return
+                    else:
+                        await respond_with_responder(
+                            request, responder, info.type, info.length
+                        )
+                        return
 
         logger.debug("We don't have a thumbnail of that size. Generating")
 
@@ -344,7 +356,15 @@ class ThumbnailProvider:
         )
 
         if file_path:
-            await respond_with_file(request, desired_type, file_path)
+            if for_federation:
+                await respond_with_multipart_responder(
+                    self.hs.get_clock(),
+                    request,
+                    FileResponder(open(file_path, "rb")),
+                    media_info,
+                )
+            else:
+                await respond_with_file(request, desired_type, file_path)
         else:
             logger.warning("Failed to generate thumbnail")
             raise SynapseError(400, "Failed to generate thumbnail.")
@@ -360,9 +380,10 @@ class ThumbnailProvider:
         desired_type: str,
         max_timeout_ms: int,
         ip_address: str,
+        use_federation: bool,
     ) -> None:
         media_info = await self.media_repo.get_remote_media_info(
-            server_name, media_id, max_timeout_ms, ip_address
+            server_name, media_id, max_timeout_ms, ip_address, use_federation
         )
         if not media_info:
             respond_404(request)
@@ -424,12 +445,13 @@ class ThumbnailProvider:
         m_type: str,
         max_timeout_ms: int,
         ip_address: str,
+        use_federation: bool,
     ) -> None:
         # TODO: Don't download the whole remote file
         # We should proxy the thumbnail from the remote server instead of
         # downloading the remote file and generating our own thumbnails.
         media_info = await self.media_repo.get_remote_media_info(
-            server_name, media_id, max_timeout_ms, ip_address
+            server_name, media_id, max_timeout_ms, ip_address, use_federation
         )
         if not media_info:
             return
@@ -448,6 +470,7 @@ class ThumbnailProvider:
             media_info.filesystem_id,
             url_cache=False,
             server_name=server_name,
+            for_federation=False,
         )
 
     async def _select_and_respond_with_thumbnail(
@@ -461,7 +484,9 @@ class ThumbnailProvider:
         media_id: str,
         file_id: str,
         url_cache: bool,
+        for_federation: bool,
         server_name: Optional[str] = None,
+        media_info: Optional[LocalMedia] = None,
     ) -> None:
         """
         Respond to a request with an appropriate thumbnail from the previously generated thumbnails.
@@ -476,6 +501,8 @@ class ThumbnailProvider:
             file_id: The ID of the media that a thumbnail is being requested for.
             url_cache: True if this is from a URL cache.
             server_name: The server name, if this is a remote thumbnail.
+            for_federation: whether the request is from the federation /thumbnail request
+            media_info: metadata about the media being requested.
         """
         logger.debug(
             "_select_and_respond_with_thumbnail: media_id=%s desired=%sx%s (%s) thumbnail_infos=%s",
@@ -511,13 +538,20 @@ class ThumbnailProvider:
 
             responder = await self.media_storage.fetch_media(file_info)
             if responder:
-                await respond_with_responder(
-                    request,
-                    responder,
-                    file_info.thumbnail.type,
-                    file_info.thumbnail.length,
-                )
-                return
+                if for_federation:
+                    assert media_info is not None
+                    await respond_with_multipart_responder(
+                        self.hs.get_clock(), request, responder, media_info
+                    )
+                    return
+                else:
+                    await respond_with_responder(
+                        request,
+                        responder,
+                        file_info.thumbnail.type,
+                        file_info.thumbnail.length,
+                    )
+                    return
 
             # If we can't find the thumbnail we regenerate it. This can happen
             # if e.g. we've deleted the thumbnails but still have the original
@@ -558,12 +592,18 @@ class ThumbnailProvider:
                 )
 
             responder = await self.media_storage.fetch_media(file_info)
-            await respond_with_responder(
-                request,
-                responder,
-                file_info.thumbnail.type,
-                file_info.thumbnail.length,
-            )
+            if for_federation:
+                assert media_info is not None
+                await respond_with_multipart_responder(
+                    self.hs.get_clock(), request, responder, media_info
+                )
+            else:
+                await respond_with_responder(
+                    request,
+                    responder,
+                    file_info.thumbnail.type,
+                    file_info.thumbnail.length,
+                )
         else:
             # This might be because:
             # 1. We can't create thumbnails for the given media (corrupted or