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

3 files changed, 65 insertions, 1 deletions

diff --git a/synapse/federation/transport/server/__init__.py b/synapse/federation/transport/server/__init__.py
index c44e5daa47..5f997040d0 100644
--- a/synapse/federation/transport/server/__init__.py
+++ b/synapse/federation/transport/server/__init__.py
@@ -33,6 +33,7 @@ from synapse.federation.transport.server.federation import (
     FEDERATION_SERVLET_CLASSES,
     FederationAccountStatusServlet,
     FederationMediaDownloadServlet,
+    FederationMediaThumbnailServlet,
     FederationUnstableClientKeysClaimServlet,
 )
 from synapse.http.server import HttpServer, JsonResource
@@ -316,7 +317,10 @@ def register_servlets(
             ):
                 continue
 
-            if servletclass == FederationMediaDownloadServlet:
+            if (
+                servletclass == FederationMediaDownloadServlet
+                or servletclass == FederationMediaThumbnailServlet
+            ):
                 if not hs.config.server.enable_media_repo:
                     continue
 
diff --git a/synapse/federation/transport/server/_base.py b/synapse/federation/transport/server/_base.py
index e124481474..9094201da0 100644
--- a/synapse/federation/transport/server/_base.py
+++ b/synapse/federation/transport/server/_base.py
@@ -363,6 +363,8 @@ class BaseFederationServlet:
                             if (
                                 func.__self__.__class__.__name__  # type: ignore
                                 == "FederationMediaDownloadServlet"
+                                or func.__self__.__class__.__name__  # type: ignore
+                                == "FederationMediaThumbnailServlet"
                             ):
                                 response = await func(
                                     origin, content, request, *args, **kwargs
@@ -375,6 +377,8 @@ class BaseFederationServlet:
                         if (
                             func.__self__.__class__.__name__  # type: ignore
                             == "FederationMediaDownloadServlet"
+                            or func.__self__.__class__.__name__  # type: ignore
+                            == "FederationMediaThumbnailServlet"
                         ):
                             response = await func(
                                 origin, content, request, *args, **kwargs
diff --git a/synapse/federation/transport/server/federation.py b/synapse/federation/transport/server/federation.py
index ec957768d4..b075a86f68 100644
--- a/synapse/federation/transport/server/federation.py
+++ b/synapse/federation/transport/server/federation.py
@@ -46,11 +46,13 @@ from synapse.http.servlet import (
     parse_boolean_from_args,
     parse_integer,
     parse_integer_from_args,
+    parse_string,
     parse_string_from_args,
     parse_strings_from_args,
 )
 from synapse.http.site import SynapseRequest
 from synapse.media._base import DEFAULT_MAX_TIMEOUT_MS, MAXIMUM_ALLOWED_MAX_TIMEOUT_MS
+from synapse.media.thumbnailer import ThumbnailProvider
 from synapse.types import JsonDict
 from synapse.util import SYNAPSE_VERSION
 from synapse.util.ratelimitutils import FederationRateLimiter
@@ -826,6 +828,59 @@ class FederationMediaDownloadServlet(BaseFederationServerServlet):
         )
 
 
+class FederationMediaThumbnailServlet(BaseFederationServerServlet):
+    """
+    Implementation of new federation media `/thumbnail` endpoint outlined in MSC3916. Returns
+    a multipart/mixed response consisting of a JSON object and the requested media
+    item. This endpoint only returns local media.
+    """
+
+    PATH = "/media/thumbnail/(?P<media_id>[^/]*)"
+    RATELIMIT = True
+
+    def __init__(
+        self,
+        hs: "HomeServer",
+        ratelimiter: FederationRateLimiter,
+        authenticator: Authenticator,
+        server_name: str,
+    ):
+        super().__init__(hs, authenticator, ratelimiter, server_name)
+        self.media_repo = self.hs.get_media_repository()
+        self.dynamic_thumbnails = hs.config.media.dynamic_thumbnails
+        self.thumbnail_provider = ThumbnailProvider(
+            hs, self.media_repo, self.media_repo.media_storage
+        )
+
+    async def on_GET(
+        self,
+        origin: Optional[str],
+        content: Literal[None],
+        request: SynapseRequest,
+        media_id: str,
+    ) -> None:
+
+        width = parse_integer(request, "width", required=True)
+        height = parse_integer(request, "height", required=True)
+        method = parse_string(request, "method", "scale")
+        # TODO Parse the Accept header to get an prioritised list of thumbnail types.
+        m_type = "image/png"
+        max_timeout_ms = parse_integer(
+            request, "timeout_ms", default=DEFAULT_MAX_TIMEOUT_MS
+        )
+        max_timeout_ms = min(max_timeout_ms, MAXIMUM_ALLOWED_MAX_TIMEOUT_MS)
+
+        if self.dynamic_thumbnails:
+            await self.thumbnail_provider.select_or_generate_local_thumbnail(
+                request, media_id, width, height, method, m_type, max_timeout_ms, True
+            )
+        else:
+            await self.thumbnail_provider.respond_local_thumbnail(
+                request, media_id, width, height, method, m_type, max_timeout_ms, True
+            )
+        self.media_repo.mark_recently_accessed(None, media_id)
+
+
 FEDERATION_SERVLET_CLASSES: Tuple[Type[BaseFederationServlet], ...] = (
     FederationSendServlet,
     FederationEventServlet,
@@ -858,4 +913,5 @@ FEDERATION_SERVLET_CLASSES: Tuple[Type[BaseFederationServlet], ...] = (
     FederationMakeKnockServlet,
     FederationAccountStatusServlet,
     FederationMediaDownloadServlet,
+    FederationMediaThumbnailServlet,
 )