Apply a timeout to reading the body when fetching a file. (#11784)

This prevents the URL preview code from reading
a stream forever.

changelog.d/11784.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug that media streams could cause long-lived connections when generating URL previews.

synapse/http/client.py

@@ -731,15 +731,24 @@ async def get_file(
         # straight back in again
 
         try:
-            length = await make_deferred_yieldable(
-                read_body_with_max_size(response, output_stream, max_size)
-            )
+            d = read_body_with_max_size(response, output_stream, max_size)
+
+            # Ensure that the body is not read forever.
+            d = timeout_deferred(d, 30, self.hs.get_reactor())
+
+            length = await make_deferred_yieldable(d)
         except BodyExceededMaxSize:
             raise SynapseError(
                 HTTPStatus.BAD_GATEWAY,
                 "Requested file is too large > %r bytes" % (max_size,),
                 Codes.TOO_LARGE,
             )
+        except defer.TimeoutError:
+            raise SynapseError(
+                HTTPStatus.BAD_GATEWAY,
+                "Requested file took too long to download",
+                Codes.TOO_LARGE,
+            )
         except Exception as e:
             raise SynapseError(
                 HTTPStatus.BAD_GATEWAY, ("Failed to download remote body: %s" % e)