Handle remote download responses with `UNKNOWN_LENGTH` more gracefully (#17439)

Prior to this PR, remote downloads which did not provide a
`content-length` were decremented from the remote download ratelimiter
at the max allowable size, leading to excessive ratelimiting - see
https://github.com/element-hq/synapse/issues/17394.

This PR adds a linearizer to limit concurrent remote downloads to 6 per
IP address, and decrements remote downloads without a `content-length`
from the ratelimiter *after* the download is complete and the response
length is known.

Also adds logic to ensure that responses with a known length respect the
`max_download_size`.

1 files changed, 45 insertions, 5 deletions

diff --git a/tests/rest/client/test_media.py b/tests/rest/client/test_media.py
index 7f2caed7d5..466c5a0b70 100644
--- a/tests/rest/client/test_media.py
+++ b/tests/rest/client/test_media.py
@@ -1809,13 +1809,19 @@ class RemoteDownloadLimiterTestCase(unittest.HomeserverTestCase):
         )
         assert channel.code == 200
 
+    @override_config(
+        {
+            "remote_media_download_burst_count": "87M",
+        }
+    )
     @patch(
         "synapse.http.matrixfederationclient.read_multipart_response",
         read_multipart_response_30MiB,
     )
-    def test_download_ratelimit_max_size_sub(self) -> None:
+    def test_download_ratelimit_unknown_length(self) -> None:
         """
-        Test that if no content-length is provided, the default max size is applied instead
+        Test that if no content-length is provided, ratelimiting is still applied after
+        media is downloaded and length is known
         """
 
         # mock out actually sending the request
@@ -1831,8 +1837,9 @@ class RemoteDownloadLimiterTestCase(unittest.HomeserverTestCase):
 
         self.client._send_request = _send_request  # type: ignore
 
-        # ten requests should go through using the max size (500MB/50MB)
-        for i in range(10):
+        # first 3 will go through (note that 3rd request technically violates rate limit but
+        # that since the ratelimiting is applied *after* download it goes through, but next one fails)
+        for i in range(3):
             channel2 = self.make_request(
                 "GET",
                 f"/_matrix/client/v1/media/download/remote.org/abc{i}",
@@ -1841,7 +1848,7 @@ class RemoteDownloadLimiterTestCase(unittest.HomeserverTestCase):
             )
             assert channel2.code == 200
 
-        # eleventh will hit ratelimit
+        # 4th will hit ratelimit
         channel3 = self.make_request(
             "GET",
             "/_matrix/client/v1/media/download/remote.org/abcd",
@@ -1850,6 +1857,39 @@ class RemoteDownloadLimiterTestCase(unittest.HomeserverTestCase):
         )
         assert channel3.code == 429
 
+    @override_config({"max_upload_size": "29M"})
+    @patch(
+        "synapse.http.matrixfederationclient.read_multipart_response",
+        read_multipart_response_30MiB,
+    )
+    def test_max_download_respected(self) -> None:
+        """
+        Test that the max download size is enforced - note that max download size is determined
+        by the max_upload_size
+        """
+
+        # mock out actually sending the request, returns a 30MiB response
+        async def _send_request(*args: Any, **kwargs: Any) -> IResponse:
+            resp = MagicMock(spec=IResponse)
+            resp.code = 200
+            resp.length = 31457280
+            resp.headers = Headers(
+                {"Content-Type": ["multipart/mixed; boundary=gc0p4Jq0M2Yt08jU534c0p"]}
+            )
+            resp.phrase = b"OK"
+            return resp
+
+        self.client._send_request = _send_request  # type: ignore
+
+        channel = self.make_request(
+            "GET",
+            "/_matrix/client/v1/media/download/remote.org/abcd",
+            shorthand=False,
+            access_token=self.tok,
+        )
+        assert channel.code == 502
+        assert channel.json_body["errcode"] == "M_TOO_LARGE"
+
     def test_file_download(self) -> None:
         content = io.BytesIO(b"file_to_stream")
         content_uri = self.get_success(