Allow bigger responses to `/federation/v1/state` (#12877)

* Refactor HTTP response size limits

Rather than passing a separate `max_response_size` down the stack, make it an
attribute of the `parser`.

* Allow bigger responses on `federation/v1/state`

`/state` can return huge responses, so we need to handle that.

4 files changed, 19 insertions, 32 deletions

diff --git a/changelog.d/12877.bugfix b/changelog.d/12877.bugfix
new file mode 100644
index 0000000000..1ecf448baf
--- /dev/null
+++ b/changelog.d/12877.bugfix
@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.54 which could sometimes cause exceptions when handling federated traffic.
diff --git a/synapse/federation/transport/client.py b/synapse/federation/transport/client.py
index 9ce06dfa28..25df1905c6 100644
--- a/synapse/federation/transport/client.py
+++ b/synapse/federation/transport/client.py
@@ -49,11 +49,6 @@ from synapse.types import JsonDict
 
 logger = logging.getLogger(__name__)
 
-# Send join responses can be huge, so we set a separate limit here. The response
-# is parsed in a streaming manner, which helps alleviate the issue of memory
-# usage a bit.
-MAX_RESPONSE_SIZE_SEND_JOIN = 500 * 1024 * 1024
-
 
 class TransportLayerClient:
     """Sends federation HTTP requests to other servers"""
@@ -349,7 +344,6 @@ class TransportLayerClient:
             path=path,
             data=content,
             parser=SendJoinParser(room_version, v1_api=True),
-            max_response_size=MAX_RESPONSE_SIZE_SEND_JOIN,
         )
 
     async def send_join_v2(
@@ -372,7 +366,6 @@ class TransportLayerClient:
             args=query_params,
             data=content,
             parser=SendJoinParser(room_version, v1_api=False),
-            max_response_size=MAX_RESPONSE_SIZE_SEND_JOIN,
         )
 
     async def send_leave_v1(
@@ -1360,6 +1353,11 @@ class SendJoinParser(ByteParser[SendJoinResponse]):
 
     CONTENT_TYPE = "application/json"
 
+    # /send_join responses can be huge, so we override the size limit here. The response
+    # is parsed in a streaming manner, which helps alleviate the issue of memory
+    # usage a bit.
+    MAX_RESPONSE_SIZE = 500 * 1024 * 1024
+
     def __init__(self, room_version: RoomVersion, v1_api: bool):
         self._response = SendJoinResponse([], [], event_dict={})
         self._room_version = room_version
@@ -1427,6 +1425,9 @@ class _StateParser(ByteParser[StateRequestResponse]):
 
     CONTENT_TYPE = "application/json"
 
+    # As with /send_join, /state responses can be huge.
+    MAX_RESPONSE_SIZE = 500 * 1024 * 1024
+
     def __init__(self, room_version: RoomVersion):
         self._response = StateRequestResponse([], [])
         self._room_version = room_version
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index 0b9475debd..db44721ef5 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -92,9 +92,6 @@ incoming_responses_counter = Counter(
     "synapse_http_matrixfederationclient_responses", "", ["method", "code"]
 )
 
-# a federation response can be rather large (eg a big state_ids is 50M or so), so we
-# need a generous limit here.
-MAX_RESPONSE_SIZE = 100 * 1024 * 1024
 
 MAX_LONG_RETRIES = 10
 MAX_SHORT_RETRIES = 3
@@ -116,6 +113,11 @@ class ByteParser(ByteWriteable, Generic[T], abc.ABC):
     the content type doesn't match we fail the request.
     """
 
+    # a federation response can be rather large (eg a big state_ids is 50M or so), so we
+    # need a generous limit here.
+    MAX_RESPONSE_SIZE: int = 100 * 1024 * 1024
+    """The largest response this parser will accept."""
+
     @abc.abstractmethod
     def finish(self) -> T:
         """Called when response has finished streaming and the parser should
@@ -203,7 +205,6 @@ async def _handle_response(
     response: IResponse,
     start_ms: int,
     parser: ByteParser[T],
-    max_response_size: Optional[int] = None,
 ) -> T:
     """
     Reads the body of a response with a timeout and sends it to a parser
@@ -215,15 +216,12 @@ async def _handle_response(
         response: response to the request
         start_ms: Timestamp when request was made
         parser: The parser for the response
-        max_response_size: The maximum size to read from the response, if None
-            uses the default.
 
     Returns:
         The parsed response
     """
 
-    if max_response_size is None:
-        max_response_size = MAX_RESPONSE_SIZE
+    max_response_size = parser.MAX_RESPONSE_SIZE
 
     try:
         check_content_type_is(response.headers, parser.CONTENT_TYPE)
@@ -240,7 +238,7 @@ async def _handle_response(
             "{%s} [%s] JSON response exceeded max size %i - %s %s",
             request.txn_id,
             request.destination,
-            MAX_RESPONSE_SIZE,
+            max_response_size,
             request.method,
             request.uri.decode("ascii"),
         )
@@ -772,7 +770,6 @@ class MatrixFederationHttpClient:
         backoff_on_404: bool = False,
         try_trailing_slash_on_400: bool = False,
         parser: Literal[None] = None,
-        max_response_size: Optional[int] = None,
     ) -> Union[JsonDict, list]:
         ...
 
@@ -790,7 +787,6 @@ class MatrixFederationHttpClient:
         backoff_on_404: bool = False,
         try_trailing_slash_on_400: bool = False,
         parser: Optional[ByteParser[T]] = None,
-        max_response_size: Optional[int] = None,
     ) -> T:
         ...
 
@@ -807,7 +803,6 @@ class MatrixFederationHttpClient:
         backoff_on_404: bool = False,
         try_trailing_slash_on_400: bool = False,
         parser: Optional[ByteParser] = None,
-        max_response_size: Optional[int] = None,
     ):
         """Sends the specified json data using PUT
 
@@ -843,8 +838,6 @@ class MatrixFederationHttpClient:
                 enabled.
             parser: The parser to use to decode the response. Defaults to
                 parsing as JSON.
-            max_response_size: The maximum size to read from the response, if None
-                uses the default.
 
         Returns:
             Succeeds when we get a 2xx HTTP response. The
@@ -895,7 +888,6 @@ class MatrixFederationHttpClient:
             response,
             start_ms,
             parser=parser,
-            max_response_size=max_response_size,
         )
 
         return body
@@ -984,7 +976,6 @@ class MatrixFederationHttpClient:
         ignore_backoff: bool = False,
         try_trailing_slash_on_400: bool = False,
         parser: Literal[None] = None,
-        max_response_size: Optional[int] = None,
     ) -> Union[JsonDict, list]:
         ...
 
@@ -999,7 +990,6 @@ class MatrixFederationHttpClient:
         ignore_backoff: bool = ...,
         try_trailing_slash_on_400: bool = ...,
         parser: ByteParser[T] = ...,
-        max_response_size: Optional[int] = ...,
     ) -> T:
         ...
 
@@ -1013,7 +1003,6 @@ class MatrixFederationHttpClient:
         ignore_backoff: bool = False,
         try_trailing_slash_on_400: bool = False,
         parser: Optional[ByteParser] = None,
-        max_response_size: Optional[int] = None,
     ):
         """GETs some json from the given host homeserver and path
 
@@ -1043,9 +1032,6 @@ class MatrixFederationHttpClient:
             parser: The parser to use to decode the response. Defaults to
                 parsing as JSON.
 
-            max_response_size: The maximum size to read from the response. If None,
-                uses the default.
-
         Returns:
             Succeeds when we get a 2xx HTTP response. The
             result will be the decoded JSON body.
@@ -1090,7 +1076,6 @@ class MatrixFederationHttpClient:
             response,
             start_ms,
             parser=parser,
-            max_response_size=max_response_size,
         )
 
         return body
diff --git a/tests/http/test_fedclient.py b/tests/http/test_fedclient.py
index 638babae69..006dbab093 100644
--- a/tests/http/test_fedclient.py
+++ b/tests/http/test_fedclient.py
@@ -26,7 +26,7 @@ from twisted.web.http import HTTPChannel
 
 from synapse.api.errors import RequestSendFailed
 from synapse.http.matrixfederationclient import (
-    MAX_RESPONSE_SIZE,
+    JsonParser,
     MatrixFederationHttpClient,
     MatrixFederationRequest,
 )
@@ -609,9 +609,9 @@ class FederationClientTests(HomeserverTestCase):
         while not test_d.called:
             protocol.dataReceived(b"a" * chunk_size)
             sent += chunk_size
-            self.assertLessEqual(sent, MAX_RESPONSE_SIZE)
+            self.assertLessEqual(sent, JsonParser.MAX_RESPONSE_SIZE)
 
-        self.assertEqual(sent, MAX_RESPONSE_SIZE)
+        self.assertEqual(sent, JsonParser.MAX_RESPONSE_SIZE)
 
         f = self.failureResultOf(test_d)
         self.assertIsInstance(f.value, RequestSendFailed)