Indicate what endpoint came back with a JSON response we were unable to parse

changelog.d/14097.misc

@@ -0,0 +1 @@
+Indicate what endpoint came back with a JSON response we were unable to parse.

synapse/http/servlet.py

@@ -35,6 +35,7 @@
 from twisted.web.server import Request
 
 from synapse.api.errors import Codes, SynapseError
+from synapse.http import redact_uri
 from synapse.http.server import HttpServer
 from synapse.types import JsonDict, RoomAlias, RoomID
 from synapse.util import json_decoder
@@ -664,7 +665,13 @@ def parse_json_value_from_request(
     try:
         content = json_decoder.decode(content_bytes.decode("utf-8"))
     except Exception as e:
-        logger.warning("Unable to parse JSON: %s (%s)", e, content_bytes)
+        logger.warning(
+            "Unable to parse JSON from %s %s response: %s (%s)",
+            request.method.decode("ascii", errors="replace"),
+            redact_uri(request.uri.decode("ascii", errors="replace")),
+            e,
+            content_bytes,
+        )
         raise SynapseError(
             HTTPStatus.BAD_REQUEST, "Content not JSON.", errcode=Codes.NOT_JSON
         )

tests/http/test_servlet.py

@@ -35,11 +35,13 @@
 
 def make_request(content):
     """Make an object that acts enough like a request."""
-    request = Mock(spec=["content"])
+    request = Mock(spec=["method", "uri", "content"])
 
     if isinstance(content, dict):
         content = json.dumps(content).encode("utf8")
 
+    request.method = bytes("STUB_METHOD", "ascii")
+    request.uri = bytes("/test_stub_uri", "ascii")
     request.content = BytesIO(content)
     return request