Honor no-content semantics for 204 and 304 responses

src/h2/stream.py

@@ -1365,8 +1365,12 @@ def _initialize_content_length(self, headers: Iterable[Header]) -> None:
             return
 
         content_length = None
+        response_status: bytes | None = None
 
         for n, v in headers:
+            if n == b":status":
+                response_status = v
+
             if n == b"content-length":
                 if not v.isdigit():
                     # https://www.rfc-editor.org/rfc/rfc9110.html#name-content-length
@@ -1385,7 +1389,12 @@ def _initialize_content_length(self, headers: Iterable[Header]) -> None:
                     msg = f"Conflicting content-length headers: {content_length} and {parsed_content_length}"
                     raise ProtocolError(msg)
 
-        self._expected_content_length = content_length
+        # These responses have no content, so Content-Length is not the size
+        # of any DATA frames (RFC 9113, § 8.1.1).
+        if response_status in {b"204", b"304"}:
+            self._expected_content_length = 0
+        else:
+            self._expected_content_length = content_length
 
     def _track_content_length(self, length: int, end_stream: bool) -> None:
         """

tests/test_invalid_content_lengths.py

@@ -115,6 +115,59 @@ def test_content_length_with_non_digit_value(self, frame_factory, value) -> None
         )
         assert c.data_to_send() == expected_frame.serialize()
 
+    @pytest.mark.parametrize("status", ["204", "304"])
+    @pytest.mark.parametrize("end_stream_on_headers", [True, False])
+    def test_no_content_response_with_content_length(self, frame_factory, status, end_stream_on_headers) -> None:
+        """Content-Length does not imply a body for a no-content response."""
+        c = h2.connection.H2Connection()
+        c.initiate_connection()
+        c.send_headers(
+            1,
+            [(":method", "GET"), (":scheme", "https"), (":authority", "example.com"), (":path", "/")],
+            end_stream=True,
+        )
+
+        headers = frame_factory.build_headers_frame(
+            headers=[(":status", status), ("content-length", "15")],
+            flags=["END_STREAM"] if end_stream_on_headers else [],
+        )
+        frames = headers.serialize()
+        if not end_stream_on_headers:
+            data = frame_factory.build_data_frame(data=b"", flags=["END_STREAM"])
+            frames += data.serialize()
+
+        events = c.receive_data(frames)
+        assert isinstance(events[0], h2.events.ResponseReceived)
+        assert isinstance(events[-1], h2.events.StreamEnded)
+
+    @pytest.mark.parametrize(
+        "response_headers",
+        [
+            [(":status", "204")],
+            [(":status", "204"), ("content-length", "15")],
+            [(":status", "304"), ("content-length", "15")],
+        ],
+    )
+    def test_no_content_response_rejects_content(self, frame_factory, response_headers) -> None:
+        """204 and 304 responses have no content, regardless of Content-Length."""
+        c = h2.connection.H2Connection()
+        c.initiate_connection()
+        c.send_headers(
+            1,
+            [(":method", "GET"), (":scheme", "https"), (":authority", "example.com"), (":path", "/")],
+            end_stream=True,
+        )
+
+        headers = frame_factory.build_headers_frame(
+            headers=response_headers,
+        )
+        data = frame_factory.build_data_frame(data=b"x", flags=["END_STREAM"])
+        with pytest.raises(h2.exceptions.InvalidBodyLengthError) as exp:
+            c.receive_data(headers.serialize() + data.serialize())
+
+        assert exp.value.expected_length == 0
+        assert exp.value.actual_length == 1
+
     @pytest.mark.parametrize(
         "request_headers",
         [