Ignore the reserved bit in PUSH_PROMISE identifiers

CHANGELOG.rst

@@ -19,7 +19,8 @@ dev
 
 **Bugfixes**
 
--
+- Mask the reserved bit in PUSH_PROMISE stream identifiers when parsing and
+  serializing.
 
 6.1.0 (2025-01-22)
 ------------------

src/hyperframe/frame.py

@@ -515,7 +515,7 @@ def _body_repr(self) -> str:
     def serialize_body(self) -> bytes:
         padding_data = self.serialize_padding_data()
         padding = b"\0" * self.pad_length
-        data = _STRUCT_L.pack(self.promised_stream_id)
+        data = _STRUCT_L.pack(self.promised_stream_id & 0x7FFFFFFF)
         return b"".join([padding_data, data, self.data, padding])
 
     def parse_body(self, data: memoryview) -> None:
@@ -529,6 +529,9 @@ def parse_body(self, data: memoryview) -> None:
             msg = "Invalid PUSH_PROMISE body"
             raise InvalidFrameError(msg) from err
 
+        # Ignore the reserved bit, RFC 9113, Section 6.6.
+        self.promised_stream_id &= 0x7FFFFFFF
+
         self.data = (
             data[padding_data_length + 4:len(data)-self.pad_length].tobytes()
         )

tests/test_reserved_promise.py

@@ -0,0 +1,31 @@
+import struct
+
+import pytest
+
+from hyperframe.exceptions import InvalidDataError
+from hyperframe.frame import PushPromiseFrame
+
+
+@pytest.mark.parametrize("stream_id", [2, 0x7FFFFFFE])
+@pytest.mark.parametrize("padded", [False, True])
+def test_parse_promised_stream_reserved_bit(stream_id, padded):
+    frame = PushPromiseFrame(1)
+    body = struct.pack("!L", stream_id | 0x80000000) + b"headers"
+    if padded:
+        frame.flags.add("PADDED")
+        body = b"\x01" + body + b"\x00"
+    frame.parse_body(memoryview(body))
+    assert frame.promised_stream_id == stream_id
+    assert frame.data == b"headers"
+
+
+def test_reserved_bit_does_not_make_zero_stream_valid():
+    frame = PushPromiseFrame(1)
+    with pytest.raises(InvalidDataError):
+        frame.parse_body(memoryview(b"\x80\x00\x00\x00"))
+
+
+@pytest.mark.parametrize("stream_id", [2, 0x7FFFFFFE])
+def test_serialize_promised_stream_reserved_bit(stream_id):
+    frame = PushPromiseFrame(1, promised_stream_id=stream_id | 0x80000000)
+    assert frame.serialize_body() == struct.pack("!L", stream_id)