From e7254fb79178af74d5ca58851be0075094c23d3d Mon Sep 17 00:00:00 2001
From: Ryan Duguid <152749594+ryanduguid@users.noreply.github.com>
Date: Thu, 8 Oct 2026 09:10:09 +1100
Subject: [PATCH] Cover frame header field boundaries and invalid sizes

---
 tests/test_frame_header.py | 55 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)
 create mode 100644 tests/test_frame_header.py

diff --git a/tests/test_frame_header.py b/tests/test_frame_header.py
new file mode 100644
index 0000000..207bf47
--- /dev/null
+++ b/tests/test_frame_header.py
@@ -0,0 +1,55 @@
+import pytest
+
+from hyperframe.exceptions import InvalidFrameError, UnknownFrameError
+from hyperframe.frame import DataFrame, ExtensionFrame, Frame
+
+
+@pytest.mark.parametrize('length', [0, 0xFF, 0x100, 0xFFFF, 0x10000, 0xFFFFFF])
+def test_frame_header_length_boundaries(length):
+    header = length.to_bytes(3, 'big') + b'\x00\x00\x00\x00\x00\x01'
+    frame, parsed_length = Frame.parse_frame_header(memoryview(header))
+
+    assert isinstance(frame, DataFrame)
+    assert parsed_length == length
+    assert frame.stream_id == 1
+
+
+@pytest.mark.parametrize('stream_id', [1, 0xFF, 0x100, 0xFFFF, 0x10000, 0x7FFFFFFF])
+@pytest.mark.parametrize('reserved', [0, 0x80000000])
+def test_frame_header_preserves_stream_id_bits(stream_id, reserved):
+    header = b'\x00' * 5 + (stream_id | reserved).to_bytes(4, 'big')
+    frame, length = Frame.parse_frame_header(memoryview(header))
+
+    assert frame.stream_id == stream_id
+    assert length == 0
+
+
+@pytest.mark.parametrize('strict', [False, True])
+@pytest.mark.parametrize('frame_type,length,flags', [
+    (0x0B, 0, 0x80),
+    (0x80, 0x10000, 0xFF),
+    (0xFF, 0xFFFFFF, 0x7F),
+])
+def test_unknown_frame_header_boundaries(frame_type, length, flags, strict):
+    header = length.to_bytes(3, 'big') + bytes([frame_type, flags]) + b'\x00\x00\x00\x01'
+
+    if strict:
+        with pytest.raises(UnknownFrameError) as excinfo:
+            Frame.parse_frame_header(memoryview(header), strict=True)
+        assert excinfo.value.frame_type == frame_type
+        assert excinfo.value.length == length
+        return
+
+    frame, parsed_length = Frame.parse_frame_header(memoryview(header))
+    assert isinstance(frame, ExtensionFrame)
+    assert frame.type == frame_type
+    assert frame.flag_byte == flags
+    assert parsed_length == length
+    assert frame.stream_id == 1
+
+
+@pytest.mark.parametrize('header_length', [*range(9), 10, 18])
+def test_frame_header_requires_exactly_nine_bytes(header_length):
+    header = b'\x00' * 8 + b'\x01' + b'\x00' * 9
+    with pytest.raises(InvalidFrameError):
+        Frame.parse_frame_header(memoryview(header)[:header_length])