Fix setting identifier truncation in SettingsFrame.serialize_body

CHANGELOG.rst

@@ -4,6 +4,10 @@ Release History
 dev
 ---
 
+**API Changes (Backward Compatible)**
+
+- Setting Identifier are now correctly serialized as 16-bit values, instead of 8-bit.
+
 **API Changes (Backward Incompatible)**
 
 -

src/hyperframe/frame.py

@@ -457,8 +457,11 @@ def _body_repr(self) -> str:
         return f"settings={self.settings}"
 
     def serialize_body(self) -> bytes:
-        return b"".join([_STRUCT_HL.pack(setting & 0xFF, value)
-                         for setting, value in self.settings.items()])
+        return b"".join([_STRUCT_HL.pack(
+            setting_identifier & 0xFFFF, # Settings identifiers are 16 bits, mask them appropriately, RFC 9113, Section 6.5.1
+            setting_value & 0xFFFFFFFF,  # Settings values are 32 bits, mask them appropriately, RFC 9113, Section 6.5.1
+        )
+                         for setting_identifier, setting_value in self.settings.items()])
 
     def parse_body(self, data: memoryview) -> None:
         if "ACK" in self.flags and len(data) > 0:

tests/test_frames.py

@@ -426,6 +426,29 @@ def test_short_settings_frame_errors(self):
         with pytest.raises(InvalidDataError):
             decode_frame(self.serialized[:-2])
 
+    def test_settings_frame_with_large_setting_ids(self):
+        f = SettingsFrame()
+        f.settings[0xFFFE] = 12345
+        f.settings[0xFFFF] = 54321
+
+        s = f.serialize()
+        new_f = decode_frame(s)
+
+        assert new_f.settings[0xFFFE] == 12345
+        assert new_f.settings[0xFFFF] == 54321
+
+    def test_settings_frame_serializes_with_large_setting_ids(self):
+        f = SettingsFrame()
+        f.settings[0xFFFE] = 12345
+        f.settings[0xFFFF] = 54321
+
+        s = f.serialize()
+        assert s == (
+            b'\x00\x00\x0C\x04\x00\x00\x00\x00\x00' +  # Frame header
+            b'\xFF\xFE\x00\x00\x30\x39' +              # Setting ID 0xFFFE
+            b'\xFF\xFF\x00\x00\xd4\x31'                # Setting ID 0xFFFF
+        )
+
 
 class TestPushPromiseFrame:
     def test_repr(self):