fix(protocol): raise on a short _read_unpack for every size (#2279) (#2349)

Problem: for a size with no struct format code, _read_unpack ran
int.from_bytes on whatever arrived, so a short read returned a smaller
integer where sizes 1, 2, 4 and 8 raise StructError.

Fix: check the length on the int.from_bytes path and raise the same
StructError without quiet; quiet still returns the partial value.

Test: test_read_unpack_short_read_fails_for_every_size.

Author: JarryShaw

pcapkit/protocols/protocol.py

@@ -1767,8 +1767,7 @@ def _read_unpack(self, size: 'int' = 1, *, signed: 'bool' = False,
 
         Raises:
             StructError: If the buffer is empty, whatever ``quiet`` says, or if
-                unpack (:func:`struct.unpack`) failed, and :exc:`struct.error`
-                raised, without ``quiet``.
+                fewer than ``size`` octets were read, without ``quiet``.
 
         """
         endian = '<' if lilendian else '>'
@@ -1788,6 +1787,10 @@ def _read_unpack(self, size: 'int' = 1, *, signed: 'bool' = False,
             raise StructError('unpack: empty buffer', quiet=True, eof=True)
 
         if kind is None:
+            # A short read fails here as :func:`struct.unpack` makes it fail
+            # for the sizes that have a format code (#2279).
+            if len(mem) < size and not quiet:
+                raise StructError(f'{self.__class__.__name__}: unpack failed')
             end = 'little' if lilendian else 'big'  # type: Literal['little', 'big']
             buf = int.from_bytes(mem, end, signed=signed)
         else:

tests/protocols/test_protocol_base_unit.py

@@ -234,6 +234,25 @@ class Number(enum.IntEnum):
         with self.assertRaises(ProtocolNotImplemented):
             DummyProtocol._make_index('missing')
 
+    def test_read_unpack_short_read_fails_for_every_size(self) -> None:
+        # #2279: a size with no struct format code read a short buffer as a
+        # smaller integer instead of failing as sizes 1, 2, 4 and 8 do.
+        DummyProtocol, _, _ = self._make_protocol_class()
+        from pcapkit.utilities.exceptions import StructError
+
+        proto = DummyProtocol(packet=b'\x01')
+        for size in (2, 3, 4, 5, 8):
+            for signed in (False, True):
+                with self.subTest(size=size, signed=signed):
+                    proto._file = io.BytesIO(bytes(range(1, size)))
+                    with self.assertRaises(StructError) as context:
+                        proto._read_unpack(size, signed=signed)
+                    self.assertFalse(context.exception.eof)
+
+                    proto._file = io.BytesIO(bytes(range(1, size)))
+                    self.assertEqual(proto._read_unpack(size, signed=signed, quiet=True),
+                                     int.from_bytes(bytes(range(1, size)), 'big', signed=signed))
+
     def test_register_analyze_and_next_layer_paths(self) -> None:
         DummyProtocol, DummyData, _ = self._make_protocol_class()
         from pcapkit.corekit.module import ModuleDescriptor