Reject a truncated iNES header instead of parsing past its end

polyfile/nes.py

@@ -18,6 +18,8 @@ def parse_ines_header(header, parent=None):
     magic = header[:4]
     if magic != b'NES\x1A':
         raise InvalidMatch(f"Expected \"NES\\x1A\" magic at the beginning of the file, but found {magic!r}")
+    if len(header) < 16:
+        raise InvalidMatch(f"Expected a 16 byte iNES header, but the file only contains {len(header)} bytes")
     parent = Submatch(
         name='iNESHeader',
         match_obj=header,

tests/unit/test_nes.py

@@ -0,0 +1,36 @@
+from unittest import TestCase
+
+from polyfile.nes import parse_ines_header
+from polyfile.polyfile import InvalidMatch, Match, Matcher
+
+
+VALID_HEADER = b"NES\x1a" + bytes([1, 0, 0, 0]) + bytes(8)
+
+
+class INESHeaderTest(TestCase):
+    def _root(self) -> Match:
+        return Match(name="root", match_obj=b"", relative_offset=0, matcher=Matcher())
+
+    def test_rejects_bad_magic(self):
+        with self.assertRaises(InvalidMatch):
+            list(parse_ines_header(b"NOPE" + bytes(12), self._root()))
+
+    def test_rejects_truncated_header(self):
+        """A header shorter than 16 bytes is not a usable iNES header.
+
+        Lengths 4 to 10 used to raise IndexError, which the parser dispatcher
+        catches as an unexpected error, logs, and then drops the rest of the
+        submatches. Lengths 11 to 15 raised nothing at all and produced an
+        iNESHeader claiming to be 16 bytes with a short UnusedPadding.
+        """
+        for length in range(4, 16):
+            with self.subTest(length=length):
+                with self.assertRaises(InvalidMatch):
+                    list(parse_ines_header(VALID_HEADER[:length], self._root()))
+
+    def test_accepts_full_header(self):
+        names = [m.name for m in parse_ines_header(VALID_HEADER, self._root())]
+
+        self.assertEqual(names[0], "iNESHeader")
+        for expected in ("iNESMagic", "PRGSize", "CHRSize", "UnusedPadding"):
+            self.assertIn(expected, names)