Fix check_pickle to seek to file start and use correct opcode count API (#253)

check_pickle() had three bugs:
  - No file.seek(0), so results depended on file position left by prior
    calls (_is_zipfile, tarfile.is_tarfile)
  - Used .opcodes() which is a @Property, not a method — calling it always
    raised TypeError, forcing every call through the StackedPickle fallback
  - StackedPickle path returned True unconditionally without min_length check

Fix by seeking to start, using len(Pickled.load(file)) instead of
len(Pickled.load(file).opcodes()), and validating StackedPickle results.

Also fix test_legacy_pickle_properties to test the actual legacy pickle
file instead of duplicating test_v1_3_properties. This test would fail
without the check_pickle fix.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: thomas-chauchefoin-tob

fickling/polyglot.py

@@ -143,13 +143,15 @@ def check_numpy(file):  # returns isNumpy,isNumpyPickle
 
 def check_pickle(file, min_length=0):
     """Checks if a file can be pickled; this does not directly determine the file is a pickle"""
+    file.seek(0)
     try:
-        opcodes = Pickled.load(file).opcodes()
-        return len(opcodes) > min_length
+        p = Pickled.load(file)
+        return len(p) > min_length
     except Exception:  # noqa
+        file.seek(0)
         try:
-            StackedPickle.load(file)
-            return True
+            sp = StackedPickle.load(file)
+            return any(len(p) > min_length for p in sp.pickled)
         except Exception:  # noqa
             return False
 

test/test_polyglot.py

@@ -196,10 +196,8 @@ def test_v1_3_properties(self):
         self.assertEqual(properties, proper_result)
 
     def test_legacy_pickle_properties(self):
-        properties = polyglot.find_file_properties(self.filename_v1_3)
-        proper_result = _make_properties(
-            is_torch_zip=True, is_standard_zip=True, has_data_pkl=True, has_version=True
-        )
+        properties = polyglot.find_file_properties(self.filename_legacy_pickle)
+        proper_result = _make_properties(is_valid_pickle=True)
         self.assertEqual(properties, proper_result)
 
     @unittest.skipIf(_lacks_torch_jit_support, "PyTorch 2.9.1 JIT broken with Python 3.14+")