For the auto-pickle checksum, allow MD5 again since it was used by Cython 0.x...

For the auto-pickle checksum, allow MD5 again since it was used by Cython 0.x pickles. Otherwise switch to SHA-256 since that should be safe enough to stay available in Python's hashlib module for another while.

Closes https://github.com/cython/cython/issues/4680

Cython/Compiler/ParseTreeTransforms.py

@@ -2036,7 +2036,21 @@ if VALUE is not None:
                     e.type.create_to_py_utility_code(env)
                     e.type.create_from_py_utility_code(env)
             all_members_names = sorted([e.name for e in all_members])
-            checksum = '0x%s' % hashlib.sha1(' '.join(all_members_names).encode('utf-8')).hexdigest()[:7]
+
+            # Cython 0.x used MD5 for the checksum, which a few Python installations remove for security reasons.
+            # SHA-256 should be ok for years to come, but early Cython 3.0 alpha releases used SHA-1,
+            # which may not be.
+            checksum_algos = [hashlib.sha256, hashlib.sha1]
+            try:
+                checksum_algos.append(hashlib.md5)
+            except AttributeError:
+                pass
+
+            member_names_string = ' '.join(all_members_names).encode('utf-8')
+            checksums = [
+                '0x' + mkchecksum(member_names_string).hexdigest()[:7]
+                for mkchecksum in checksum_algos
+            ]
             unpickle_func_name = '__pyx_unpickle_%s' % node.punycode_class_name
 
             # TODO(robertwb): Move the state into the third argument
@@ -2045,9 +2059,9 @@ if VALUE is not None:
                 def %(unpickle_func_name)s(__pyx_type, long __pyx_checksum, __pyx_state):
                     cdef object __pyx_PickleError
                     cdef object __pyx_result
-                    if __pyx_checksum != %(checksum)s:
+                    if __pyx_checksum not in %(checksums)s:
                         from pickle import PickleError as __pyx_PickleError
-                        raise __pyx_PickleError, "Incompatible checksums (%%s vs %(checksum)s = (%(members)s))" %% __pyx_checksum
+                        raise __pyx_PickleError, "Incompatible checksums (%%s vs %(checksums)s = (%(members)s))" %% __pyx_checksum
                     __pyx_result = %(class_name)s.__new__(__pyx_type)
                     if __pyx_state is not None:
                         %(unpickle_func_name)s__set_state(<%(class_name)s> __pyx_result, __pyx_state)
@@ -2059,7 +2073,7 @@ if VALUE is not None:
                         __pyx_result.__dict__.update(__pyx_state[%(num_members)d])
                 """ % {
                     'unpickle_func_name': unpickle_func_name,
-                    'checksum': checksum,
+                    'checksums': "(%s)" % ', '.join(checksums),
                     'members': ', '.join(all_members_names),
                     'class_name': node.class_name,
                     'assignments': '; '.join(
@@ -2092,7 +2106,7 @@ if VALUE is not None:
                     %(unpickle_func_name)s__set_state(self, __pyx_state)
                 """ % {
                     'unpickle_func_name': unpickle_func_name,
-                    'checksum': checksum,
+                    'checksum': checksums[0],
                     'members': ', '.join('self.%s' % v for v in all_members_names) + (',' if len(all_members_names) == 1 else ''),
                     # Even better, we could check PyType_IS_GC.
                     'any_notnone_members' : ' or '.join(['self.%s is not None' % e.name for e in all_members if e.type.is_pyobject] or ['False']),

tests/run/reduce_pickle.pyx

@@ -62,6 +62,9 @@ cdef class B:
     def __reduce__(self):
         return makeObj, (type(self), {'x': self.x, 'y': self.y})
 
+    def __eq__(self, other):
+        return isinstance(other, B) and (<B>other).x == self.x and (<B>other).y == self.y
+
 def makeObj(obj_type, kwds):
     return obj_type(**kwds)
 
@@ -304,3 +307,24 @@ if sys.version_info[:2] >= (3, 5):
         """
         def my_method(self, x):
             return x
+
+
+# Pickled with Cython 0.29.28 (using MD5 for the checksum).
+OLD_MD5_PICKLE = b'''\
+\x80\x04\x95:\x00\x00\x00\x00\x00\x00\x00\x8c\rreduce_pickle\
+\x94\x8c\x07makeObj\x94\x93\x94h\x00\x8c\x01B\x94\x93\x94}\
+\x94(\x8c\x01x\x94K%\x8c\x01y\x94M\x85\x01u\x86\x94R\x94.\
+'''
+
+try:
+    from hashlib import md5
+except ImportError:
+    pass
+else:
+    def unpickle_old_0_29_28():
+        """
+        >>> import pickle
+        >>> b = pickle.loads(OLD_MD5_PICKLE)
+        >>> b == B(x=37, y=389) or b
+        True
+        """