bpo-29514: Check magic number for bugfix release

Lib/importlib/_bootstrap_external.py

@@ -248,6 +248,21 @@ def _write_atomic(path, data, mode=0o666):
 #
 # Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
 # in PC/launcher.c must also be updated.
+#
+# In general, the MAGIC_NUMBER should not change for maintenance releases
+# although this may be required under exceptional circumstances. When
+# each release reaches candidate status, an entry in EXPECTED_MAGIC_NUMBERS
+# should be added for this release. This value is tested against the actual
+# MAGIC_NUMBER to ensure that if a change is required within a minor
+# release, the exception is first discussed with python-dev and relevant
+# community stakeholders such as OS distribution package maintainers
+# are properly informed of the change.
+
+EXPECTED_MAGIC_NUMBERS = {
+    (2, 7): 62211,
+    (3, 5): 3350,
+    (3, 6): 3379
+}
 
 MAGIC_NUMBER = (3390).to_bytes(2, 'little') + b'\r\n'
 _RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c

Lib/importlib/util.py

@@ -4,6 +4,7 @@
 from ._bootstrap import _resolve_name
 from ._bootstrap import spec_from_loader
 from ._bootstrap import _find_spec
+from ._bootstrap_external import EXPECTED_MAGIC_NUMBERS
 from ._bootstrap_external import MAGIC_NUMBER
 from ._bootstrap_external import cache_from_source
 from ._bootstrap_external import decode_source

Lib/test/test_importlib/test_release.py

@@ -0,0 +1,57 @@
+import importlib.util as util
+import sys
+import unittest
+
+
+class ImportModuleReleaseTests(unittest.TestCase):
+    """
+    Test release compatibility issues relating to importlib
+    """
+    def test_magic_number(self):
+        """
+        Each python minor release should generally have a MAGIC_NUMBER
+        that does not change once the release reaches candidate status.
+
+        Once a release reaches candidate status, an entry should be
+        added to EXPECTED_MAGIC_NUMBERS. This test will then check that
+        the actual MAGIC_NUMBER matches the expected value for the
+        release.
+
+        In exceptional cases, it may be required to change the MAGIC_NUMBER
+        for a maintenance release. In this case the change should be
+        discussed in dev-python. If a change is required, community
+        stakeholders such as OS package maintainers must be notified
+        in advance. Such exceptional releases will then require an
+        adjustment to this test case.
+        """
+        if sys.version_info.releaselevel not in ('final', 'candidate'):
+            return
+
+        release = (sys.version_info.major, sys.version_info.minor)
+
+        msg = (
+            "Candidate and final releases require an entry in "
+            "importlib.util.EXPECTED_MAGIC_NUMBERS. Set the expected "
+            "magic number to the current MAGIC_NUMBER to continue "
+            "with the release."
+        )
+        self.assertIn(release, util.EXPECTED_MAGIC_NUMBERS, msg=msg)
+        expected = util.EXPECTED_MAGIC_NUMBERS[release]
+        actual = int.from_bytes(util.MAGIC_NUMBER[:2], 'little')
+
+        # Adjust for exceptional releases
+        if release == (3, 5):
+            expected = 3351 # Changed in 3.5.3 issue 27286
+
+        msg = (
+            "The MAGIC_NUMBER {0} does not match the expected value "
+            "{1} for release {2}. Changing the MAGIC_NUMBER for a "
+            "maintenance release requires discussion in dev-python and "
+            "notification of community stakeholders."
+            .format(actual, expected, release)
+        )
+        self.assertEqual(expected, actual, msg)
+
+
+if __name__ == '__main__':
+    unittest.main()