[3.6] bpo-29514: Check magic number for bugfix releases (#2157)

Add a test to check the current MAGIC_NUMBER against the
expected number for the release if the current release is
at candidate or final level. On test failure, describe to
the developer the procedure for changing the magic number.

This ensures that pre-merge CI will automatically pick up
on magic number changes in maintenance releases (and
explain why those are problematic), rather than relying on
all core developers to be aware of the implications of
such changes.

Author: ncoghlan

Lib/test/test_importlib/test_util.py

@@ -4,6 +4,7 @@
 machinery = util.import_importlib('importlib.machinery')
 importlib_util = util.import_importlib('importlib.util')
 
+import importlib.util
 import os
 import pathlib
 import string
@@ -757,5 +758,48 @@ def test_source_from_cache_path_like_arg(self):
  ) = util.test_both(PEP3147Tests, util=importlib_util)
 
 
+class MagicNumberTests(unittest.TestCase):
+    """
+    Test release compatibility issues relating to importlib
+    """
+    @unittest.skipUnless(
+        sys.version_info.releaselevel in ('final', 'release'),
+        'only applies to candidate or final python release levels'
+    )
+    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, the value of the constant
+        EXPECTED_MAGIC_NUMBER in this test should be changed.
+        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 python-dev. 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.
+        """
+        EXPECTED_MAGIC_NUMBER = 3379
+        actual = int.from_bytes(importlib.util.MAGIC_NUMBER[:2], 'little')
+
+        msg = (
+            "To avoid breaking backwards compatibility with cached bytecode "
+            "files that can't be automatically regenerated by the current "
+            "user, candidate and final releases require the current  "
+            "importlib.util.MAGIC_NUMBER to match the expected "
+            "magic number in this test. Set the expected "
+            "magic number in this test to the current MAGIC_NUMBER to "
+            "continue with the release.\n\n"
+            "Changing the MAGIC_NUMBER for a maintenance release "
+            "requires discussion in python-dev and notification of "
+            "community stakeholders."
+        )
+        self.assertEqual(EXPECTED_MAGIC_NUMBER, actual, msg)
+
+
 if __name__ == '__main__':
     unittest.main()