bpo-27286 fixed a problem where BUILD_MAP_UNPACK_WITH_CALL could
be emitted with an incorrect oparg value, causing the eval loop
to access the wrong stack entry when attempting to read the
function name.
The associated magic number change caused significant problems when
attempting to upgrade to 3.5.3 for anyone that relies on pre-cached
bytecode remaining valid across maintenance releases.
This patch restores the ability to import legacy bytecode generated
by 3.5.0, 3.5.1 or 3.5.2, and modifies the eval loop to
avoid any harmful consequences from the potentially malformed legacy
bytecode.
Original import patch by Petr Viktorin, eval loop patch by Serhiy Storchaka,
and tests and integration by Nick Coghlan.
Known limitations of the current implementation:
- documentation changes are incomplete
- there's a reference leak I haven't tracked down yet
The leak is most visible by running:
./python -m test -R3:3 test_importlib
However, you can also see it by running:
./python -X showrefcount
Importing the array or _testmultiphase modules, and
then deleting them from both sys.modules and the local
namespace shows significant increases in the total
number of active references each cycle. By contrast,
with _testcapi (which continues to use single-phase
initialisation) the global refcounts stabilise after
a couple of cycles.
