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[WIP] bpo-31070: Fix importlib _get_module_lock() race #3029

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[WIP] bpo-31070: Fix importlib _get_module_lock() race #3029

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29 changes: 25 additions & 4 deletions Lib/importlib/_bootstrap.py
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Original file line number Diff line number Diff line change
Expand Up @@ -152,29 +152,50 @@ def __exit__(self, *args, **kwargs):
self._lock.release()


def _remove_module_lock(name):
item_lock, lock_wr = _module_locks.pop(name)
item_lock.release()


# The following two functions are for consumption by Python/import.c.

def _get_module_lock(name):
"""Get or create the module lock for a given module name.

Acquire/release internally the global import lock to protect
_module_locks."""

_imp.acquire_lock()
try:
try:
lock = _module_locks[name]()
item_lock, lock_wr = _module_locks[name]
except KeyError:
lock_wr = None

if lock_wr is not None:
lock = lock_wr()

if lock is None:
# Current thread sees that a module lock was created, is now
# destroyed, but its weak reference callback was not called
# yet. Wait until _module_locks[name] is deleted.
item_lock.acquire()
item_lock.release()
else:
lock = None

if lock is None:
if _thread is None:
lock = _DummyModuleLock(name)
item_lock = _DummyModuleLock(name)
else:
lock = _ModuleLock(name)
item_lock = _thread.allocate_lock()
item_lock.acquire()

def cb(_):
del _module_locks[name]
_module_locks[name] = _weakref.ref(lock, cb)
_remove_module_lock(name)
lock_wr = _weakref.ref(lock, cb)
_module_locks[name] = (item_lock, lock_wr)
finally:
_imp.release_lock()

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