[issue30891] Sometimes, test_concurrency() of test_import fails with AttributeError: module 'package' has no attribute 'submodule' on AMD64 Windows8.1 Refleaks 3.6

[Nick Coghlan]

Nick Coghlan added the comment:

No, there's no global lock on the import system these days.

Instead, imports are supposed to be atomic on a per-module basis, but issue 30814 showed that some of the accesses to the shared state management (which *is* still supposed to be protected by the import lock) could get into trouble given concurrent imports from different threads.

However, the new test exercises an interesting sequence of per-module lock acquisition, since each thread effectively runs through this sequence:

- import package # Test case
- import package.submodule # Package init
- import package # Parent package import

That moves back-and-forth between C & Python in a few different places, and hence may have been relying on the GIL to keep the parts executed in C atomic.

So now I'm wondering whether or not this new problem might be due to moving the _imp.acquire_lock() calls into Python code as part of issue 30814, as that creates a window near the start of _find_and_load() where the thread may end up holding neither the GIL *nor* the import lock. By contrast, when the import lock was acquired in C, then by the time the Python code starts running (potentially releasing the GIL), the import lock has already been acquired.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30891>
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