GH-83065: Fix import deadlock by implementing hierarchical module locking #137196
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…le locking Fix a deadlock in the import system that occurs when concurrent threads import modules at different levels of the same package hierarchy while `__init__.py` files have circular imports. The deadlock scenario (correctly analyzed by @emmatyping): - Thread 1 imports `package.subpackage`, which imports `package.subpackage.module` in its `__init__.py` - Thread 2 imports `package.subpackage.module`, which needs to ensure `package.subpackage` is loaded first - Each thread holds a lock the other needs, causing deadlock The fix introduces _HierarchicalLockManager that acquires all necessary module locks in a consistent order (parent before child) for nested modules. This ensures all threads acquire locks in the same order, preventing circular wait conditions. 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <noreply@anthropic.com>
gpshead
commented
Jul 29, 2025
| # Only acquire lock if module is not already fully loaded | ||
| module = sys.modules.get(module_name) | ||
| if (module is None or | ||
| getattr(getattr(module, "__spec__", None), "_initializing", False)): |
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is this the right way to do this?
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In _get_module_lock a sentinel object is the default for sys.modules.get(), so that might be better. Though I think using None as a sentinel should be fine?
| def _get_module_chain(self, name): | ||
| """Get all modules in the hierarchy from root to leaf. | ||
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| For example: 'a.b.c' -> ['a', 'a.b', 'a.b.c'] |
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do we already have a function to do this? also - it's a function and does not need to be a method.
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I don't feel qualified to say this PR is correct, as import logic has a lot of nasty reentrant behavior I haven't dove into, but the PR does look right, based on what I imagined an implementation of a solution might be. |
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Summary
Fix a deadlock in the import system that occurs when concurrent threads import modules at different levels of the same package hierarchy while
__init__.pyfiles have circular imports.The Problem
As correctly analyzed by @emmatyping in the issue, the deadlock occurs when:
package.subpackage, which importspackage.subpackage.modulein its__init__.pypackage.subpackage.module, which needs to ensurepackage.subpackageis loaded first_DeadlockErrorThe Solution
This PR introduces
_HierarchicalLockManagerthat acquires all necessary module locks in a consistent order (parent modules before child modules) for nested module imports. This ensures all threads acquire locks in the same hierarchical order, preventing the circular wait condition that causes deadlocks.Key Changes:
_HierarchicalLockManagerclass inLib/importlib/_bootstrap.py_find_and_load()to use hierarchical locking for nested modules (those containing '.')test_hierarchical_import_deadlockto prevent regression(human edit: Please take Claude's performance claims with a grain of salt here... it made some of its own but we need to run real ones.)
Performance Impact
Benchmarking shows minimal performance impact:
Test Plan
Fixes #83065
🤖 Generated with Claude Code