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Gregory P. Smith
7159b152a0
Since gh-83065 (3.15.0), the first import of pkg.mod kept pkg's module lock until pkg/mod.py had finished executing. Threads that needed only the finished pkg stayed blocked for that long, and concurrent first imports could raise _DeadlockError or a spurious "partially initialized module" ImportError, or hang, with no import cycle involved. Parent packages are now imported, or waited for if another thread is initializing them, before the submodule's module lock is taken, so only that lock is held while the submodule executes. The lock ordering deadlock fixed by gh-83065 stays fixed. importlib.import_module() now returns the partially initialized module in a circular import split across threads, as the import statement already did, instead of raising _DeadlockError. That was long-standing behavior rather than part of the 3.15.0 regression.