Fix flaky Windows write-lock: retry on PermissionError#36
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_file_lock only treated FileExistsError as a contended lock. On Windows, os.open(O_CREAT|O_EXCL) raises PermissionError when the lock file already exists or is being unlinked by another holder, so under concurrent writes the lock would propagate that error instead of retrying — making test_files_core flaky on Windows CI. Catch PermissionError alongside FileExistsError in the acquire loop (the standard portable file-lock pattern) and add a regression test that forces a transient PermissionError on the exclusive open.
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_file_lock only treated FileExistsError as a contended lock. On Windows, os.open(O_CREAT|O_EXCL) raises PermissionError when the lock file already exists or is being unlinked by another holder, so under concurrent writes the lock would propagate that error instead of retrying — making test_files_core flaky on Windows CI.
Catch PermissionError alongside FileExistsError in the acquire loop (the standard portable file-lock pattern) and add a regression test that forces a transient PermissionError on the exclusive open.