fix: add Windows file locking to SessionIndex - #78
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SessionIndex._locked() only implemented file locking via POSIX fcntl, which is unavailable on Windows (fcntl is None there). This left the documented read-modify-write guarantee completely unenforced on Windows: concurrent session writes silently drop each other's data, and concurrent os.replace() calls in save() can raise PermissionError. Add an msvcrt.locking() fallback for Windows. The lock file's single required byte is written once in __init__, before any concurrent access exists, because Windows enforces this lock as mandatory: a bare read() on the lock file while another thread holds it would itself raise PermissionError. Verified with tests/test_robustness.py::TestSessionIndexDurability (previously lost half of 80 concurrently created sessions; now passes consistently across repeated runs) and the full test suite on Windows.
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Summary
SessionIndex._locked()only implemented file locking via POSIXfcntl, which isNoneon Windows. This silently disabled the locking that the docstring on_synchronizedpromises, so concurrent session writes on Windows can drop each other's data andsave()'sos.replace()can raisePermissionError.msvcrt.locking()fallback for Windows, matching the existingfcntl.flockbehavior on POSIX.__init__(before any concurrent access exists) rather than on each lock acquisition, because Windows enforcesmsvcrtlocks as mandatory — a bareread()on the lock file while another thread holds the lock would itself raisePermissionError.Test plan
tests/test_robustness.py::TestSessionIndexDurability::test_concurrent_creates_do_not_lose_sessions— previously lost roughly half of 80 concurrently created sessions on Windows; now passes consistently across repeated runs.pytest tests/suite on Windows: 182/184 passing (remaining 2 failures are pre-existing, unrelated test-portability issues — a hardcoded/tmppath and a test reading a file without specifying UTF-8 encoding — not touched by this change).