Module shared_condvar 

SharedCondvar: cross-process condition variable built on top of CrossProcessWaker.

Classic Mesa-style condvar interface: waiters check a user-owned predicate, park if not satisfied, and resume when a notifier advances the predicate AND calls notify_*. The substrate uses a monotonic generation counter so each wait parks at target = current_gen + 1; every notify_* bumps the generation and fires wake_(one_)up_to(new_gen), which wakes parked waiters whose target <= new_gen.

Cross-process semantics

Two processes mmap the same condvar base; both call wait / notify_* directly. On Linux the wake call crosses the process boundary via SHARED futex (keyed by inode + offset, so two different mmaps of the same file page DO match). On Windows / macOS the primitive runs intra-process via WaitOnAddress / spin fallback.

Intra-process sharing: use Arc::clone, NOT create+open

Within ONE process, share a single SharedCondvar through Arc<SharedCondvar> + Arc::clone. Calling create and then open on the same path in the same process produces two independent mmaps with different virtual-address ranges aliased to the same file pages. Windows WaitOnAddress is keyed by virtual address, so a notify_* on the second handle does NOT reach a wait on the first handle - the wake hashtable lookup misses on the differing virtual address. Linux SHARED futex keys by the underlying file page, which works across separate mmaps, but the rule “use one Arc<SharedCondvar> per process” is cross-platform safe.

The open constructor is exclusively for joiners in SEPARATE processes that need to find the file the creator already initialised.

Predicate ownership

The condvar does NOT own the predicate atom; the caller passes a closure that returns the current predicate value. This matches parking_lot::Condvar::wait_while semantics and lets the same condvar guard predicates held in any cross-process atom (SharedAtomicU32, a field in a SharedCell, an offset into an MMF struct, etc.).

Structs

SharedCondvar

Cross-process condition variable. Mesa-style: callers re-check the predicate after each wake. Internally backed by one CrossProcessWaker plus a generation counter in mmap.

Enums

CondvarError

Errors returned by SharedCondvar operations.