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Channels and synchronization - Contracts and edge cases

Platform spec article

Channels and synchronization - Contracts and edge cases

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Runtime must mirror corelib Result semantics at the builtin boundary (no panic for closed/cancelled channel ops).

Duplicate Close is safe. Receive on closed empty queue returns Closed. Send after Close returns Closed.

Hub.WaitReceive with zero registrations returns HubError::Empty. Hub uses round-robin readiness scan (normative).

Multiple fibers may Receive on the same channel; each message goes to one receiver (FIFO).

Cancellation wakes parked fibers with Cancelled after OnCancelled on the child fiber.

Pointer-payload channels (Phase B)

Section titled “Pointer-payload channels (Phase B)”

The runtime exposes pointer-payload channel ops at the ABI: channel_send_ptr, channel_try_send_ptr, channel_receive_ptr, channel_try_receive_ptr. These ops share the existing ChannelId table and the same FIFO semantics described above; only the payload moves a GC-managed pointer instead of a raw i64.

Implementations must:

  • Register the in-flight pointer as an external GC handle before pushing the handle id onto the internal queue, and drop the registration on both the receiver side and on any error path. The pointer is reachable to the GC for the full duration it sits in the queue.
  • Apply the Dijkstra insertion write barrier (gc_write_barrier) on both the send and the receive sides. The barrier is correct regardless of whether the runtime is in Phase A or Phase B; in Phase A it is effectively a no-op.
  • Return Closed when no active mutator is registered, rather than leaking the external handle.
  • When called from an OS thread that is not a Beskid fiber (a Phase B mutator registered via attach_phase_b_mutator), back-pressure on a bounded queue must not call into the fiber scheduler. The runtime instead falls back to try_send / try_receive polling with std::thread::yield_now.