ActiveSeamSlot

A self-healing single-active-session slot for a Loom/factory that hosts one live Seam at a time.

In plain terms: some fabrics — Apple Multipeer, a raw radio link — can only drive one session per device. Such a factory must reject a second weave while a session is live, yet become reusable the instant that session dies, without the caller having to force-quit or manually reset anything. ActiveSeamSlot is that guard, packaged once.

Torn-aware guard (the structural fix)

The naive version of this guard is check(slot == null) plus a hand-wired onTerminated callback that the seam fires on terminal death to clear the slot. That side-channel is a standing obligation every future single-session fabric can forget — and a fabric that forgets it wedges: the slot stays occupied forever, so the factory can never be reused.

ActiveSeamSlot removes the obligation. A claim succeeds when the slot is empty or its current occupant has latched SeamState.Torn — the next occupy consults the seam's own terminal state rather than a side-channel. Because a seam latches Torn on self-driven death (a remote drop, a failed join) as well as on an explicit close(), the slot heals on the next claim with no per-fabric wiring. A fabric that latches Torn but does no explicit slot release is still correctly reusable. (Mirrors MuxClientLoom, which treats a Torn base seam as free.)

Ordering — null the slot before native teardown

grabAndRelease atomically clears the slot and returns the previous occupant so the caller can run native teardown (disconnect() / mc_session_close) after the slot is already empty. A re-entrant terminal callback that races the teardown therefore sees an empty (or already-moved-on) slot and short-circuits.

Thread-safety

All slot state is guarded by an atomicfu reentrantLock — correct under a genuinely multi-threaded dispatcher, not single-thread confinement. occupy performs the guard check and the install as one atomic operation, so two concurrent claims can never both succeed. The build lambda runs under the lock; it must not suspend (fabric session construction is synchronous native work, never a coroutine suspension point).

Parameters