fix(async): preserve cancellation and add revision-aware job gate

packages/async/src/job_state.zigadded+44-0

Changes to packages/async/src/job_state.zig

		@@ -0,0 +1,44 @@
	1	+//! Shared atomic task transitions. Cancellation cannot be undone by an in-flight
	2	+//! poll. Terminal states are immutable. Storage must outlive the polling owner.
	3	+const std = ;
	4	+pub const State = enum(u8) { Pending, Running, Completed, Failed, Cancelled };
	5	+pub fn terminal(state: State) bool {
	6	+ return switch (state) {
	7	+ .Completed, .Failed, .Cancelled => true,
	8	+ else => false,
	9	+ };
	10	+}
	11	+pub fn cancel(state: *std.atomic.Value(State)) bool {
	12	+ var current = state.load(.acquire);
	13	+ while (!terminal(current)) {
	14	+ current = state.cmpxchgStrong(current, .Cancelled, .acq_rel, .acquire) orelse return true;
	15	+ }
	16	+ return false;
	17	+}
	18	+/// Called only by the poll owner, after publishing any result payload.
	19	+pub fn finishPoll(state: *std.atomic.Value(State), ready: bool) bool {
	20	+ const next: State = if (ready) .Completed else .Pending;
	21	+ if (state.cmpxchgStrong(.Running, next, .release, .monotonic)) |actual| return terminal(actual);
	22	+ return ready;
	23	+}
	24	+test "cancel between poll reservation and completion cannot be overwritten" {
	25	+ for ([_]bool{ false, true }) |ready| {
	26	+ var state = std.atomic.Value(State).init(.Running);
	27	+ try std.testing.expect(cancel(&state));
	28	+ try std.testing.expect(finishPoll(&state, ready));
	29	+ try std.testing.expectEqual(State.Cancelled, state.load(.acquire));
	30	+ }
	31	+}
	32	+test "completion and failure are immutable and pending polls remain resumable" {
	33	+ for ([_]State{ .Completed, .Failed, .Cancelled }) |terminal_state| {
	34	+ var state = std.atomic.Value(State).init(terminal_state);
	35	+ try std.testing.expect(!cancel(&state));
	36	+ try std.testing.expectEqual(terminal_state, state.load(.acquire));
	37	+ }
	38	+ var state = std.atomic.Value(State).init(.Running);
	39	+ try std.testing.expect(!finishPoll(&state, false));
	40	+ try std.testing.expectEqual(State.Pending, state.load(.acquire));
	41	+ state.store(.Running, .release);
	42	+ try std.testing.expect(finishPoll(&state, true));
	43	+ try std.testing.expectEqual(State.Completed, state.load(.acquire));
	44	+}