groupAwait

fn groupAwait(userdata: ?*anyopaque, type_erased: *Io.Group, initial_token: *anyopaque) Io.Cancelable!void {
    _ = initial_token; // we need to load `token` *after* the group finishes
    if (builtin.single_threaded) unreachable; // nothing to await
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    const g: Group = .{ .ptr = type_erased };

    var num_completed: std.atomic.Value(u32) = .init(0);
    g.awaiter().* = &num_completed;

    const pre_await_status = g.status().fetchOr(.{
        .num_running = 0,
        .have_awaiter = true,
        .canceled = false,
    }, .acq_rel); // acquire results if complete; release `g.awaiter()`

    assert(!pre_await_status.have_awaiter);
    assert(!pre_await_status.canceled);
    if (pre_await_status.num_running == 0) {
        // Already done. Since the group is finished, it's illegal to spawn more tasks in it
        // until we return, so we can access `g.status()` non-atomically.
        g.status().raw.have_awaiter = false;
        return;
    }

    while (Thread.futexWait(&num_completed.raw, 0, null)) {
        switch (num_completed.load(.acquire)) { // acquire task results
            0 => continue,
            1 => break,
            else => unreachable, // group was reused before `await` returned
        }
    } else |err| switch (err) {
        error.Canceled => {
            const pre_cancel_status = g.status().fetchOr(.{
                .num_running = 0,
                .have_awaiter = false,
                .canceled = true,
            }, .acq_rel); // acquire results if complete; release `g.awaiter()`
            assert(pre_cancel_status.have_awaiter);
            assert(!pre_cancel_status.canceled);

            // Even if `pre_cancel_status.num_running == 0`, we still need to wait for the signal,
            // because in that case the last member of the group is already trying to modify it.
            // However, if we know everything is done, we *can* skip signaling blocked threads.
            const skip_signals = pre_cancel_status.num_running == 0;
            g.waitForCancelWithSignaling(t, &num_completed, skip_signals);

            // The group is finished, so it's illegal to spawn more tasks in it until we return, so
            // we can access `g.status()` non-atomically.
            g.status().raw.canceled = false;
            g.status().raw.have_awaiter = false;
            return error.Canceled;
        },
    }

    // The group is finished, so it's illegal to spawn more tasks in it until we return, so
    // we can access `g.status()` non-atomically.
    g.status().raw.have_awaiter = false;
}