concurrent

fn concurrent(
    userdata: ?*anyopaque,
    result_len: usize,
    result_alignment: Alignment,
    context: []const u8,
    context_alignment: Alignment,
    start: *const fn (context: *const anyopaque, result: *anyopaque) void,
) Io.ConcurrentError!*std.Io.AnyFuture {
    assert(result_alignment.compare(.lte, Fiber.max_result_align)); // TODO
    assert(context_alignment.compare(.lte, Fiber.max_context_align)); // TODO
    assert(result_len <= Fiber.max_result_size); // TODO
    assert(context.len <= Fiber.max_context_size); // TODO

    const ev: *Evented = @ptrCast(@alignCast(userdata));
    const fiber = Fiber.create(ev) catch |err| switch (err) {
        error.OutOfMemory => return error.ConcurrencyUnavailable,
    };

    const closure: *AsyncClosure = .fromFiber(fiber);
    fiber.* = .{
        .required_align = {},
        .evented = ev,
        .context = switch (builtin.cpu.arch) {
            .aarch64 => .{
                .sp = @intFromPtr(closure),
                .fp = 0,
                .pc = @intFromPtr(&AsyncClosure.entry),
            },
            .x86_64 => .{
                .rsp = @intFromPtr(closure) - 8,
                .rbp = 0,
                .rip = @intFromPtr(&AsyncClosure.entry),
            },
            else => |arch| @compileError("unimplemented architecture: " ++ @tagName(arch)),
        },
        .link = .{ .awaiter = null },
        .awaiting_group = undefined,
        .cancel_status = .unrequested,
        .cancel_protection = .unblocked,
    };
    closure.* = .{
        .evented = ev,
        .fiber = fiber,
        .start = start,
        .result_align = result_alignment,
    };
    @memcpy(closure.contextPointer(), context);

    ev.queue.async(fiber, &Fiber.@"resume");
    return @ptrCast(fiber);
}