processSpawn

fn processSpawn(userdata: ?*anyopaque, options: process.SpawnOptions) process.SpawnError!process.Child {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    const spawned = try ev.spawn(options);
    var cancel_region: CancelRegion = .initBlocked();
    defer cancel_region.deinit();
    defer ev.closeAsync(spawned.err_fd);

    // Wait for the child to report any errors in or before `execvpe`.
    var child_err: ForkBailError = undefined;
    ev.readAll(&cancel_region, spawned.err_fd, @ptrCast(&child_err)) catch |read_err| {
        switch (read_err) {
            error.Canceled => unreachable, // blocked
            error.EndOfStream => {
                // Write end closed by CLOEXEC at the time of the `execvpe` call,
                // indicating success.
            },
            else => {
                // Problem reading the error from the error reporting pipe. We
                // don't know if the child is alive or dead. Better to assume it is
                // alive so the resource does not risk being leaked.
            },
        }
        return .{
            .id = spawned.pid,
            .thread_handle = {},
            .stdin = spawned.stdin,
            .stdout = spawned.stdout,
            .stderr = spawned.stderr,
            .request_resource_usage_statistics = options.request_resource_usage_statistics,
        };
    };
    return child_err;
}