processSpawnPosix

fn processSpawnPosix(userdata: ?*anyopaque, options: process.SpawnOptions) process.SpawnError!process.Child {
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    const spawned = try spawnPosix(t, options);
    defer closeFd(spawned.err_fd);

    // Wait for the child to report any errors in or before `execvpe`.
    if (readIntFd(spawned.err_fd)) |child_err_int| {
        const child_err: process.SpawnError = @errorCast(@errorFromInt(child_err_int));
        return child_err;
    } else |read_err| switch (read_err) {
        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,
    };
}