d watcher polls process file descriptors (pidfds) to await child
    process termination. In some respects, PidfdChildWatcher is a "Goldilocks"
    child watcher implementation. It doesn't require signals or threads, doesn't
    interfere with any processes launched outside the event loop, and scales
    linearly with the number of subprocesses launched by the event loop. The
    main disadvantage is that pidfds are specific to Linux, and only work on
    recent (5.3+) kernels.
    c