Merge "Fix race condition caused by fd reused for DnsResolver"

import android.annotation.Nullable;

import android.os.CancellationSignal;

import android.os.Looper;

import android.os.MessageQueue;

import android.system.ErrnoException;

import android.util.Log;

    private void registerFDListener(@NonNull Executor executor,

            @NonNull FileDescriptor queryfd, @NonNull Callback<? super byte[]> answerCallback,

            @Nullable CancellationSignal cancellationSignal, @NonNull Object lock) {

        Looper.getMainLooper().getQueue().addOnFileDescriptorEventListener(

        final MessageQueue mainThreadMessageQueue = Looper.getMainLooper().getQueue();

        mainThreadMessageQueue.addOnFileDescriptorEventListener(

                queryfd,

                FD_EVENTS,

                (fd, events) -> {

                    // b/134310704

                    // Unregister fd event listener before resNetworkResult is called to prevent

                    // race condition caused by fd reused.

                    // For example when querying v4 and v6, it's possible that the first query ends

                    // and the fd is closed before the second request starts, which might return

                    // the same fd for the second request. By that time, the looper must have

                    // unregistered the fd, otherwise another event listener can't be registered.

                    mainThreadMessageQueue.removeOnFileDescriptorEventListener(fd);

                    executor.execute(() -> {

                        DnsResponse resp = null;

                        ErrnoException exception = null;

                        }

                        answerCallback.onAnswer(resp.answerbuf, resp.rcode);

                    });

                    // Unregister this fd listener

                    // The file descriptor has already been unregistered, so it does not really

                    // matter what is returned here. In spirit 0 (meaning "unregister this FD")

                    // is still the closest to what the looper needs to do. When returning 0,

                    // Looper knows to ignore the fd if it has already been unregistered.

                    return 0;

                });

    }