val index = serialTaskQueue.indexOfFirst { it.isReady() }
    if (index == -1) return null

    val nextTask = serialTaskQueue.removeAt(index)
    currentTask = nextTask
    contextSwitchCount++
    nextTask.start()
    return nextTask
  }

  private interface SerialTask {
    /** Returns true if this task is ready to start. */
    fun isReady() = true

  private lateinit var socket: BufferedSocket
  private var connection: RealConnection? = null

  /** True if this connection is ready for use, including TCP, tunnels, and TLS. */
  override val isReady: Boolean
    get() = protocol != null

  private fun copy(
    attempt: Int = this.attempt,
    tunnelRequest: Request? = this.tunnelRequest,
    connectionSpecIndex: Int = this.connectionSpecIndex,

                while (true) {
                    final int read = fis.read(b[i]);
                    synchronized (w) {
                        w.checkException();
                        while (!w.isReady()) {
                            try {
                                w.wait();
                            } catch (final InterruptedException ie) {

            @Override
            public void write(int b) throws IOException {
                outputStream.write(b);
            }

            @Override
            public boolean isReady() {
                return true;
            }

            @Override
            public void setWriteListener(WriteListener listener) {
            }
        };

        synchronized (w) {
            long deadline = System.currentTimeMillis() + 2000;
            while (!w.isReady() && System.currentTimeMillis() < deadline) {
                w.wait(10);
            }
            assertTrue(w.isReady(), "WriterThread should be ready to accept work");

            // Act: submit a write
            w.write(payload, 3, out);
        }

        doExtensiveHealthChecks = doExtensiveHealthChecks,
        address = address,
        call = call,
        routes = routes,
        requireMultiplexed = planToReplace != null && planToReplace.isReady,
      ) ?: return null

    // If we coalesced our connection, remember the replaced connection's route. That way if the
    // coalesced connection later fails we don't waste a valid route.