[REG 5.5 -> 5.6] QAbstractSocket::socketDescriptor() is no longer set while in ConnectingState

Since https://codereview.qt-project.org/102610 (Qt 5.5 change), socketDescriptor() returns -1 while in ConnectingState.

This makes it impossible to implement a non-blocking non-event-loop-based connect (basically, polling).

The application code that I'm looking at, and which worked in Qt3, Qt4 and Qt < 5.5, is calling connectToHost() followed by socketDescriptor() and then - possibly multiple times, if the function is called again - it calls select() to check for writability (with a 1ms timeout since 0 wouldn't work).

In pseudo code,

// Caller can call this multiple times, for polling (from a thread without an event loop)
bool MySocket::connect(const QHostAddress & addr, quint16 port)
{
    if (state() != ConnectingState) {
        QTcpSocket::connectToHost(addr, port);
    }
    if (state() == ConnectedState) {
        return true;
    }
    int socket_fd = socketDescriptor();
    int n = select(socket_fd + 1, NULL, &wset, NULL, &tval);
    if (n == 0) return false;
    if (n < 0) { report error, return false; }
    if (getsockopt(socket_fd, SOL_SOCKET, SO_ERROR, &sock_error, &len) != 0 || sock_error != 0) { report error, return false; }
    return true;
}

The whole approach no longer works now that socketDescriptor() is -1, but I see no alternative either.

Calling waitForConnected(0 or 1) leads to a disconnect in case of timeout, so that function cannot be called for such polling.

QNativeSocketEngine::connectToHost/waitForWrite looks interesting, but there's no way to ask for the engine of a QTcpSocket, right?

Any chance to make socketDescriptor() useful again while in ConnectingState ?

Alternatively something like a waitForConnected(0, DoNotDisconnectOnError) would help.