Full read buffer triggers infinite busy loop in QAbstractSocket

Following the mailing list post here: http://lists.trolltech.com/pipermail/qt-interest/2010-December/029781.html

While using KDE I discovered that downloading from a high-bandwith server can cause the kio_http process to stop responding while using 100% CPU. I tracked the problem down to a 'forever' loop in qabstractsocket.cpp. The offending lines are in QAbstractSocket::waitForReadyRead. In Qt 4.6.3 and 4.7.1 the loop is at lines 1832--1859 and (last I checked) in master the lines begin at 1862.

In that loop are the lines:

if (readyToRead) {
     if (d->canReadNotification())
         return true;
 }

The call to canReadNotification() returns false if the read buffer has no spare space, so the busy loop will not exit at the following line like it normally would. Assuming the socket remains connected, the loop does not terminate at all. (This also means that the calling program will be unable to flush the buffer.)

While the caller should not request Qt to add to a full buffer (I will file a bug with KDE about this, once I can narrow down the cause), Qt should not enter an infinite busy loop because of it, but instead return with some kind of error.