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bonjour: use Qt::AutoConnection for BonjourServiceResolver's QSocketN…
…otifier slot. Using a QueuedConnection for the slot had the unpleasant side effect that the QSocketNotifier could have its activated() slot invoked even though no data was waiting to be read. In our case, this could cause a deadlock inside Avahi's libdns_sd compatibility library. I've settled on using Qt::AutoConnection to be consitent with the rest of the code base. The Bonjour code should always be invoked from the main thread, so in this case Qt::AutoConnection will always mean Qt::DirectConnection. Why does this happen? Qt seems to process events before invoking queued slot invocations. If the Qt event loop finds that the file descriptor that our QSocketNotifier is providing notification for is ready for reading, it queues up an invocation of the activated() slot for the next event loop iteration (because we use a QueuedConnection). As mentioned above, because Qt seems to poll() FDs before invoking queued-up slots, the end result is that an invocation of the activated() slot for a given QSocketNotifier's file descriptor can be queued up in the very same event loop iteration that a read() is performed for the exact same file descriptor. After performing the read(), the queued-up activated() slot invocation is no longer valid, and can wreak havoc, which in our case causes a deadlock in the Avahi libdns_sd code. The flow below describes the event loop iterations in more detail: 1st event loop iteration ------------------------ * poll() is invoked; the QSocketNotifier's FD is ready for reading. * An invocation of the activated() slot is queued up, to be executed next time we enter the event loop (due to Qt::QueuedConnection). 2nd event loop iteration ------------------------ * poll() is invoked; the QSocketNotifier's FD is _still_ ready for reading. * An invocation of the activated() slot is again queued up, to be executed in the 3rd iteration. * The queued-up slot invocation from the 1st iteration is invoked. (read() is called.) 3rd event loop iteration ------------------------ * poll is invoked(); the QSocketNotifier's FD has nothing to read anymore. Everything was read in the activated() slot that was invoked in the 2nd iteration. * The queued-up slot invocation from the 2nd iteration is invoked. This time, the read() syscall will block, because there is nothing to read.
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