Clearing hotcue while previewing it causes play button to get stuck.

I've pretty much nailed down this bug. It happens when a Hotcue is activated (the track was previously paused) and then you hit the play button. Judging from the code this might have been done more as a *feature* than it being an actual bug. For reference, this is the actual offending code, from src/engine/cuecontrol.cpp[649]:

void CueControl::cuePlay(double v) {
    QMutexLocker lock(&m_mutex);

    if (m_bPreviewing && !v) {
    // we're previewing? Then stop previewing and go into normal play mode.
        m_pPlayButton->set(1.0);
        m_bPreviewing = false;
    }

    if (m_bPreviewingHotcue && !v) {
        m_pPlayButton->set(1.0);
        m_bHotcueCancel = true;
    }

    lock.unlock();
}

Where it checks for m_bPreviewingHotcue is where it gets hung up.

Looking through the logs apparently this was a *feature*. This fixes the long standing issue of getting the track to resume when previewing using cue and then pressing the play button.

Another logic race condition in the engine...

This isn't related to the hotcue cancel feature. It's a race condition between the hotcue_active and hotcue_clear controls. The hotcue_activate call previews the hotcue, and before it is released, hotcue_clear detaches the Cue from the hotcue. Once hotcue_activate gets released, the logic that would normally stop the player and seek it back to where it started is not triggered because the hotcue is cleared of its Cue.

I have a patch that I need to test, but I think it should take care of this corner case.

The fix works for me.

This happened to me yesterday on my branch based on trunk revno: 3292 (Sat 2012-08-11).
I was playing around with my American Audio VMS2 hotcues. I still have to verify whether this is a bug in mixxx code or just an error in my controller script. There are a lot of buttons involved in reproducing this.
Most annoying: even reloading the track will resume to 'play' mode instantaneously. There seems to be no easy way to return to normal behaviour. I guess simply pressing 'pause' should in any case pause the track. Instead it also resumes to 'play' mode. Pressing 'play' will start the track from the beginning, ignoring all cue-points.

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/5914