In rare cases it can happen that the GUI thread results in
a call to DropReferences(), while the backend (RCU) still has a
reference to the track and processes the track.
However the call to DropReferences, DiskIO processor will
have cleared the pointer to _track, leading to segfaults when
the processor runs.
Since the DIO processor is owned by the track, one cannot directly
pass a shared_ptr<Track>. That would keep the Track around forever.
However the DIO processor cannot exist without a track passing
a reference is acceptable.
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