Session::process_audition calls Graph::swap_process_chain()
to handle any pending graph changes (notably route removal
depends the graph to switch chains to drop references).
However this must not change the Graph::_trigger_queue
(which is refilled when processing resumes).
Previously routes were added to the trigger_queue, leading to
an inconsistent Graph state. When the terminal-count was reached
the trigger-queue was not empty. Process threads ran after processing
already completed and or concurrently with processing.
A common result of that was:
delayline.cc:70: virtual void ARDOUR::DelayLine::run(ARDOUR::BufferSet&, ARDOUR::samplepos_t, ARDOUR::samplepos_t, double, ARDOUR::pframes_t, bool): Assertion `lm.locked ()' failed.
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