The crashed previously because:
A VCA is-a Automatable is-a Evoral::ControlSet
After VCA's d'tor completes ~Automatable runs and emits signal to
DropReferences of all master-controls.
This in turn calls SlavableAutomationControl::master_going_away()
for slaved parameters for the given master-control
In ::master_going_away() the weak-pointer reference to the master's
AutomationControl (owned by the destroyed VCA) is still valid.
Execution is in the d'tor of Automatable which is-a ControlSet and
the ContolSet keeps a reference to the Control and hence also to the
AutomationControl which is-a Evoral::Control.
So master_going_away() locks a boost::shared_ptr<ARDOUR::AutomationControl>
which is actually the MuteControl owned by the VCA.
It calls SlavableAutomationControl::remove_master() which
in turn calls MuteControl::pre_remove_master() which uses the
MuteMaster API to retrieve the value. The MuteMaster however is the
VCA that has just been destroyed.
The solution is twofold:
1) emit "drop_references" from the VCA d'tor itself,
before the VCA is destroyed.
2) disconnect a slaved control from the master's drop_references signal
when un-assigning a master-control.
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