VCAManager::create_vca sets PI::order while holding the
VCAManager:lock mutex. PI order changes emit a "changed" signal
which in turn can result in querying a list of all strips
(reassign_track_numbers) which requires the VCA mutex:
See also 729ff35faf
```
#2 Glib::Threads::Mutex::Lock::Lock(Glib::Threads::Mutex&) (this=0x7fffffffb070, mutex=...) at /usr/include/glibmm-2.4/glibmm/threads.h:687
#3 ARDOUR::VCAManager::vcas[abi:cxx11]() const (this=0x5555599b6d10) at ../libs/ardour/vca_manager.cc:77
#4 ARDOUR::Session::get_stripables(std::__cxx11::list<boost::shared_ptr<ARDOUR::Stripable>, std::allocator<boost::shared_ptr<ARDOUR::Stripable> > >&, ARDOUR::PresentationInfo::Flag) const (this=0x55555bf03910, sl=std::__cxx11::list = {...}, fl=127) at ../libs/ardour/session.cc:3949
#5 ARDOUR::Session::ensure_stripable_sort_order() (this=0x55555bf03910) at ../libs/ardour/session.cc:2634
#6 ARDOUR::Session::notify_presentation_info_change(PBD::PropertyChange const&) (this=0x55555bf03910, what_changed=...) at ../libs/ardour/session.cc:7016
[ .. connect same thread ..]
#13 ARDOUR::PresentationInfo::send_static_change(PBD::PropertyChange const&) (what_changed=...) at ../libs/ardour/presentation_info.cc:113
#14 ARDOUR::PresentationInfo::set_order(unsigned int) (this=0x5555619ceca0, order=5) at ../libs/ardour/presentation_info.cc:288
#15 ARDOUR::Stripable::set_presentation_order(unsigned int) (this=0x5555619ce8a0, order=5) at ../libs/ardour/stripable.cc:55
#16 ARDOUR::VCAManager::create_vca(unsigned int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)
(this=0x5555599b6d10, howmany=1, name_template="VCA %n") at ../libs/ardour/vca_manager.cc:105
```
Robin Gareus
e33d131174
