If connecting ports using the port-engine fails,
ardour forgets the connection.
Internal backends only produced an error if a port was already
connected, when using ::connect (handle, other), but
ignore already existing connection when using port-names.
Various ports are connected twice when the engine connects
at session load. This worked fine for as long as the engine
was never stopped (saving the session asks the port-engine),
but failed when the engine went away and internal representation
is used.
"While 'atomic' has a volatile qualifier, this is a historical
artifact and the pointer passed to it should not be volatile."
Furthermore "It is very important that all accesses to a
particular integer or pointer be performed using only this API"
(from https://developer.gnome.org/glib/2.68/glib-Atomic-Operations.html)
Hence initialization of atomic variables is changed to also use
this API, instead of directly initializing the value.
This also fixes a few cases where atomic variables were
accessed directly.
see also libs/pbd/pbd/g_atomic_compat.h
PortAudio uses what it calls 'default suggested latencies' but in callback streaming mode, they can result in wildly inaccurate buffer sizing (e.g. the user requests a buffer size of 128 but PortAudio actually instructs ASIO to use a much bigger size).
What we do now is to improve PortAudio's suggested latency calculation by basing it on the actual buffer size requested by the user.
The backend holds `_port_callback_mutex` while disconnecting ports.
In some cases disconnecting a port can drop the last reference
resulting in a port-deletion from the connection handler.
This in turn will eventually aquire the `_port_callback_mutex`
and deadlock.
This is now circumvented by using atomic operations instead of
taking a lock to set the `_port_change_flag`.
The flag is also used to trigger a latency update in some cases,
atomic is preferable to taking a lock to set this flag.
--
Full bt: https://paste.debian.net/1184056/
Short:
#1 in pthread_mutex_lock ()
#2 in ARDOUR::PortEngineSharedImpl::port_connect_add_remove_callback()
#3 in ARDOUR::BackendPort::~BackendPort()
#4 in ARDOUR::DummyPort::~DummyPort()
#6 in ARDOUR::DummyAudioPort::~DummyAudioPort()
#7 in boost::checked_delete<ARDOUR::BackendPort>(ARDOUR::BackendPort*)
#12 in boost::shared_ptr<ARDOUR::ProtoPort>::reset()
#13 in ARDOUR::Port::drop()
#14 in ARDOUR::Port::~Port()
#15 in ARDOUR::AudioPort::~AudioPort()
#17 in ARDOUR::AudioEngine::add_pending_port_deletion(ARDOUR::Port*)
#20 in boost::detail::sp_counted_base::release()
#37 in ARDOUR::PortManager::connect_callback() at libs/ardour/port_manager.cc:788
#38 in ARDOUR::DummyAudioBackend::main_process_thread() at libs/backends/dummy/dummy_audiobackend.cc:1018
