Fix concurrency issue when adding/removing ports of latent plugins

This fixes an issue when changing port-configuration
or toggle strict-i/o of latent multi-out plugins.

`Session::auto_connect_thread_run` can run concurrently
while ports are added/removed from the GUI thread.
The latter invalidates IO::_port PortSet, while
the former thread iterates over ports.

This also changes the IO-Mutex into a RWLock to allow
concurrent access when possible.

libs/ardour/ardour/io.h

@@ -206,7 +206,7 @@ class LIBARDOUR_API IO : public SessionObject
 	bool     _sendish;
 
 private:
-	mutable Glib::Threads::Mutex io_lock;
+	mutable Glib::Threads::RWLock _io_lock;
 	PortSet   _ports;
 
 	void reestablish_port_subscriptions ();

libs/ardour/io.cc

@@ -99,11 +99,10 @@ IO::IO (Session& s, const XMLNode& node, DataType dt, bool sendish)
 
 IO::~IO ()
 {
-	Glib::Threads::Mutex::Lock lm (io_lock);
-
 	DEBUG_TRACE (DEBUG::Ports, string_compose ("IO %1 unregisters %2 ports\n", name(), _ports.num_ports()));
 
 	BLOCK_PROCESS_CALLBACK ();
+	Glib::Threads::RWLock::WriterLock wl (_io_lock);
 
 	for (PortSet::iterator i = _ports.begin(); i != _ports.end(); ++i) {
 		_session.engine().unregister_port (*i);
@@ -122,12 +121,13 @@ IO::connection_change (boost::shared_ptr<Port> a, boost::shared_ptr<Port> b)
 	   we assume that its safely locked by our own ::disconnect().
 	*/
 
-	Glib::Threads::Mutex::Lock tm (io_lock, Glib::Threads::TRY_LOCK);
+	Glib::Threads::RWLock::WriterLock wl (_io_lock, Glib::Threads::TRY_LOCK);
 
-	if (tm.locked()) {
+	if (wl.locked()) {
 		/* we took the lock, so we cannot be here from inside
 		 * ::disconnect()
 		 */
+		wl.release (); // release lock before emitting signal
 		if (_ports.contains (a) || _ports.contains (b)) {
 			changed (IOChange (IOChange::ConnectionsChanged), this); /* EMIT SIGNAL */
 		}
@@ -158,7 +158,7 @@ IO::disconnect (boost::shared_ptr<Port> our_port, string other_port, void* src)
 	}
 
 	{
-		Glib::Threads::Mutex::Lock lm (io_lock);
+		Glib::Threads::RWLock::ReaderLock rl (_io_lock);
 
 		/* check that our_port is really one of ours */
 
@@ -192,7 +192,7 @@ IO::connect (boost::shared_ptr<Port> our_port, string other_port, void* src)
 	}
 
 	{
-		Glib::Threads::Mutex::Lock lm (io_lock);
+		Glib::Threads::RWLock::ReaderLock rl (_io_lock);
 
 		/* check that our_port is really one of ours */
 
@@ -247,7 +247,7 @@ IO::remove_port (boost::shared_ptr<Port> port, void* src)
 		BLOCK_PROCESS_CALLBACK ();
 
 		{
-			Glib::Threads::Mutex::Lock lm (io_lock);
+			Glib::Threads::RWLock::WriterLock wl (_io_lock);
 
 			if (_ports.remove(port)) {
 				change.type = IOChange::Type (change.type | IOChange::ConfigurationChanged);
@@ -318,7 +318,7 @@ IO::add_port (string destination, void* src, DataType type)
 
 
 		{
-			Glib::Threads::Mutex::Lock lm (io_lock);
+			Glib::Threads::RWLock::WriterLock wl (_io_lock);
 
 			/* Create a new port */
 
@@ -364,7 +364,7 @@ int
 IO::disconnect (void* src)
 {
 	{
-		Glib::Threads::Mutex::Lock lm (io_lock);
+		Glib::Threads::RWLock::ReaderLock rl (_io_lock);
 
 		for (PortSet::iterator i = _ports.begin(); i != _ports.end(); ++i) {
 			i->disconnect_all ();
@@ -487,7 +487,7 @@ IO::ensure_ports (ChanCount count, bool clear, void* src)
 	change.before = _ports.count ();
 
 	{
-		Glib::Threads::Mutex::Lock im (io_lock);
+		Glib::Threads::RWLock::WriterLock wl (_io_lock);
 		if (ensure_ports_locked (count, clear, changed)) {
 			return -1;
 		}
@@ -536,7 +536,7 @@ IO::state () const
 {
 	XMLNode* node = new XMLNode (state_node_name);
 	int n;
-	Glib::Threads::Mutex::Lock lm (io_lock);
+	Glib::Threads::RWLock::WriterLock wl (_io_lock);
 
 	node->set_property ("name", name());
 	node->set_property ("id", id ());
@@ -1237,7 +1237,7 @@ IO::latency () const
 {
 	samplecnt_t max_latency = 0;
 
-	/* io lock not taken - must be protected by other means */
+	Glib::Threads::RWLock::ReaderLock rl (_io_lock);
 
 	for (PortSet::const_iterator i = _ports.begin(); i != _ports.end(); ++i) {
 		samplecnt_t latency;
@@ -1285,7 +1285,16 @@ IO::public_latency () const
 samplecnt_t
 IO::connected_latency (bool for_playback) const
 {
-	/* io lock not taken - must be protected by other means */
+	/* may be called concurrently with processing via
+	 *
+	 * Session::auto_connect_thread_run ()
+	 * -> Session::update_latency_compensation ()
+	 * -> Session::update_route_latency ()
+	 * -> Route::update_signal_latency ()
+	 * -> IO::connected_latency ()
+	 */
+	Glib::Threads::RWLock::ReaderLock rl (_io_lock);
+
 	samplecnt_t max_latency = 0;
 	bool connected = false;
 
@@ -1325,7 +1334,7 @@ IO::connect_ports_to_bundle (boost::shared_ptr<Bundle> c, bool exclusive,
 	BLOCK_PROCESS_CALLBACK ();
 
 	{
-		Glib::Threads::Mutex::Lock lm2 (io_lock);
+		Glib::Threads::RWLock::ReaderLock rl (_io_lock);
 
 		if (exclusive) {
 			for (PortSet::iterator i = _ports.begin(); i != _ports.end(); ++i) {
@@ -1347,7 +1356,7 @@ IO::disconnect_ports_from_bundle (boost::shared_ptr<Bundle> c, void* src)
 	BLOCK_PROCESS_CALLBACK ();
 
 	{
-		Glib::Threads::Mutex::Lock lm2 (io_lock);
+		Glib::Threads::RWLock::ReaderLock rl (_io_lock);
 
 		c->disconnect (_bundle, _session.engine());
 
@@ -1738,6 +1747,6 @@ IO::physically_connected () const
 bool
 IO::has_port (boost::shared_ptr<Port> p) const
 {
-	Glib::Threads::Mutex::Lock lm (io_lock);
+	Glib::Threads::RWLock::ReaderLock rl (_io_lock);
 	return _ports.contains (p);
 }