services/pipewire: destroy qml ifaces early to avoid user callbacks

Consumers of defaultAudio*Changed signals can run code between
safeDestroy being called and PwObjectIface destruction due to
signal connection order. This change destroys ifaces earlier so they
are nulled by the time a changed signal is fired from destruction,
preventing access between ~PwNode() and ~QObject() completion.

Fixes #116 #122 #124

src/services/pipewire/qml.cpp

@@ -18,6 +18,16 @@
 
 namespace qs::service::pipewire {
 
+PwObjectIface::PwObjectIface(PwBindableObject* object): QObject(object), object(object) {
+	// We want to destroy the interface before QObject::destroyed is fired, as handlers
+	// connected before PwObjectIface will run first and emit signals that hit user code,
+	// which can then try to reference the iface again after ~PwNode() has been called but
+	// before ~QObject() has finished.
+	QObject::connect(object, &PwBindableObject::destroying, this, &PwObjectIface::onObjectDestroying);
+}
+
+void PwObjectIface::onObjectDestroying() { delete this; }
+
 void PwObjectIface::ref() {
 	this->refcount++;
 

src/services/pipewire/qml.hpp

@@ -36,7 +36,7 @@ class PwObjectIface
 	Q_OBJECT;
 
 public:
-	explicit PwObjectIface(PwBindableObject* object): QObject(object), object(object) {};
+	explicit PwObjectIface(PwBindableObject* object);
 	// destructor should ONLY be called by the pw object destructor, making an unref unnecessary
 	~PwObjectIface() override = default;
 	Q_DISABLE_COPY_MOVE(PwObjectIface);
@@ -44,6 +44,9 @@ public:
 	void ref() override;
 	void unref() override;
 
+private slots:
+	void onObjectDestroying();
+
 private:
 	quint32 refcount = 0;
 	PwBindableObject* object;