core/generation: destroy the engine after the root's children

changelog/next.md

@@ -11,3 +11,4 @@
 - Fixed `WlSessionLockSurface.screen` being null and never updating with a real screen on monitor plug.
 - Fixed `NotificationAction.text` not accepting text updates.
 - Fixed PipeWire route volume writes for devices without a reported volume step.
+- Fixed a crash when quitting or reloading while a `Loader` was still loading asynchronously.

src/core/generation.cpp

@@ -11,6 +11,7 @@
 #include <qlist.h>
 #include <qlogging.h>
 #include <qloggingcategory.h>
+#include <qnamespace.h>
 #include <qobject.h>
 #include <qqmlcontext.h>
 #include <qqmlengine.h>
@@ -83,25 +84,36 @@ void EngineGeneration::destroy() {
 	}
 
 	if (this->root != nullptr) {
+		// prevent further js execution during teardown of the root's children, gc and the engine.
 		QObject::connect(this->root, &QObject::destroyed, this, [this]() {
-			// prevent further js execution between garbage collection and engine destruction.
 			this->engine->setInterrupted(true);
-
-			g_generations.remove(this->engine);
-
-			// Garbage is not collected during engine destruction.
-			this->engine->collectGarbage();
-
-			delete this->engine;
-			this->engine = nullptr;
-
-			auto terminate = this->shouldTerminate;
-			auto code = this->exitCode;
-			delete this;
-
-			if (terminate) QCoreApplication::exit(code);
 		});
 
+		// QObject::destroyed is emitted before the root's children are destroyed.
+		// The engine must outlive them, as objects such as Loaders with in-flight
+		// incubations access it in their destructors.
+		QObject::connect(
+		    this->root,
+		    &QObject::destroyed,
+		    this,
+		    [this]() {
+			    g_generations.remove(this->engine);
+
+			    // Garbage is not collected during engine destruction.
+			    this->engine->collectGarbage();
+
+			    delete this->engine;
+			    this->engine = nullptr;
+
+			    auto terminate = this->shouldTerminate;
+			    auto code = this->exitCode;
+			    delete this;
+
+			    if (terminate) QCoreApplication::exit(code);
+		    },
+		    Qt::QueuedConnection
+		);
+
 		this->root->deleteLater();
 		this->root = nullptr;
 	} else {