Create QApplication/QCoreApplication only once

Fixes a crash that can occur due to QEvents being unexpectedly
deleted when an extra audqt::init() + cleanup() pair is called at
startup, as when the skins-qt plugin fails to load.

src/libaudcore/internal.h

@@ -94,6 +94,9 @@ bool iface_plugin_set_current(PluginHandle * plugin);
 
 void interface_run();
 
+/* mainloop.cc */
+void mainloop_cleanup();
+
 /* playback.cc */
 /* do not call these; use aud_drct_play/stop() instead */
 void playback_play(int seek_time, bool pause);

src/libaudcore/mainloop.cc

@@ -353,6 +353,7 @@ static bool cleanup_node(QueuedFuncNode * node)
     delete node;
     return true;
 }
+
 // inhibit all future callbacks at shutdown
 static void enter_lockdown() { in_lockdown = true; }
 
@@ -374,10 +375,12 @@ EXPORT void mainloop_run()
         static int dummy_argc = 1;
         static char * dummy_argv[] = {app_name, nullptr};
 
-        if (qApp) // did audqt create a QApplication already?
-            qApp->exec();
-        else
-            QCoreApplication(dummy_argc, dummy_argv).exec();
+        // Create QCoreApplication instance if running in headless mode
+        // (otherwise audqt will have created a QApplication already).
+        if (!qApp)
+            new QCoreApplication(dummy_argc, dummy_argv);
+
+        qApp->exec();
     }
     else
 #endif
@@ -402,3 +405,13 @@ EXPORT void mainloop_quit()
         g_main_loop_quit(glib_mainloop);
     }
 }
+
+void mainloop_cleanup()
+{
+#ifdef USE_QT
+    if (aud_get_mainloop_type() == MainloopType::Qt)
+    {
+        delete qApp;
+    }
+#endif
+}

src/libaudcore/runtime.cc

@@ -364,6 +364,10 @@ EXPORT void aud_cleanup()
     scanner_cleanup();
     record_cleanup();
 
+    /* In Qt mode, this deletes the QApplication. This must be done
+     * after shutting down any GUI plugins but before unloading plugin
+     * modules (which will indirectly unload Qt shared libraries). */
+    mainloop_cleanup();
     stop_plugins_one();
 
     art_cleanup();

src/libaudqt/audqt.cc

@@ -114,6 +114,12 @@ EXPORT void init()
         return;
 
     aud_config_set_defaults("audqt", audqt_defaults);
+    log_init();
+
+    // The QApplication instance is created only once and is not deleted
+    // by audqt::cleanup(). If it already exists, we are done here.
+    if (qApp)
+        return;
 
 #if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0) && defined(_WIN32)
     QApplication::setHighDpiScaleFactorRoundingPolicy(
@@ -178,8 +184,6 @@ EXPORT void init()
     QApplication::setFont(QApplication::font("QSmallFont"), "QTreeView");
     QApplication::setFont(QApplication::font("QTipLabel"), "QStatusBar");
 #endif
-
-    log_init();
 }
 
 EXPORT void cleanup()
@@ -196,7 +200,9 @@ EXPORT void cleanup()
 
     log_cleanup();
 
-    delete qApp;
+    // We do not delete the QApplication here due to issues that arise
+    // if it is deleted and then re-created. Instead, it is deleted
+    // later during shutdown; see mainloop_cleanup() in libaudcore.
 }
 
 EXPORT QGradientStops dark_bg_gradient(const QColor & base)