Add an opt-out for the single-process behavior

This would have been useful for testing multiple times and now even came up
in a forum discussion.

tray/application/main.cpp

@@ -159,6 +159,7 @@ int runApplication(int argc, const char *const *argv)
     connectionArg.setRequiredValueCount(Argument::varValueCount);
     ConfigValueArgument configPathArg("config-dir-path", '\0', "specifies the path to the configuration directory", { "path" });
     configPathArg.setEnvironmentVariable(PROJECT_VARNAME_UPPER "_CONFIG_DIR");
+    ConfigValueArgument newInstanceArg("new-instance", '\0', "disable the usual single-process behavior");
     Argument &widgetsGuiArg = qtConfigArgs.qtWidgetsGuiArg();
     widgetsGuiArg.addSubArgument(&windowedArg);
     widgetsGuiArg.addSubArgument(&showWebUiArg);
@@ -166,6 +167,7 @@ int runApplication(int argc, const char *const *argv)
     widgetsGuiArg.addSubArgument(&waitForTrayArg);
     widgetsGuiArg.addSubArgument(&connectionArg);
     widgetsGuiArg.addSubArgument(&configPathArg);
+    widgetsGuiArg.addSubArgument(&newInstanceArg);
 
     parser.setMainArguments({ &qtConfigArgs.qtWidgetsGuiArg(), &parser.noColorArg(), &parser.helpArg() });
     parser.parseArgs(argc, argv);
@@ -187,7 +189,7 @@ int runApplication(int argc, const char *const *argv)
         SET_QT_APPLICATION_INFO;
         QApplication application(argc, const_cast<char **>(argv));
         QGuiApplication::setQuitOnLastWindowClosed(false);
-        SingleInstance singleInstance(argc, argv);
+        SingleInstance singleInstance(argc, argv, newInstanceArg.isPresent());
         networkAccessManager().setParent(&singleInstance);
         QObject::connect(&singleInstance, &SingleInstance::newInstance, &runApplication);
         Settings::restore();

tray/application/singleinstance.cpp

@@ -17,10 +17,14 @@ using namespace CppUtilities::EscapeCodes;
 
 namespace QtGui {
 
-SingleInstance::SingleInstance(int argc, const char *const *argv, QObject *parent)
+SingleInstance::SingleInstance(int argc, const char *const *argv, bool newInstance, QObject *parent)
     : QObject(parent)
     , m_server(nullptr)
 {
+    if (newInstance) {
+        return;
+    }
+
     const QString appId(QCoreApplication::applicationName() % QStringLiteral(" by ") % QCoreApplication::organizationName());
 
     // check for previous instance

tray/application/singleinstance.h

@@ -14,7 +14,7 @@ namespace QtGui {
 class SingleInstance : public QObject {
     Q_OBJECT
 public:
-    SingleInstance(int argc, const char *const *argv, QObject *parent = nullptr);
+    SingleInstance(int argc, const char *const *argv, bool newInstance = false, QObject *parent = nullptr);
 
 Q_SIGNALS:
     void newInstance(int argc, const char *const *argv);