NOISSUE preserve minecraft.jar while migrating Legacy instances

It can be manually modded. It must be preserved when it's the only jar around.

api/logic/minecraft/legacy/LegacyInstance.cpp

@@ -44,6 +44,26 @@ LegacyInstance::LegacyInstance(SettingsObjectPtr globalSettings, SettingsObjectP
 	settings->registerSetting("CustomBaseJar", "");
 }
 
+QString LegacyInstance::mainJarToPreserve() const
+{
+	bool customJar = m_settings->get("UseCustomBaseJar").toBool();
+	if(customJar)
+	{
+		auto base = baseJar();
+		if(QFile::exists(base))
+		{
+			return base;
+		}
+	}
+	auto runnable = runnableJar();
+	if(QFile::exists(runnable))
+	{
+		return runnable;
+	}
+	return QString();
+}
+
+
 QString LegacyInstance::baseJar() const
 {
 	bool customJar = m_settings->get("UseCustomBaseJar").toBool();

api/logic/minecraft/legacy/LegacyInstance.h

@@ -64,6 +64,9 @@ class MULTIMC_LOGIC_EXPORT LegacyInstance : public BaseInstance
 	/// the default custom base jar of this instance
 	QString defaultCustomBaseJar() const;
 
+	// the main jar that we actually want to keep when migrating the instance
+	QString mainJarToPreserve() const;
+
 	/*!
 	 * Whether or not custom base jar is used
 	 */

api/logic/minecraft/legacy/LegacyUpgradeTask.cpp

@@ -92,10 +92,10 @@ void LegacyUpgradeTask::copyFinished()
 		components->buildingFromScratch();
 		components->setComponentVersion("net.minecraft", preferredVersionNumber, true);
 
-		if(legacyInst->shouldUseCustomBaseJar())
+		QString jarPath = legacyInst->mainJarToPreserve();
+		if(!jarPath.isNull())
 		{
-			QString jarPath = legacyInst->customBaseJar();
-			qDebug() << "Base jar is custom! : " << jarPath;
+			qDebug() << "Preserving base jar! : " << jarPath;
 			// FIXME: handle case when the jar is unreadable?
 			// TODO: check the hash, if it's the same as the upstream jar, do not do this
 			components->installCustomJar(jarPath);