CLI: disable async crash-recovery backup in headless mode (fixes segfault)

QETProject schedules an asynchronous crash-recovery backup on construction
(writeBackup() -> QtConcurrent::run(QET::writeToFile, ..., &m_backup_file)).
In one-shot CLI mode the QETProject is destroyed as soon as the command
returns, while that background write still references its m_backup_file
member — an intermittent use-after-free segfault during teardown (~1 in 6
runs; observed on --resave and --set-titleblock).

A crash-recovery backup is meaningless for a short-lived headless command,
so add QETProject::setBackupEnabled(false), called from the CLI entry in
main(). writeBackup() then early-returns, so no background write is ever
launched. Fixes the crash for all CLI commands. See #492.

sources/main.cpp

@@ -19,6 +19,7 @@
 #include "machine_info.h"
 #include "qet.h"
 #include "qetapp.h"
+#include "qetproject.h"
 #include "singleapplication.h"
 #include "utils/macosxopenevent.h"
 #include "utils/qetsettings.h"
@@ -206,6 +207,10 @@ QGuiApplication::setHighDpiScaleFactorRoundingPolicy(QetSettings::hdpiScaleFacto
 			raw_args << QString::fromLocal8Bit(argv[i]);
 		if (CLIExport::isExportRequest(raw_args)) {
 			QApplication export_app(argc, argv);
+			// No crash-recovery backups in one-shot CLI mode: the backup write
+			// runs on a background thread referencing the project and races the
+			// process exit (intermittent segfault in QET::writeToFile).
+			QETProject::setBackupEnabled(false);
 			return CLIExport::run(export_app.arguments());
 		}
 	}

sources/qetproject.cpp

@@ -43,6 +43,13 @@
 
 static int BACKUP_INTERVAL = 1200000; //interval in ms of backup = 20min
 
+bool QETProject::m_backup_enabled = true;
+
+void QETProject::setBackupEnabled(bool enabled)
+{
+	m_backup_enabled = enabled;
+}
+
 /**
 	@brief QETProject::QETProject
 	Create a empty project
@@ -1783,6 +1790,8 @@ void QETProject::addDiagram(Diagram *diagram, int pos)
 */
 void QETProject::writeBackup()
 {
+	if (!m_backup_enabled)
+		return;
 #ifdef BUILD_WITHOUT_KF5
 #else
 #	if QT_VERSION < QT_VERSION_CHECK(6, 0, 0) // ### Qt 6: remove

sources/qetproject.h

@@ -105,6 +105,12 @@ class QETProject : public QObject
 		QVersionNumber declaredQElectroTechVersion();
 		void setTitle(const QString &);
 
+		/// Enable/disable the asynchronous crash-recovery backup for all
+		/// projects.  Disabled by the headless CLI: the backup write runs on a
+		/// background thread referencing the project, and a short-lived CLI
+		/// process can destroy the project before the write finishes (crash).
+		static void setBackupEnabled(bool enabled);
+
 			///DEFAULT PROPERTIES
 		BorderProperties defaultBorderProperties() const;
 		void             setDefaultBorderProperties(const BorderProperties &);
@@ -241,6 +247,8 @@ class QETProject : public QObject
 
 	// attributes
 	private:
+			/// When false, writeBackup() is a no-op (set by the headless CLI)
+		static bool m_backup_enabled;
 			/// File path this project is saved to
 		QString m_file_path;
 			/// Current state of the project