Handle SIGINT (LMMS#7698)

Handle SIGINT
Allows Ctrl + C to close LMMS for unix-like environments that 
---------

Co-authored-by: Fawn Sannar <rubiefawn@gmail.com>

Author: tresf

include/GuiApplication.h

@@ -26,6 +26,7 @@
 #define LMMS_GUI_GUI_APPLICATION_H
 
 #include <QObject>
+#include <QSocketNotifier>
 
 #include "lmms_export.h"
 #include "lmmsconfig.h"
@@ -53,10 +54,13 @@ class LMMS_EXPORT GuiApplication : public QObject
 	~GuiApplication() override;
 
 	static GuiApplication* instance();
+	static void sigintHandler(int);
 #ifdef LMMS_BUILD_WIN32
 	static QFont getWin32SystemFont();
 #endif
 
+	void createSocketNotifier();
+
 	MainWindow* mainWindow() { return m_mainWindow; }
 	MixerView* mixerView() { return m_mixerView; }
 	SongEditorWindow* songEditor() { return m_songEditor; }
@@ -67,11 +71,15 @@ class LMMS_EXPORT GuiApplication : public QObject
 	AutomationEditorWindow* automationEditor() { return m_automationEditor; }
 	ControllerRackView* getControllerRackView() { return m_controllerRackView; }
 
+	//! File descriptors for unix socketpair, used to receive SIGINT
+	static inline int s_sigintFd[2];
+
 public slots:
 	void displayInitProgress(const QString &msg);
 
 private slots:
 	void childDestroyed(QObject *obj);
+	void sigintOccurred();
 
 private:
 	static GuiApplication* s_instance;
@@ -86,6 +94,7 @@ private slots:
 	MicrotunerConfig* m_microtunerConfig;
 	ControllerRackView* m_controllerRackView;
 	QLabel* m_loadingProgressLabel;
+	QSocketNotifier* m_sigintNotifier;
 };
 
 // Short-hand function

src/core/main.cpp

@@ -55,7 +55,7 @@
 #include <sys/prctl.h>
 #endif
 
-#include <csignal>
+#include <csignal>  // To register the signal handler
 
 #include "MainApplication.h"
 #include "ConfigManager.h"
@@ -76,12 +76,12 @@
 #include <fenv.h> // For feenableexcept
 #include <execinfo.h> // For backtrace and backtrace_symbols_fd
 #include <unistd.h> // For STDERR_FILENO
-#include <csignal> // To register the signal handler
 #endif
 
 
 #ifdef LMMS_DEBUG_FPE
-void signalHandler( int signum ) {
+void sigfpeHandler(int signum)
+{
 
 	// Get a back trace
 	void *array[10];
@@ -315,8 +315,9 @@ int main( int argc, char * * argv )
 
 	// Install the trap handler
 	// register signal SIGFPE and signal handler
-	signal(SIGFPE, signalHandler);
+	signal(SIGFPE, sigfpeHandler);
 #endif
+	signal(SIGINT, gui::GuiApplication::sigintHandler);
 
 #ifdef LMMS_BUILD_WIN32
 	// Don't touch redirected streams here

src/gui/GuiApplication.cpp

@@ -41,14 +41,22 @@
 #include "SongEditor.h"
 
 #include <QApplication>
+#include <QDebug>
 #include <QDir>
 #include <QtGlobal>
 #include <QLabel>
 #include <QMessageBox>
 #include <QSplashScreen>
+#include <QSocketNotifier>
+#include <csignal>
 
 #ifdef LMMS_BUILD_WIN32
+#include <io.h>
+#include <stdio.h>
 #include <windows.h>
+#else
+#include <sys/socket.h>
+#include <unistd.h>
 #endif
 
 namespace lmms
@@ -75,6 +83,9 @@ GuiApplication* GuiApplication::instance()
 
 GuiApplication::GuiApplication()
 {
+	// Immediately register our SIGINT handler
+	createSocketNotifier();
+
 	// prompt the user to create the LMMS working directory (e.g. ~/Documents/lmms) if it doesn't exist
 	if ( !ConfigManager::inst()->hasWorkingDir() &&
 		QMessageBox::question( nullptr,
@@ -240,6 +251,52 @@ void GuiApplication::childDestroyed(QObject *obj)
 	}
 }
 
+/** \brief Called from main when SIGINT is fired
+ *
+ * Unix signal handlers can only call async-signal-safe functions:
+ *  write(fd) --> QSocketNotifier --> SLOT sigintOccurred()
+ *
+ * See https://doc.qt.io/qt-6/unix-signals.html
+ */
+void GuiApplication::sigintHandler(int)
+{
+#ifdef LMMS_BUILD_WIN32
+	char message[] = "Sorry, SIGINT is unhandled on this platform\n";
+	std::ignore = _write(_fileno(stderr), message, sizeof(message));
+#else
+	char a = 1;
+	std::ignore = ::write(s_sigintFd[0], &a, sizeof(a));
+#endif
+}
+
+// Create our unix signal notifiers
+void GuiApplication::createSocketNotifier()
+{
+#ifdef LMMS_BUILD_WIN32
+	// no-op
+#else
+	if (::socketpair(AF_UNIX, SOCK_STREAM, 0, s_sigintFd))
+	{
+		qFatal("Couldn't create SIGINT socketpair");
+		return;
+	}
+
+	// Listen on the file descriptor for SIGINT
+	m_sigintNotifier = new QSocketNotifier(s_sigintFd[1], QSocketNotifier::Read, this);
+	connect(m_sigintNotifier, SIGNAL(activated(QSocketDescriptor)), this, SLOT(sigintOccurred()), Qt::QueuedConnection);
+#endif
+}
+
+// Handle the shutdown event
+void GuiApplication::sigintOccurred()
+{
+	m_sigintNotifier->setEnabled(false);
+	qDebug() << "Shutting down...";
+	// cleanup, etc
+	qApp->exit(3);
+	m_sigintNotifier->setEnabled(true);
+}
+
 #ifdef LMMS_BUILD_WIN32
 /*!
  * @brief Returns the Windows System font.