Fixed race condition #3! Library version 2.1; Explained implementation in README

README.md

@@ -32,12 +32,19 @@ The ```Show Up``` signal
 ------------------------
 The SingleApplication class implements a ```showUp()``` signal. You can bind to that signal to raise your application's window when a new instance had been started.
 
-Known bugs
+Implementation
-----------
+--------------
-The ```SingleApplication``` class works really well for the scenario 
+The library is implemented with a QSharedMemory block which is thread safe and guarantees a race condition will not occur. It also uses a QLocalSocket to notify the main process that a new instance had been spawned and thus invoke the ```showUp()``` signal.
-when a 
+
-user starts your application twice, but the implementation it uses does not garantee it will work if two processes are started simultaniously (with a millisecond delay, which is only done programatically), in which case two instances might coexist on some platforms.
+To handle an issue with Unix systems, where the operating system owns the shared memory block and if the program crashes the memory remains untouched, the library binds to the following signals and closes the program with error code = ```128 + signum``` where signum is the number representation of the signal listed below.
-_The issue is a work in proggress and should be fixed soon._
+
+* ```SIGINT ``` - ```2```
+* ```SIGILL ``` - ```4```
+* ```SIGABRT``` - ```6```
+* ```SIGFPE ``` - ```8```
+* ```SIGSEGV``` - ```11```
+* ```SIGTERM``` - ```15```
+
 
 License
 -------

singleapplication.cpp

@@ -1,6 +1,73 @@
 #include "singleapplication.h"
+#include <QSharedMemory>
+#include <QLocalSocket>
+#include <QLocalServer>
+#include <QMutex>
 #include <cstdlib>
 
+#ifdef Q_OS_UNIX
+    #include <signal.h>
+    #include <unistd.h>
+#endif
+
+class SingleApplicationPrivate {
+public:
+    SingleApplicationPrivate(SingleApplication *q_ptr) : q_ptr(q_ptr) { }
+
+    void startServer(QString &serverName)
+    {
+        // Start a QLocalServer to listen for connections
+        server = new QLocalServer();
+        server->removeServer(serverName);
+        server->listen(serverName);
+        QObject::connect(server, SIGNAL(newConnection()), q_ptr, SLOT(slotConnectionEstablished()));
+    }
+
+#ifdef Q_OS_UNIX
+    void crashHandler()
+    {
+        // This guarantees the program will work even with multiple
+        // instances of SingleApplication in different threads
+        // Which in my opinion is idiotic, but lets handle that too
+        {
+            sharedMemMutex.lock();
+            sharedMem.append(memory);
+            sharedMemMutex.unlock();
+        }
+        // Handle any further termination signals to ensure the
+        // QSharedMemory block is deleted even if the process crashes
+        signal(SIGSEGV, SingleApplicationPrivate::terminate);
+        signal(SIGABRT, SingleApplicationPrivate::terminate);
+        signal(SIGFPE, SingleApplicationPrivate::terminate);
+        signal(SIGILL, SingleApplicationPrivate::terminate);
+        signal(SIGINT, SingleApplicationPrivate::terminate);
+        signal(SIGTERM, SingleApplicationPrivate::terminate);
+    }
+
+    static void terminate(int signum)
+    {
+        while( ! sharedMem.empty() ) {
+            delete sharedMem.back();
+            sharedMem.pop_back();
+        }
+        ::exit(128 + signum);
+    }
+
+    static QList<QSharedMemory*> sharedMem;
+    static QMutex sharedMemMutex;
+#endif
+
+    QSharedMemory *memory;
+    SingleApplication *q_ptr;
+    QLocalServer  *server;
+    QLocalSocket  *socket;
+};
+
+#ifdef Q_OS_UNIX
+    QList<QSharedMemory*> SingleApplicationPrivate::sharedMem;
+    QMutex SingleApplicationPrivate::sharedMemMutex;
+#endif
+
 /**
  * @brief Constructor. Checks and fires up LocalServer or closes the program
  * if another instance already exists
@@ -8,23 +75,40 @@
  * @param argv
  */
 SingleApplication::SingleApplication(int &argc, char *argv[])
-    : QApplication(argc, argv)
+    : QApplication(argc, argv), d_ptr(new SingleApplicationPrivate(this))
 {
     QString serverName = QApplication::organizationName() + QApplication::applicationName();
     serverName.replace(QRegExp("[^\\w\\-. ]"), "");
 
-    // Attempt to connect to the LocalServer
+    // Garantee thread safe behaviour with a shared memory block
-    socket = new QLocalSocket();
+    d_ptr->memory = new QSharedMemory(serverName);
-    socket->connectToServer(serverName);
+
-    if(socket->waitForConnected(1000)){
+    // Create a shared memory block with a minimum size of 1 byte
+    if( d_ptr->memory->create(1, QSharedMemory::ReadOnly) )
+    {
+#ifdef Q_OS_UNIX
+        // Handle any further termination signals to ensure the
+        // QSharedMemory block is deleted even if the process crashes
+        d_ptr->crashHandler();
+#endif
+        // Successful creation means that no main process exists
+        // So we start a Local Server to listen for connections
+        d_ptr->startServer(serverName);
+    } else {
+        // Connect to the Local Server of the main process to notify it
+        // that a new process had been started
+        d_ptr->socket = new QLocalSocket();
+        d_ptr->socket->connectToServer(serverName);
+
+        // Even though a shared memory block exists, the original application might have crashed
+        // So only after a successful connection is the second instance terminated
+        if( d_ptr->socket->waitForConnected(100) )
+        {
             ::exit(EXIT_SUCCESS); // Terminate the program using STDLib's exit function
         } else {
-        // If the connection is insuccessful, this is the main process
+            delete d_ptr->memory;
-        // So we create a Local Server
+            ::exit(EXIT_SUCCESS);
-        server = new QLocalServer();
+        }
-        server->removeServer(serverName);
-        server->listen(serverName);
-        QObject::connect(server, SIGNAL(newConnection()), this, SLOT(slotConnectionEstablished()));
     }
 }
 
@@ -33,7 +117,8 @@ SingleApplication::SingleApplication(int &argc, char *argv[])
  */
 SingleApplication::~SingleApplication()
 {
-    server->close();
+    delete d_ptr->memory;
+    d_ptr->server->close();
 }
 
 /**
@@ -41,7 +126,7 @@ SingleApplication::~SingleApplication()
  */
 void SingleApplication::slotConnectionEstablished()
 {
-    QLocalSocket *socket = server->nextPendingConnection();
+    QLocalSocket *socket = d_ptr->server->nextPendingConnection();
     socket->close();
     delete socket;
     emit showUp();

singleapplication.h

@@ -2,8 +2,8 @@
 #define SINGLE_APPLICATION_H
 
 #include <QApplication>
-#include <QLocalSocket>
+
-#include <QLocalServer>
+class SingleApplicationPrivate;
 
 /**
  * @brief The SingleApplication class handles multipe instances of the same Application
@@ -23,9 +23,7 @@ private slots:
   void slotConnectionEstablished();
 
 private:
-  QLocalSocket *socket;
+  SingleApplicationPrivate *d_ptr;
-  QLocalServer *server;
-
 };
 
 #endif // SINGLE_APPLICATION_H