Mitigated possible race condition during app startup

singleapplication.cpp

@@ -74,17 +74,28 @@ SingleApplication::SingleApplication( int &argc, char *argv[], bool allowSeconda
     // Create a shared memory block
     if( d->memory->create( sizeof( InstancesInfo ) )){
         // Initialize the shared memory block
-        d->memory->lock();
+        if( ! d->memory->lock() ){
+          qCritical() << "SingleApplication: Unable to lock memory block after create.";
+          abortSafely();
+        }
         d->initializeMemoryBlock();
+        qDebug() << "SingleApplication: Created and initialized new memory block.";
     } else {
+        if( d->memory->error() == QSharedMemory::AlreadyExists ){
           // Attempt to attach to the memory segment
           if( ! d->memory->attach() ){
               qCritical() << "SingleApplication: Unable to attach to shared memory block.";
-            qCritical() << d->memory->errorString();
-            delete d;
-            ::exit( EXIT_FAILURE );
+              abortSafely();
+          }
+          if( ! d->memory->lock() ){
+            qCritical() << "SingleApplication: Unable to lock memory block after attach.";
+            abortSafely();
+          }
+          qDebug() << "SingleApplication: Attached to new memory block.";
+        } else {
+          qCritical() << "SingleApplication: Unable create block.";
+          abortSafely();
         }
-        d->memory->lock();
     }
 
     auto *inst = static_cast<InstancesInfo*>( d->memory->data() );
@@ -93,9 +104,13 @@ SingleApplication::SingleApplication( int &argc, char *argv[], bool allowSeconda
 
     // Make sure the shared memory block is initialised and in consistent state
     while( true ){
+      qDebug() << "SingleApplication: Verifying block checksum.";
+
       // If the shared memory block's checksum is valid continue
       if( d->blockChecksum() == inst->checksum ) break;
 
+      qDebug() << "SingleApplication: Invalid block checksum. Waiting.";
+
       // If more than 5s have elapsed, assume the primary instance crashed and
       // assume it's position
       if( time.elapsed() > 5000 ){
@@ -106,36 +121,48 @@ SingleApplication::SingleApplication( int &argc, char *argv[], bool allowSeconda
       // Otherwise wait for a random period and try again. The random sleep here
       // limits the probability of a collision between two racing apps and
       // allows the app to initialise faster
-      d->memory->unlock();
+      if( ! d->memory->unlock() ){
+        qDebug() << "SingleApplication: Unable to unlock memory for random wait.";
+        qDebug() << d->memory->errorString();
+      }
 #if QT_VERSION >= QT_VERSION_CHECK( 5, 10, 0 )
       QThread::sleep( QRandomGenerator::global()->bounded( 8u, 18u ));
 #else
       qsrand( QDateTime::currentMSecsSinceEpoch() % std::numeric_limits<uint>::max() );
       QThread::sleep( 8 + static_cast <unsigned long>( static_cast <float>( qrand() ) / RAND_MAX * 10 ));
 #endif
-      d->memory->lock();
+      if( ! d->memory->lock() ){
+        qCritical() << "SingleApplication: Unable to lock memory after random wait.";
+        abortSafely();
+      }
     }
 
     if( inst->primary == false ){
         d->startPrimary();
-        d->memory->unlock();
+        if( ! d->memory->unlock() ){
+          qDebug() << "SingleApplication: Unable to unlock memory after primary start.";
+          qDebug() << d->memory->errorString();
+        }
         return;
     }
 
     // Check if another instance can be started
     if( allowSecondary ){
-        inst->secondary += 1;
-        inst->checksum = d->blockChecksum();
-        d->instanceNumber = inst->secondary;
         d->startSecondary();
         if( d->options & Mode::SecondaryNotification ){
             d->connectToPrimary( timeout, SingleApplicationPrivate::SecondaryInstance );
         }
-        d->memory->unlock();
+        if( ! d->memory->unlock() ){
+          qDebug() << "SingleApplication: Unable to unlock memory after secondary start.";
+          qDebug() << d->memory->errorString();
+        }
         return;
     }
 
-    d->memory->unlock();
+    if( ! d->memory->unlock() ){
+      qDebug() << "SingleApplication: Unable to unlock memory at end of execution.";
+      qDebug() << d->memory->errorString();
+    }
 
     d->connectToPrimary( timeout, SingleApplicationPrivate::NewInstance );
 
@@ -235,3 +262,16 @@ bool SingleApplication::sendMessage( const QByteArray &message, int timeout )
     d->socket->flush();
     return dataWritten;
 }
+
+/**
+ * Cleans up the shared memory block and exits with a failure.
+ * This function halts program execution.
+ */
+void SingleApplication::abortSafely()
+{
+    Q_D( SingleApplication );
+
+    qCritical() << d->memory->errorString();
+    delete d;
+    ::exit( EXIT_FAILURE );
+}

singleapplication.h

@@ -140,6 +140,7 @@ Q_SIGNALS:
 private:
     SingleApplicationPrivate *d_ptr;
     Q_DECLARE_PRIVATE(SingleApplication)
+    void abortSafely();
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS(SingleApplication::Options)

singleapplication_p.cpp

@@ -161,6 +161,17 @@ void SingleApplicationPrivate::startPrimary()
 {
     Q_Q(SingleApplication);
 
+    // Reset the number of connections
+    auto *inst = static_cast <InstancesInfo*>( memory->data() );
+
+    inst->primary = true;
+    inst->primaryPid = q->applicationPid();
+    qstrncpy( inst->primaryUser, getUsername().toUtf8().data(), sizeof(inst->primaryUser) );
+    inst->checksum = blockChecksum();
+    instanceNumber = 0;
+
+    qDebug() << "SingleApplication: Started as a primary instance.";
+
     // Successful creation means that no main process exists
     // So we start a QLocalServer to listen for connections
     QLocalServer::removeServer( blockServerName );
@@ -181,20 +192,17 @@ void SingleApplicationPrivate::startPrimary()
         this,
         &SingleApplicationPrivate::slotConnectionEstablished
     );
-
-    // Reset the number of connections
-    auto *inst = static_cast <InstancesInfo*>( memory->data() );
-
-    inst->primary = true;
-    inst->primaryPid = q->applicationPid();
-    qstrncpy( inst->primaryUser, getUsername().toUtf8().data(), sizeof(inst->primaryUser) );
-    inst->checksum = blockChecksum();
-
-    instanceNumber = 0;
 }
 
 void SingleApplicationPrivate::startSecondary()
 {
+  auto *inst = static_cast <InstancesInfo*>( memory->data() );
+
+  inst->secondary += 1;
+  inst->checksum = blockChecksum();
+  instanceNumber = inst->secondary;
+
+  qDebug() << "SingleApplication: Started as a secondary instance.";
 }
 
 void SingleApplicationPrivate::connectToPrimary( int msecs, ConnectionType connectionType )
@@ -411,6 +419,7 @@ void SingleApplicationPrivate::readInitMessageBody( QLocalSocket *sock )
         ( connectionType == SecondaryInstance &&
           options & SingleApplication::Mode::SecondaryNotification ) )
     {
+        qDebug() << "SingleApplication: instanceStarted()";
         Q_EMIT q->instanceStarted();
     }
 

singleapplication_p.h

@@ -41,8 +41,8 @@ struct InstancesInfo {
     bool primary;
     quint32 secondary;
     qint64 primaryPid;
-    quint16 checksum;
     char primaryUser[128];
+    quint16 checksum; // Must be the last field
 };
 
 struct ConnectionInfo {