fix(core): improve singleton crash recovery (#1799)

src/app/main.cpp

@@ -274,7 +274,12 @@ int main(int argc, char *argv[])
 
     using Zeal::Core::ApplicationSingleton;
     QScopedPointer<ApplicationSingleton> appSingleton(new ApplicationSingleton());
-    if (appSingleton->isSecondary()) {
+    if (appSingleton->state() == ApplicationSingleton::State::Failed) {
+        QTextStream(stderr) << "Failed to initialize application singleton." << '\n';
+        return EXIT_FAILURE;
+    }
+
+    if (appSingleton->state() == ApplicationSingleton::State::Secondary) {
 #ifdef Q_OS_WINDOWS
         ::AllowSetForegroundWindow(appSingleton->primaryPid());
 #endif

src/libs/core/applicationsingleton.cpp

@@ -45,8 +45,7 @@ ApplicationSingleton::ApplicationSingleton(QObject *parent)
 
     m_sharedMemory = new QSharedMemory(m_id, this);
 
-    m_isPrimary = m_sharedMemory->create(sizeof(SharedData));
-    if (m_isPrimary) {
+    if (m_sharedMemory->create(sizeof(SharedData))) {
         setupPrimary();
         return;
     }
@@ -95,31 +94,34 @@ ApplicationSingleton::ApplicationSingleton(QObject *parent)
             }
         }
 
-        m_isPrimary = m_sharedMemory->create(sizeof(SharedData));
-        if (m_isPrimary) {
+        if (m_sharedMemory->create(sizeof(SharedData))) {
             setupPrimary();
             return;
         }
     }
 #endif
 
-    // Fall back to secondary if we couldn't reclaim the segment.
+    // If both create() and attach() failed, the shared memory segment is in
+    // a broken state (e.g., stale backing file after a crash). Clean up and
+    // try to become primary one last time.
     if (!m_sharedMemory->attach(QSharedMemory::ReadOnly)) {
         qCWarning(log) << "Cannot attach to the shared memory segment:" << m_sharedMemory->errorString();
+
+        if (m_sharedMemory->create(sizeof(SharedData))) {
+            setupPrimary();
+            return;
+        }
+
+        qCWarning(log) << "Cannot create shared memory segment:" << m_sharedMemory->errorString();
         return;
     }
 
     setupSecondary();
 }
 
-bool ApplicationSingleton::isPrimary() const
+ApplicationSingleton::State ApplicationSingleton::state() const
 {
-    return m_isPrimary;
-}
-
-bool ApplicationSingleton::isSecondary() const
-{
-    return !m_isPrimary;
+    return m_state;
 }
 
 qint64 ApplicationSingleton::primaryPid() const
@@ -130,7 +132,7 @@ qint64 ApplicationSingleton::primaryPid() const
 bool ApplicationSingleton::sendMessage(QByteArray &data, int timeout)
 {
     // No support for primary to secondary communication.
-    if (m_isPrimary) {
+    if (m_state != State::Secondary) {
         return false;
     }
 
@@ -146,6 +148,7 @@ bool ApplicationSingleton::sendMessage(QByteArray &data, int timeout)
 
 void ApplicationSingleton::setupPrimary()
 {
+    m_state = State::Primary;
     m_primaryPid = QCoreApplication::applicationPid();
 
     qCInfo(log, "Starting as a primary instance. (PID: %lld)", m_primaryPid);
@@ -177,6 +180,8 @@ void ApplicationSingleton::setupPrimary()
 
 void ApplicationSingleton::setupSecondary()
 {
+    m_state = State::Secondary;
+
     m_sharedMemory->lock();
     auto sd = static_cast<SharedData *>(m_sharedMemory->data());
     m_primaryPid = sd->primaryPid;

src/libs/core/applicationsingleton.h

@@ -16,10 +16,15 @@ class ApplicationSingleton final : public QObject
     Q_OBJECT
     Q_DISABLE_COPY_MOVE(ApplicationSingleton)
 public:
+    enum class State {
+        Primary,
+        Secondary,
+        Failed,
+    };
+
     explicit ApplicationSingleton(QObject *parent = nullptr);
 
-    bool isPrimary() const;
-    bool isSecondary() const;
+    State state() const;
     qint64 primaryPid() const;
 
     bool sendMessage(QByteArray &data, int timeout = 500);
@@ -35,7 +40,7 @@ private:
 
     QString m_id;
 
-    bool m_isPrimary = false;
+    State m_state = State::Failed;
     qint64 m_primaryPid = 0;
 
     QSharedMemory *m_sharedMemory = nullptr;