[plasma-session] Avoid hypothetical race condition starting a service

Summary:
If for some reason a service is started before StartServiceJob runs, we
will end up blocking forever.

Test Plan: Logged in

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27471

startkde/plasma-session/startup.cpp

@@ -417,7 +417,8 @@ void AutoStartAppsJob::start() {
 StartServiceJob::StartServiceJob(const QString &process, const QStringList &args, const QString &serviceId):
     KJob(),
     m_process(process),
-    m_args(args)
+    m_args(args),
+    m_serviceId(serviceId)
 {
     auto watcher = new QDBusServiceWatcher(serviceId, QDBusConnection::sessionBus(), QDBusServiceWatcher::WatchForRegistration, this);
     connect(watcher, &QDBusServiceWatcher::serviceRegistered, this, &StartServiceJob::emitResult);
@@ -425,6 +426,12 @@ StartServiceJob::StartServiceJob(const QString &process, const QStringList &args
 
 void StartServiceJob::start()
 {
+    if (QDBusConnection::sessionBus().interface()->isServiceRegistered(m_serviceId)) {
+        qCDebug(PLASMA_SESSION) << m_process << "already running";
+        emitResult();
+        return;
+    }
+    qCDebug(PLASMA_SESSION) << "Starting " << m_process << m_args;
     QProcess::startDetached(m_process, m_args);
 }
 

startkde/plasma-session/startup.h

@@ -92,6 +92,7 @@ public:
 private:
     const QString m_process;
     const QStringList m_args;
+    const QString m_serviceId;
 };
 
 class RestoreSessionJob: public KJob