Cancel loading in EffectsLoader on tear down

When unloading all effects we should make sure that also the ones
scheduled for loading are canceled. For this a new method clear()
is added to the AbstractEffectLoader and all inheriting classes which
clears the current load queue. For the scripted and plugin effect loader
it should also cancel the future, but that's not yet implemented.

effectloader.cpp

@@ -178,6 +178,11 @@ QString BuiltInEffectLoader::internalName(const QString& name) const
     return name.toLower();
 }
 
+void BuiltInEffectLoader::clear()
+{
+    m_queue->clear();
+}
+
 static const QString s_nameProperty = QStringLiteral("X-KDE-PluginInfo-Name");
 static const QString s_jsConstraint = QStringLiteral("[X-Plasma-API] == 'javascript'");
 static const QString s_serviceType = QStringLiteral("KWin/Effect");
@@ -289,6 +294,12 @@ KPluginMetaData ScriptedEffectLoader::findEffect(const QString &name) const
 }
 
 
+void ScriptedEffectLoader::clear()
+{
+    // TODO: cancel future
+    m_queue->clear();
+}
+
 PluginEffectLoader::PluginEffectLoader(QObject *parent)
     : AbstractEffectLoader(parent)
     , m_queue(new EffectLoadQueue< PluginEffectLoader, KPluginMetaData>(this))
@@ -448,6 +459,12 @@ void PluginEffectLoader::setPluginSubDirectory(const QString &directory)
     m_pluginSubDirectory = directory;
 }
 
+void PluginEffectLoader::clear()
+{
+    // TODO: cancel future
+    m_queue->clear();
+}
+
 EffectLoader::EffectLoader(QObject *parent)
     : AbstractEffectLoader(parent)
 {
@@ -513,4 +530,11 @@ void EffectLoader::setConfig(KSharedConfig::Ptr config)
     }
 }
 
+void EffectLoader::clear()
+{
+    for (auto it = m_loaders.constBegin(); it != m_loaders.constEnd(); ++it) {
+        (*it)->clear();
+    }
+}
+
 } // namespace KWin

effectloader.h

@@ -162,6 +162,11 @@ public:
      */
     virtual bool isEffectSupported(const QString &name) const = 0;
 
+    /**
+     * @brief Clears the load queue, that is all scheduled Effects are discarded from loading.
+     **/
+    virtual void clear() = 0;
+
 Q_SIGNALS:
     /**
      * @brief The loader emits this signal when it successfully loaded an effect.
@@ -236,10 +241,17 @@ public:
         m_queue.enqueue(value);
         scheduleDequeue();
     }
+    void clear()
+    {
+        m_queue.clear();
+        m_dequeueScheduled = false;
+    }
 protected:
     void dequeue() override
     {
-        Q_ASSERT(!m_queue.isEmpty());
+        if (m_queue.isEmpty()) {
+            return;
+        }
         m_dequeueScheduled = false;
         const auto pair = m_queue.dequeue();
         m_effectLoader->loadEffect(pair.first, pair.second);
@@ -274,6 +286,7 @@ public:
     bool isEffectSupported(const QString &name) const override;
     QStringList listOfKnownEffects() const override;
 
+    void clear() override;
     void queryAndLoadAll() override;
     bool loadEffect(const QString& name) override;
     bool loadEffect(BuiltInEffect effect, LoadEffectFlags flags);
@@ -300,6 +313,7 @@ public:
     bool isEffectSupported(const QString &name) const override;
     QStringList listOfKnownEffects() const override;
 
+    void clear() override;
     void queryAndLoadAll() override;
     bool loadEffect(const QString &name) override;
     bool loadEffect(const KPluginMetaData &effect, LoadEffectFlags flags);
@@ -322,6 +336,7 @@ public:
     bool isEffectSupported(const QString &name) const override;
     QStringList listOfKnownEffects() const override;
 
+    void clear() override;
     void queryAndLoadAll() override;
     bool loadEffect(const QString &name) override;
     bool loadEffect(const KPluginMetaData &info, LoadEffectFlags flags);
@@ -349,6 +364,7 @@ public:
     bool loadEffect(const QString &name) override;
     void queryAndLoadAll() override;
     void setConfig(KSharedConfig::Ptr config) override;
+    void clear() override;
 
 private:
     QList<AbstractEffectLoader*> m_loaders;

effects.cpp

@@ -336,6 +336,7 @@ void EffectsHandlerImpl::unloadAllEffects()
         delete effect;
     }
     loaded_effects.clear();
+    m_effectLoader->clear();
 }
 
 void EffectsHandlerImpl::setupAbstractClientConnections(AbstractClient* c)