Fix circular reference memory leaks

When dispatching the promise result, we need to clear both handlers and catchers to prevent retaining circular references when the promise is captured into the handler and/or catcher. Also refactor part of the `notify` logic.

src/qtpromise/qpromise_p.h

@@ -331,11 +331,12 @@ struct PromiseCatcher<T, std::nullptr_t, void>
 
 template <typename T> class PromiseData;
 
-template <typename T>
+template <typename T, typename F>
 class PromiseDataBase : public QSharedData
 {
 public:
     using Error = QtPromise::QPromiseError;
+    using Handler = std::pair<QPointer<QThread>, std::function<F> >;
     using Catcher = std::pair<QPointer<QThread>, std::function<void(const Error&)> >;
 
     virtual ~PromiseDataBase() {}
@@ -356,6 +357,12 @@ public:
         return !m_settled;
     }
 
+    void addHandler(std::function<F> handler)
+    {
+        QWriteLocker lock(&m_lock);
+        m_handlers.append({QThread::currentThread(), std::move(handler)});
+    }
+
     void addCatcher(std::function<void(const Error&)> catcher)
     {
         QWriteLocker lock(&m_lock);
@@ -376,15 +383,23 @@ public:
             return;
         }
 
-        if (m_error.isNull()) {
-            notify();
-            return;
-        }
+        // A promise can't be resolved multiple times so once settled, its state can't
+        // change. When fulfilled, handlers must be called (a single time) and catchers
+        // ignored indefinitely (or vice-versa when the promise is rejected), so make
+        // sure to clear both handlers AND catchers when dispatching. This also prevents
+        // shared pointer circular reference memory leaks when the owning promise is
+        // captured in the handler and/or catcher lambdas.
 
         m_lock.lockForWrite();
+        QVector<Handler> handlers(std::move(m_handlers));
         QVector<Catcher> catchers(std::move(m_catchers));
         m_lock.unlock();
 
+        if (m_error.isNull()) {
+            notify(handlers);
+            return;
+        }
+
         QSharedPointer<Error> error = m_error;
         Q_ASSERT(!error.isNull());
 
@@ -406,26 +421,21 @@ protected:
         m_settled = true;
     }
 
-    virtual void notify() = 0;
+    virtual void notify(const QVector<Handler>&) = 0;
 
 private:
     bool m_settled = false;
+    QVector<Handler> m_handlers;
     QVector<Catcher> m_catchers;
     QSharedPointer<Error> m_error;
 };
 
 template <typename T>
-class PromiseData : public PromiseDataBase<T>
+class PromiseData : public PromiseDataBase<T, void(const T&)>
 {
-    using Handler = std::pair<QPointer<QThread>, std::function<void(const T&)> >;
+    using Handler = typename PromiseDataBase<T, void(const T&)>::Handler;
 
 public:
-    void addHandler(std::function<void(const T&)> handler)
-    {
-        QWriteLocker lock(&this->m_lock);
-        m_handlers.append({QThread::currentThread(), std::move(handler)});
-    }
-
     void resolve(T&& value)
     {
         Q_ASSERT(m_value.isNull());
@@ -440,12 +450,8 @@ public:
         this->setSettled();
     }
 
-    void notify() Q_DECL_OVERRIDE
+    void notify(const QVector<Handler>& handlers) Q_DECL_OVERRIDE
     {
-        this->m_lock.lockForWrite();
-        QVector<Handler> handlers(std::move(m_handlers));
-        this->m_lock.unlock();
-
         QSharedPointer<T> value(m_value);
         Q_ASSERT(!value.isNull());
 
@@ -458,38 +464,24 @@ public:
     }
 
 private:
-    QVector<Handler> m_handlers;
     QSharedPointer<T> m_value;
 };
 
 template <>
-class PromiseData<void> : public PromiseDataBase<void>
+class PromiseData<void> : public PromiseDataBase<void, void()>
 {
-    using Handler = std::pair<QPointer<QThread>, std::function<void()> >;
+    using Handler = typename PromiseDataBase<void, void()>::Handler;
 
 public:
-    void addHandler(std::function<void()> handler)
-    {
-        QWriteLocker lock(&m_lock);
-        m_handlers.append({QThread::currentThread(), std::move(handler)});
-    }
-
     void resolve() { setSettled(); }
 
 protected:
-    void notify() Q_DECL_OVERRIDE
+    void notify(const QVector<Handler>& handlers) Q_DECL_OVERRIDE
     {
-        this->m_lock.lockForWrite();
-        QVector<Handler> handlers(std::move(m_handlers));
-        this->m_lock.unlock();
-
         for (const auto& handler: handlers) {
             qtpromise_defer(handler.second, handler.first);
         }
     }
-
-private:
-    QVector<Handler> m_handlers;
 };
 
 } // namespace QtPromise