Store QFuture exceptions as std::exception_ptr

Replaced the internal ExceptionHolder for storing QException* by
std::exception_ptr. This will allow to report and store exceptions
of types that are not derived from QException.

Task-number: QTBUG-81588
Change-Id: I96be919d8289448b3e608310e51a16cebc586301
Reviewed-by: Vitaly Fanaskov <vitaly.fanaskov@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/concurrent/qtconcurrentthreadengine.cpp

@@ -324,7 +324,7 @@ void ThreadEngineBase::handleException(const QException &exception)
 {
     if (futureInterface)
         futureInterface->reportException(exception);
-    else
+    else if (!exceptionStore.hasException())
         exceptionStore.setException(exception);
 }
 #endif

src/corelib/thread/qexception.cpp

@@ -158,65 +158,38 @@ QUnhandledException *QUnhandledException::clone() const
 
 namespace QtPrivate {
 
-class Base : public QSharedData
-{
-public:
-    Base(QException *exception)
-    : exception(exception), hasThrown(false) { }
-    ~Base() { delete exception; }
-
-    QException *exception;
-    bool hasThrown;
-};
-
-ExceptionHolder::ExceptionHolder(QException *exception)
-: base(exception ? new Base(exception) : nullptr) {}
-
-ExceptionHolder::ExceptionHolder(const ExceptionHolder &other)
-: base(other.base)
-{}
-
-void ExceptionHolder::operator=(const ExceptionHolder &other)
-{
-    base = other.base;
-}
-
-ExceptionHolder::~ExceptionHolder()
-{}
-
-QException *ExceptionHolder::exception() const
-{
-    if (!base)
-        return nullptr;
-    return base->exception;
-}
-
 void ExceptionStore::setException(const QException &e)
 {
-    if (hasException() == false)
-        exceptionHolder = ExceptionHolder(e.clone());
+    Q_ASSERT(!hasException());
+    try {
+        e.raise();
+    } catch (...) {
+        exceptionHolder = std::current_exception();
+    }
+}
+
+void ExceptionStore::setException(std::exception_ptr e)
+{
+    Q_ASSERT(!hasException());
+    exceptionHolder = e;
 }
 
 bool ExceptionStore::hasException() const
 {
-    return (exceptionHolder.exception() != nullptr);
+    return !!exceptionHolder;
 }
 
-ExceptionHolder ExceptionStore::exception()
+std::exception_ptr ExceptionStore::exception() const
 {
     return exceptionHolder;
 }
 
 void ExceptionStore::throwPossibleException()
 {
-    if (hasException() ) {
-        exceptionHolder.base->hasThrown = true;
-        exceptionHolder.exception()->raise();
-    }
+    if (hasException())
+        std::rethrow_exception(exceptionHolder);
 }
 
-bool ExceptionStore::hasThrown() const { return exceptionHolder.base->hasThrown; }
-
 } // namespace QtPrivate
 
 #endif //Q_CLANG_QDOC

src/corelib/thread/qexception.h

@@ -84,27 +84,15 @@ public:
 
 namespace QtPrivate {
 
-class Base;
-class Q_CORE_EXPORT ExceptionHolder
-{
-public:
-    ExceptionHolder(QException *exception = nullptr);
-    ExceptionHolder(const ExceptionHolder &other);
-    void operator=(const ExceptionHolder &other); // ### Qt6: copy-assign operator shouldn't return void. Remove this method and the copy-ctor, they are unneeded.
-    ~ExceptionHolder();
-    QException *exception() const;
-    QExplicitlySharedDataPointer<Base> base;
-};
-
 class Q_CORE_EXPORT ExceptionStore
 {
 public:
     void setException(const QException &e);
+    void setException(std::exception_ptr e);
     bool hasException() const;
-    ExceptionHolder exception();
+    std::exception_ptr exception() const;
     void throwPossibleException();
-    bool hasThrown() const;
-    ExceptionHolder exceptionHolder;
+    std::exception_ptr exceptionHolder;
 };
 
 } // namespace QtPrivate

src/corelib/thread/qfuture_impl.h

@@ -226,10 +226,8 @@ void Continuation<Function, ResultType, ParentResultType>::runFunction()
             }
         }
 #ifndef QT_NO_EXCEPTIONS
-    } catch (QException &e) {
-        promise.reportException(e);
     } catch (...) {
-        promise.reportException(QUnhandledException());
+        promise.reportException(std::current_exception());
     }
 #endif
     promise.reportFinished();
@@ -249,8 +247,7 @@ bool Continuation<Function, ResultType, ParentResultType>::execute()
             // interrupt the continuation chain, so don't report anything yet.
             if constexpr (!std::is_invocable_v<std::decay_t<Function>, QFuture<ParentResultType>>) {
                 promise.reportStarted();
-                const QException *e = parentFuture.d.exceptionStore().exception().exception();
-                promise.reportException(*e);
+                promise.reportException(parentFuture.d.exceptionStore().exception());
                 promise.reportFinished();
                 return false;
             }

src/corelib/thread/qfutureinterface.cpp

@@ -282,6 +282,15 @@ void QFutureInterfaceBase::reportCanceled()
 
 #ifndef QT_NO_EXCEPTIONS
 void QFutureInterfaceBase::reportException(const QException &exception)
+{
+    try {
+        exception.raise();
+    } catch (...) {
+        reportException(std::current_exception());
+    }
+}
+
+void QFutureInterfaceBase::reportException(std::exception_ptr exception)
 {
     QMutexLocker locker(&d->m_mutex);
     if (d->state.loadRelaxed() & (Canceled|Finished))

src/corelib/thread/qfutureinterface.h

@@ -90,6 +90,7 @@ public:
     void reportCanceled();
 #ifndef QT_NO_EXCEPTIONS
     void reportException(const QException &e);
+    void reportException(std::exception_ptr e);
 #endif
     void reportResultsReady(int beginIndex, int endIndex);
 

tests/auto/corelib/thread/qfuture/tst_qfuture.cpp

@@ -1418,6 +1418,23 @@ QFuture<void> createDerivedExceptionFuture()
     return f;
 }
 
+struct TestException
+{
+};
+
+QFuture<int> createCustomExceptionFuture()
+{
+    QFutureInterface<int> i;
+    i.reportStarted();
+    QFuture<int> f = i.future();
+    int r = 0;
+    i.reportResult(r);
+    auto exception = std::make_exception_ptr(TestException());
+    i.reportException(exception);
+    i.reportFinished();
+    return f;
+}
+
 void tst_QFuture::exceptions()
 {
     // test throwing from waitForFinished
@@ -1502,6 +1519,18 @@ void tst_QFuture::exceptions()
         }
         QVERIFY(caught);
     }
+
+    // Custom exceptions
+    {
+        QFuture<int> f = createCustomExceptionFuture();
+        bool caught = false;
+        try {
+            f.result();
+        } catch (const TestException &) {
+            caught = true;
+        }
+        QVERIFY(caught);
+    }
 }
 
 class MyClass
@@ -2040,46 +2069,87 @@ void tst_QFuture::thenOnExceptionFuture()
     }
 }
 
+template<class Exception, bool hasTestMsg = false>
+QFuture<void> createExceptionContinuation(QtFuture::Launch policy = QtFuture::Launch::Sync)
+{
+    QFutureInterface<void> promise;
+
+    auto then = promise.future().then(policy, [] {
+        if constexpr (hasTestMsg)
+            throw Exception("TEST");
+        else
+            throw Exception();
+    });
+
+    promise.reportStarted();
+    promise.reportFinished();
+
+    return then;
+}
+
 void tst_QFuture::thenThrows()
 {
-    // Continuation throws an exception
+    // Continuation throws a QException
     {
-        QFutureInterface<void> promise;
-
-        QFuture<void> then = promise.future().then([]() { throw QException(); });
-
-        promise.reportStarted();
-        promise.reportFinished();
+        auto future = createExceptionContinuation<QException>();
 
         bool caught = false;
         try {
-            then.waitForFinished();
-        } catch (QException &) {
+            future.waitForFinished();
+        } catch (const QException &) {
             caught = true;
         }
         QVERIFY(caught);
     }
 
+    // Continuation throws an exception derived from QException
+    {
+        auto future = createExceptionContinuation<DerivedException>();
+
+        bool caught = false;
+        try {
+            future.waitForFinished();
+        } catch (const QException &) {
+            caught = true;
+        } catch (const std::exception &) {
+            QFAIL("The exception should be caught by the above catch block.");
+        }
+
+        QVERIFY(caught);
+    }
+
+    // Continuation throws std::exception
+    {
+        auto future = createExceptionContinuation<std::runtime_error, true>();
+
+        bool caught = false;
+        try {
+            future.waitForFinished();
+        } catch (const QException &) {
+            QFAIL("The exception should be caught by the below catch block.");
+        } catch (const std::exception &e) {
+            QCOMPARE(e.what(), "TEST");
+            caught = true;
+        }
+
+        QVERIFY(caught);
+    }
+
     // Same with QtFuture::Launch::Async
     {
-        QFutureInterface<void> promise;
-
-        QFuture<void> then =
-                promise.future().then(QtFuture::Launch::Async, []() { throw QException(); });
-
-        promise.reportStarted();
-        promise.reportFinished();
+        auto future = createExceptionContinuation<QException>(QtFuture::Launch::Async);
 
         bool caught = false;
         try {
-            then.waitForFinished();
-        } catch (QException &) {
+            future.waitForFinished();
+        } catch (const QException &) {
             caught = true;
         }
         QVERIFY(caught);
     }
 }
-#endif
+
+#endif // QT_NO_EXCEPTIONS
 
 void tst_QFuture::testSingleResult(const UniquePtr &p)
 {