Store std::exception_ptr in QUnhandledException

For historical reasons Qt Concurrent reports QUnhandledException in
case if an exception that is not derived from QException is thrown
from a worker thread. Changing this behavior may not be a good idea,
since the existing user code may rely on it. Changed QUnhandledException
to wrap the std::exception_ptr to the actual exception, so that the
users can obtain the information about the thrown exception if needed.

[ChangeLog][QtCore][QUnhandledException] Improved QUnhandledException to
store the std::exception_ptr to the actual exception thrown from a
QtCocnurrent worker thread.

Change-Id: I30e7c1d3e01aff6e1ed9938c421da0a888f12066
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/concurrent/qtconcurrentrunbase.h

@@ -114,7 +114,7 @@ public:
         } catch (QException &e) {
             promise.reportException(e);
         } catch (...) {
-            promise.reportException(QUnhandledException());
+            promise.reportException(QUnhandledException(std::current_exception()));
         }
 #endif
 

src/concurrent/qtconcurrentthreadengine.cpp

@@ -196,7 +196,7 @@ void ThreadEngineBase::startBlocking()
     } catch (QException &e) {
         handleException(e);
     } catch (...) {
-        handleException(QUnhandledException());
+        handleException(QUnhandledException(std::current_exception()));
     }
 #endif
 
@@ -325,7 +325,7 @@ void ThreadEngineBase::run() // implements QRunnable.
     } catch (QException &e) {
         handleException(e);
     } catch (...) {
-        handleException(QUnhandledException());
+        handleException(QUnhandledException(std::current_exception()));
     }
 #endif
     threadExit();

src/corelib/doc/snippets/code/src_corelib_thread_qexception.cpp

@@ -83,3 +83,19 @@ void MyException::raise() const { throw *this; }
 MyException *MyException::clone() const { return new MyException(*this); }
 
 //! [3]
+
+//! [4]
+
+try {
+    auto f = QtConcurrent::run([] { throw MyException {}; });
+    // ...
+} catch (const QUnhandledException &e) {
+    try {
+        if (e.exception())
+            std::rethrow_exception(e.exception());
+    } catch (const MyException &ex) {
+        // Process 'ex'
+    }
+}
+
+//! [4]

src/corelib/thread/qexception.cpp

@@ -62,7 +62,7 @@ QT_BEGIN_NAMESPACE
     \snippet code/src_corelib_thread_qexception.cpp 1
 
     If you throw an exception that is not a subclass of QException,
-    the Qt functions will throw a QUnhandledException
+    the \l{Qt Concurrent} functions will throw a QUnhandledException
     in the receiver thread.
 
     When using QFuture, transferred exceptions will be thrown when calling the following functions:
@@ -92,12 +92,18 @@ QT_BEGIN_NAMESPACE
     \class QUnhandledException
     \inmodule QtCore
 
-    \brief The UnhandledException class represents an unhandled exception in a worker thread.
+    \brief The QUnhandledException class represents an unhandled exception in a
+    Qt Concurrent worker thread.
     \since 5.0
 
     If a worker thread throws an exception that is not a subclass of QException,
-    the Qt functions will throw a QUnhandledException
+    the \l{Qt Concurrent} functions will throw a QUnhandledException on the receiver
-    on the receiver thread side.
+    thread side. The information about the actual exception that has been thrown
+    will be saved in the QUnhandledException class and can be obtained using the
+    exception() method. For example, you can process the exception held by
+    QUnhandledException in the following way:
+
+    \snippet code/src_corelib_thread_qexception.cpp 4
 
     Inheriting from this class is not supported.
 */
@@ -127,6 +133,74 @@ QException *QException::clone() const
     return new QException(*this);
 }
 
+class QUnhandledExceptionPrivate : public QSharedData
+{
+public:
+    QUnhandledExceptionPrivate(std::exception_ptr exception) noexcept : exceptionPtr(exception) { }
+    std::exception_ptr exceptionPtr;
+};
+
+/*!
+    \fn QUnhandledException::QUnhandledException(std::exception_ptr exception = nullptr) noexcept
+    \since 6.0
+
+    Constructs a new QUnhandledException object. Saves the pointer to the actual
+    exception object if \a exception is passed.
+
+    \sa exception()
+*/
+QUnhandledException::QUnhandledException(std::exception_ptr exception) noexcept
+    : d(new QUnhandledExceptionPrivate(exception))
+{
+}
+
+/*!
+    Move-constructs a QUnhandledException, making it point to the same
+    object as \a other was pointing to.
+*/
+QUnhandledException::QUnhandledException(QUnhandledException &&other) noexcept
+    : d(std::exchange(other.d, {}))
+{
+}
+
+/*!
+    Constructs a QUnhandledException object as a copy of \a other.
+*/
+QUnhandledException::QUnhandledException(const QUnhandledException &other) noexcept
+    : d(other.d)
+{
+}
+
+/*!
+    Assigns \a other to this QUnhandledException object and returns a reference
+    to this QUnhandledException object.
+*/
+QUnhandledException &QUnhandledException::operator=(const QUnhandledException &other) noexcept
+{
+    d = other.d;
+    return *this;
+}
+
+/*!
+    \fn void QUnhandledException::swap(QUnhandledException &other)
+    \since 6.0
+
+    Swaps this QUnhandledException with \a other. This function is very fast and
+    never fails.
+*/
+
+/*!
+    \since 6.0
+
+    Returns a \l{https://en.cppreference.com/w/cpp/error/exception_ptr}{pointer} to
+    the actual exception that has been saved in this QUnhandledException. Returns a
+    \c null pointer, if it does not point to an exception object.
+*/
+std::exception_ptr QUnhandledException::exception() const
+{
+    return d->exceptionPtr;
+}
+
 QUnhandledException::~QUnhandledException() noexcept
 {
 }

src/corelib/thread/qexception.h

@@ -62,12 +62,28 @@ public:
     virtual QException *clone() const;
 };
 
-class Q_CORE_EXPORT QUnhandledException : public QException
+class QUnhandledExceptionPrivate;
+class Q_CORE_EXPORT QUnhandledException final : public QException
 {
 public:
-    ~QUnhandledException() noexcept;
+    QUnhandledException(std::exception_ptr exception = nullptr) noexcept;
+    ~QUnhandledException() noexcept override;
+
+    QUnhandledException(QUnhandledException &&other) noexcept;
+    QUnhandledException(const QUnhandledException &other) noexcept;
+
+    void swap(QUnhandledException &other) noexcept { qSwap(d, other.d); }
+
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QUnhandledException)
+    QUnhandledException &operator=(const QUnhandledException &other) noexcept;
+
     void raise() const override;
     QUnhandledException *clone() const override;
+
+    std::exception_ptr exception() const;
+
+private:
+    QSharedDataPointer<QUnhandledExceptionPrivate> d;
 };
 
 namespace QtPrivate {

tests/auto/concurrent/qtconcurrentrun/tst_qtconcurrentrun.cpp

@@ -48,6 +48,7 @@ private slots:
     void recursive();
 #ifndef QT_NO_EXCEPTIONS
     void exceptions();
+    void unhandledException();
 #endif
     void functor();
     void lambda();
@@ -890,6 +891,25 @@ void tst_QtConcurrentRun::exceptions()
 
     QVERIFY2(caught, "did not get exception");
 }
+
+void tst_QtConcurrentRun::unhandledException()
+{
+    struct Exception {};
+    bool caught = false;
+    try {
+        auto f = QtConcurrent::run([] { throw Exception {}; });
+        f.waitForFinished();
+    } catch (const QUnhandledException &e) {
+        try {
+            if (e.exception())
+                std::rethrow_exception(e.exception());
+        } catch (const Exception &) {
+            caught = true;
+        }
+    }
+
+    QVERIFY(caught);
+}
 #endif
 
 // Compiler supports decltype

tests/auto/concurrent/qtconcurrentthreadengine/tst_qtconcurrentthreadengine.cpp

@@ -430,10 +430,10 @@ public:
     QThread *blockThread;
 };
 
-class UnrelatedExceptionThrower : public ThreadEngine<void>
+class IntExceptionThrower : public ThreadEngine<void>
 {
 public:
-    UnrelatedExceptionThrower(QThread *blockThread = nullptr)
+    IntExceptionThrower(QThread *blockThread = nullptr)
     : ThreadEngine(QThreadPool::globalInstance())
     {
         this->blockThread = blockThread;
@@ -480,7 +480,7 @@ void tst_QtConcurrentThreadEngine::exceptions()
     {
         bool caught = false;
         try  {
-            QtConcurrentExceptionThrower e(0);
+            QtConcurrentExceptionThrower e(nullptr);
             e.startBlocking();
         } catch (const QException &) {
             caught = true;
@@ -492,11 +492,17 @@ void tst_QtConcurrentThreadEngine::exceptions()
     {
         bool caught = false;
         try  {
-            UnrelatedExceptionThrower *e = new UnrelatedExceptionThrower();
+            IntExceptionThrower *e = new IntExceptionThrower();
             QFuture<void> f = e->startAsynchronously();
             f.waitForFinished();
-        } catch (const QUnhandledException &) {
+        } catch (const QUnhandledException &ex) {
-            caught = true;
+            // Make sure the exception info is not lost
+            try {
+                if (ex.exception())
+                    std::rethrow_exception(ex.exception());
+            } catch (int) {
+                caught = true;
+            }
         }
         QVERIFY2(caught, "did not get exception");
     }
@@ -506,10 +512,16 @@ void tst_QtConcurrentThreadEngine::exceptions()
     {
         bool caught = false;
         try  {
-            UnrelatedExceptionThrower e(QThread::currentThread());
+            IntExceptionThrower e(QThread::currentThread());
             e.startBlocking();
-        } catch (const QUnhandledException &) {
+        } catch (const QUnhandledException &ex) {
-            caught = true;
+            // Make sure the exception info is not lost
+            try {
+                if (ex.exception())
+                    std::rethrow_exception(ex.exception());
+            } catch (int) {
+                caught = true;
+            }
         }
         QVERIFY2(caught, "did not get exception");
     }
@@ -518,10 +530,16 @@ void tst_QtConcurrentThreadEngine::exceptions()
     {
         bool caught = false;
         try  {
-            UnrelatedExceptionThrower e(0);
+            IntExceptionThrower e(nullptr);
             e.startBlocking();
-        } catch (const QUnhandledException &) {
+        } catch (const QUnhandledException &ex) {
-            caught = true;
+            // Make sure the exception info is not lost
+            try {
+                if (ex.exception())
+                    std::rethrow_exception(ex.exception());
+            } catch (int) {
+                caught = true;
+            }
         }
         QVERIFY2(caught, "did not get exception");
     }