Always cancel unfinished QPromises on destruction

If the QPromise is being destroyed, we should signal the associated
futures to stop waiting. No matter in which state the promise is, if
it's not finished, we should always cancel to avoid infinite waits.
This is also what docs state:

"The promise implicitly transitions to a canceled state on destruction
unless finish() is called beforehand by the user."

Fixes: QTBUG-101284
Pick-to: 6.3 6.2
Change-Id: I65ebfefe03b79b41afacda78a4f49938c54d8b37
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

2 files changed, 16 insertions, 8 deletions

diff --git a/src/corelib/thread/qpromise.h b/src/corelib/thread/qpromise.h
index f7abf8babc..0fd7680314 100644
--- a/src/corelib/thread/qpromise.h
+++ b/src/corelib/thread/qpromise.h
@@ -71,15 +71,9 @@ public:
     QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_MOVE_AND_SWAP(QPromise)
     ~QPromise()
     {
-        const int state = d.loadState();
-        // If QFutureInterface has no state, there is nothing to be done
-        if (state == static_cast<int>(QFutureInterfaceBase::State::NoState)) {
-            d.cleanContinuation();
-            return;
-        }
-        // Otherwise, if computation is not finished at this point, cancel
+        // If computation is not finished at this point, cancel
         // potential waits
-        if (!(state & QFutureInterfaceBase::State::Finished)) {
+        if (d.d && !(d.loadState() & QFutureInterfaceBase::State::Finished)) {
             d.cancelAndFinish(); // cancel and finalize the state
             d.cleanContinuation();
         }
diff --git a/tests/auto/corelib/thread/qpromise/tst_qpromise.cpp b/tests/auto/corelib/thread/qpromise/tst_qpromise.cpp
index 52f4daa24a..5c20287e89 100644
--- a/tests/auto/corelib/thread/qpromise/tst_qpromise.cpp
+++ b/tests/auto/corelib/thread/qpromise/tst_qpromise.cpp
@@ -64,6 +64,7 @@ private slots:
     void cancelWhenDestroyed();
 #endif
     void cancelWhenReassigned();
+    void cancelWhenDestroyedWithoutStarting();
     void finishWhenSwapped();
     void cancelWhenMoved();
     void waitUntilResumed();
@@ -505,6 +506,19 @@ void tst_QPromise::cancelWhenReassigned()
 #endif
 }
 
+void tst_QPromise::cancelWhenDestroyedWithoutStarting()
+{
+    QFuture<void> future;
+    {
+        QPromise<void> promise;
+        future = promise.future();
+    }
+    future.waitForFinished();
+    QVERIFY(!future.isStarted());
+    QVERIFY(future.isCanceled());
+    QVERIFY(future.isFinished());
+}
+
 void tst_QPromise::finishWhenSwapped()
 {
 #if !QT_CONFIG(cxx11_future)