QMLJS: Fix exception handling in promises

If an exception is thrown inside a promise's resolve or reject
handler, the promise needs to resolve into a rejected state with the
exceptions value. The value was previously not set.

Fixes: QTBUG-78554
Change-Id: Ic22faa6ef1e519e4cae6732c69bb14f7053d13da
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/qml/jsruntime/qv4promiseobject.cpp

@@ -163,6 +163,7 @@ void ReactionHandler::executeReaction(ReactionEvent *event)
         ScopedFunctionObject reaction(scope);
         if (scope.hasException()) {
             reaction = capability->d()->reject.as<QV4::FunctionObject>();
+            result = scope.engine->catchException();
         } else {
             reaction = capability->d()->resolve.as<QV4::FunctionObject>();
         }

tests/auto/qml/qqmlpromise/data/promisehandlerthrows.qml

@@ -0,0 +1,17 @@
+import QtQuick 2.12
+
+Item {
+    id: root
+    property string errorMessage
+    Component.onCompleted: () => {
+    let prom = Promise.reject("Some error")
+        .then(
+                o => {console.log("Never reached");},
+                err => {
+                    console.log("Rethrowing err");
+                    throw err;
+                }
+             )
+        .catch(err => root.errorMessage = err)
+    }
+}

tests/auto/qml/qqmlpromise/tst_qqmlpromise.cpp

@@ -83,6 +83,7 @@ private slots:
     void then_reject_non_callable();
     void then_resolve_multiple_then();
     void promiseChain();
+    void promiseHandlerThrows();
 
 private:
     void execute_test(QString testName);
@@ -285,6 +286,17 @@ void tst_qqmlpromise::promiseChain()
 
 }
 
+void tst_qqmlpromise::promiseHandlerThrows()
+{
+    QQmlEngine engine;
+    QQmlComponent component(&engine, testFileUrl("promisehandlerthrows.qml"));
+    QVERIFY(component.isReady());
+    QTest::ignoreMessage(QtDebugMsg, "Rethrowing err");
+    QScopedPointer<QObject> root(component.create());
+    QVERIFY(root);
+    QTRY_VERIFY(root->property("errorMessage") == QLatin1String("Some error"));
+}
+
 
 QTEST_MAIN(tst_qqmlpromise)