Fix QFuture::waitForFinished to wait until QFuture is started

Currently QFuture::waitForFinished() exits as soon as the future is not
in the running state. If the user calls it before
QPromise::reportStarted() is called, it will exit immediately, because
nothing is running yet. Fix the behavior to wait for the finished state.

[ChangeLog][Important Behavior Changes][QtCore] Fixed the behavior of
QFuture::waitForFinished() to wait until the future is actually in the
finished state, instead of exiting as soon as it is not in the running
state. This prevents waitForFinished() from exiting immediately, if at
the moment of calling it the future is not started yet.

Task-number: QTBUG-84867
Change-Id: I12f5e95d8200cfffa5653b6aa566a625f8320ca8
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

5 files changed, 30 insertions, 8 deletions

diff --git a/src/corelib/thread/qfuture.qdoc b/src/corelib/thread/qfuture.qdoc
index b20875c423..8235fbf190 100644
--- a/src/corelib/thread/qfuture.qdoc
+++ b/src/corelib/thread/qfuture.qdoc
@@ -404,7 +404,7 @@
 /*! \fn template <typename T> void QFuture<T>::waitForFinished()
 
     Waits for the asynchronous computation to finish (including cancel()ed
-    computations).
+    computations), i.e. until isFinished() returns \c true.
 */
 
 /*! \fn template <typename T> T QFuture<T>::result() const
diff --git a/src/corelib/thread/qfutureinterface.cpp b/src/corelib/thread/qfutureinterface.cpp
index a625e05aac..600dd9bd3f 100644
--- a/src/corelib/thread/qfutureinterface.cpp
+++ b/src/corelib/thread/qfutureinterface.cpp
@@ -422,7 +422,7 @@ void QFutureInterfaceBase::waitForResult(int resultIndex)
 void QFutureInterfaceBase::waitForFinished()
 {
     QMutexLocker lock(&d->m_mutex);
-    const bool alreadyFinished = !isRunningOrPending();
+    const bool alreadyFinished = isFinished();
     lock.unlock();
 
     if (!alreadyFinished) {
@@ -430,7 +430,7 @@ void QFutureInterfaceBase::waitForFinished()
 
         lock.relock();
 
-        while (isRunningOrPending())
+        while (!isFinished())
             d->waitCondition.wait(&d->m_mutex);
     }
 
diff --git a/src/corelib/thread/qfuturewatcher.cpp b/src/corelib/thread/qfuturewatcher.cpp
index 75f5e297e2..d59b6755a4 100644
--- a/src/corelib/thread/qfuturewatcher.cpp
+++ b/src/corelib/thread/qfuturewatcher.cpp
@@ -417,7 +417,7 @@ bool QFutureWatcherBase::isSuspended() const
 /*! \fn template <typename T> void QFutureWatcher<T>::waitForFinished()
 
     Waits for the asynchronous computation to finish (including cancel()ed
-    computations).
+    computations), i.e. until isFinished() returns \c true.
 */
 void QFutureWatcherBase::waitForFinished()
 {
diff --git a/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp b/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp
index c1b562e7f4..e6f6827f3f 100644
--- a/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp
+++ b/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp
@@ -144,6 +144,7 @@ private slots:
     void takeResultWorksForTypesWithoutDefaultCtor();
     void canceledFutureIsNotValid();
     void signalConnect();
+    void waitForFinished();
 
 private:
     using size_type = std::vector<int>::size_type;
@@ -3076,5 +3077,29 @@ void tst_QFuture::signalConnect()
     }
 }
 
+void tst_QFuture::waitForFinished()
+{
+    QFutureInterface<void> fi;
+    auto future = fi.future();
+
+    QScopedPointer<QThread> waitingThread (QThread::create([&] {
+        future.waitForFinished();
+    }));
+
+    waitingThread->start();
+
+    QVERIFY(!waitingThread->wait(200));
+    QVERIFY(!waitingThread->isFinished());
+
+    fi.reportStarted();
+    QVERIFY(!waitingThread->wait(200));
+    QVERIFY(!waitingThread->isFinished());
+
+    fi.reportFinished();
+
+    QVERIFY(waitingThread->wait());
+    QVERIFY(waitingThread->isFinished());
+}
+
 QTEST_MAIN(tst_QFuture)
 #include "tst_qfuture.moc"
diff --git a/tests/auto/corelib/thread/qpromise/snippet_qpromise.cpp b/tests/auto/corelib/thread/qpromise/snippet_qpromise.cpp
index 2975afca0f..7180baccca 100644
--- a/tests/auto/corelib/thread/qpromise/snippet_qpromise.cpp
+++ b/tests/auto/corelib/thread/qpromise/snippet_qpromise.cpp
@@ -75,11 +75,8 @@ void snippet_QPromise::basicExample()
     QPromise<int> promise;
     QFuture<int> future = promise.future();
 
-    // Note: calling reportStarted() prior to thread creation to enforce order
-    // of calls: first promise.reportStarted() then future.waitForFinished()
-    promise.reportStarted();  // notifies QFuture that the computation is started
-
     QScopedPointer<QThread> thread(QThread::create([] (QPromise<int> promise) {
+        promise.reportStarted();   // notifies QFuture that the computation is started
         promise.addResult(42);
         promise.reportFinished();  // notifies QFuture that the computation is finished
     }, std::move(promise)));