QFuture: support cancellation of continuation chain through parent

This change allows canceling the chain of continuations attached to a
future through canceling the future itself at any point of execution of
the chain.

[ChangeLog][QtCore][Important Behavior Changes] The chain of
continuations attached to a future now can be cancelled through
cancelling the future itself at any point of the execution of the chain,
as it was documented. Previously canceling the future would cancel the
chain only if it was done before the chain starts executing, otherwise
the cancellation would be ignored. Now the part of the chain that wasn't
started at the moment of cancellation will be canceled.

Task-number: QTBUG-97582
Change-Id: I4c3b3c68e34d3a044243ac9a7a9ed3c38b7cb02e
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

4 files changed, 37 insertions, 4 deletions

diff --git a/src/corelib/thread/qfuture_impl.h b/src/corelib/thread/qfuture_impl.h
index 511caafdf7..d7132caa7b 100644
--- a/src/corelib/thread/qfuture_impl.h
+++ b/src/corelib/thread/qfuture_impl.h
@@ -425,7 +425,7 @@ bool Continuation<Function, ResultType, ParentResultType>::execute()
 {
     Q_ASSERT(parentFuture.isFinished());
 
-    if (parentFuture.isCanceled()) {
+    if (parentFuture.d.isChainCanceled()) {
 #ifndef QT_NO_EXCEPTIONS
         if (parentFuture.d.hasException()) {
             // If the continuation doesn't take a QFuture argument, propagate the exception
@@ -498,7 +498,7 @@ void Continuation<Function, ResultType, ParentResultType>::create(F &&func,
         }
     };
 
-    f->d.setContinuation(std::move(continuation));
+    f->d.setContinuation(std::move(continuation), p.d);
 }
 
 template<typename Function, typename ResultType, typename ParentResultType>
@@ -527,7 +527,7 @@ void Continuation<Function, ResultType, ParentResultType>::create(F &&func,
         }
     };
 
-    f->d.setContinuation(std::move(continuation));
+    f->d.setContinuation(std::move(continuation), p.d);
 }
 
 template<typename Function, typename ResultType, typename ParentResultType>
@@ -550,7 +550,7 @@ void Continuation<Function, ResultType, ParentResultType>::create(F &&func,
         });
     };
 
-    f->d.setContinuation(std::move(continuation));
+    f->d.setContinuation(std::move(continuation), p.d);
 }
 
 template<typename Function, typename ResultType, typename ParentResultType>
diff --git a/src/corelib/thread/qfutureinterface.cpp b/src/corelib/thread/qfutureinterface.cpp
index e640114bb0..7512d6174a 100644
--- a/src/corelib/thread/qfutureinterface.cpp
+++ b/src/corelib/thread/qfutureinterface.cpp
@@ -835,7 +835,17 @@ void QFutureInterfaceBasePrivate::setState(QFutureInterfaceBase::State newState)
 
 void QFutureInterfaceBase::setContinuation(std::function<void(const QFutureInterfaceBase &)> func)
 {
+    setContinuation(func, nullptr);
+}
+
+void QFutureInterfaceBase::setContinuation(std::function<void(const QFutureInterfaceBase &)> func,
+                                           QFutureInterfaceBasePrivate *continuationFutureData)
+{
     QMutexLocker lock(&d->continuationMutex);
+
+    if (continuationFutureData)
+        continuationFutureData->parentData = d;
+
     // If the state is ready, run continuation immediately,
     // otherwise save it for later.
     if (isFinished()) {
@@ -855,6 +865,24 @@ void QFutureInterfaceBase::runContinuation() const
     }
 }
 
+bool QFutureInterfaceBase::isChainCanceled() const
+{
+    if (isCanceled())
+        return true;
+
+    auto parent = d->parentData;
+    while (parent) {
+        // If the future is in Canceled state because it had an exception, we want to
+        // continue checking the chain of parents for cancellation, otherwise if the exception
+        // is handeled inside the chain, it won't be interrupted even though cancellation has
+        // been requested.
+        if ((parent->state.loadRelaxed() & Canceled) && !parent->hasException)
+            return true;
+        parent = parent->parentData;
+    }
+    return false;
+}
+
 void QFutureInterfaceBase::setLaunchAsync(bool value)
 {
     d->launchAsync = value;
diff --git a/src/corelib/thread/qfutureinterface.h b/src/corelib/thread/qfutureinterface.h
index e813031d59..3130c91420 100644
--- a/src/corelib/thread/qfutureinterface.h
+++ b/src/corelib/thread/qfutureinterface.h
@@ -182,6 +182,8 @@ public:
     template<typename T>
     static QFutureInterfaceBase get(const QFuture<T> &future);  // implemented in qfuture.h
 
+    bool isChainCanceled() const;
+
 protected:
     // ### Qt 7: remove const from refT/derefT
     bool refT() const noexcept;
@@ -212,6 +214,8 @@ private:
 
 protected:
     void setContinuation(std::function<void(const QFutureInterfaceBase &)> func);
+    void setContinuation(std::function<void(const QFutureInterfaceBase &)> func,
+                         QFutureInterfaceBasePrivate *continuationFutureData);
     void runContinuation() const;
 
     void setLaunchAsync(bool value);
diff --git a/src/corelib/thread/qfutureinterface_p.h b/src/corelib/thread/qfutureinterface_p.h
index c92f8e2230..fd7f34826d 100644
--- a/src/corelib/thread/qfutureinterface_p.h
+++ b/src/corelib/thread/qfutureinterface_p.h
@@ -190,6 +190,7 @@ public:
     QThreadPool *m_pool = nullptr;
     // Wrapper for continuation
     std::function<void(const QFutureInterfaceBase &)> continuation;
+    QFutureInterfaceBasePrivate *parentData = nullptr;
 
     RefCount refCount = 1;
     QAtomicInt state; // reads and writes can happen unprotected, both must be atomic