QFuture: Don't use QFutureCallOutInterface for continuations

This patch replaces the QBasicFutureWatcher that was used for
continuations with context objects with a smaller QObject-based wrapper
that works directly from the actual continuation.
The idea stays the same: In order to run continuations in the thread of
a context object, we offload the continuation invocation to the
signal-slot mechanism.
Previously, we've hooked into QFuture with QFutureCallOutInterface to
emit a signal and trigger continuation invocation. However, it is much
easier and robust to emit that signal from the continuation itself.

This sidesteps the locking issues that QFutureCallOutInterface handling
presents. QFutureCallOutInterface basically requires any consumer to
only access the QFuture after all events have been posted and the
internal mutex unlocked, i.e. on the next cycle of the event loop.

Continuations do not impose this restriction; runContinuation()
explicitly unlocks the internal mutex before calling the continuation.

This fixes a deadlock when using QFuture::then(context, ...) where
the paren future is resolved from the same thread that the context
object lives in.

Fixes: QTBUG-119406
Fixes: QTBUG-119103
Fixes: QTBUG-117918
Fixes: QTBUG-119579
Fixes: QTBUG-119810
Pick-to: 6.7 6.6
Change-Id: I112b16024cde6b6ee0e4d8127392864b813df5bc
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

1 files changed, 6 insertions, 2 deletions

diff --git a/src/corelib/thread/qfutureinterface.h b/src/corelib/thread/qfutureinterface.h
index 151584e66b..fa5e8ea7ba 100644
--- a/src/corelib/thread/qfutureinterface.h
+++ b/src/corelib/thread/qfutureinterface.h
@@ -21,6 +21,7 @@ QT_BEGIN_NAMESPACE
 
 template <typename T> class QFuture;
 class QThreadPool;
+class QFutureInterfaceBase;
 class QFutureInterfaceBasePrivate;
 class QFutureWatcherBase;
 class QFutureWatcherBasePrivate;
@@ -39,8 +40,10 @@ template<class Function, class ResultType>
 class FailureHandler;
 #endif
 
+void Q_CORE_EXPORT watchContinuationImpl(const QObject *context,
+                                         QtPrivate::QSlotObjectBase *slotObj,
+                                         QFutureInterfaceBase &fi);
 }
-class QBasicFutureWatcher;
 
 class Q_CORE_EXPORT QFutureInterfaceBase
 {
@@ -178,7 +181,8 @@ private:
     friend class QtPrivate::FailureHandler;
 #endif
 
-    friend class QBasicFutureWatcher;
+    friend Q_CORE_EXPORT void QtPrivate::watchContinuationImpl(
+            const QObject *context, QtPrivate::QSlotObjectBase *slotObj, QFutureInterfaceBase &fi);
 
     template<class T>
     friend class QPromise;