QFutureInterface: add a warning when an existing continuation is overwritten

... and also extend the documentation to explain this case explicitly.

Fixes: QTBUG-107545
Pick-to: 6.5 6.2
Change-Id: I9414cc677b037989de60e97871485018e5c8a569
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

3 files changed, 27 insertions, 0 deletions

diff --git a/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp b/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp
index 091d98539b..cf85dab226 100644
--- a/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp
@@ -385,3 +385,17 @@ QFuture<QFuture<QFuture<int>>>> outerFuture;
 QFuture<int> unwrappedFuture = outerFuture.unwrap();
 
 //! [30]
+
+//! [31]
+QPromise<int> p;
+
+QFuture<int> f1 = p.future();
+f1.then([](int) { qDebug("first"); });
+
+QFuture<int> f2 = p.future();
+f2.then([](int) { qDebug("second"); });
+
+p.start();
+p.addResult(42);
+p.finish();
+//! [31]
diff --git a/src/corelib/thread/qfuture.qdoc b/src/corelib/thread/qfuture.qdoc
index 570f7de8e3..50ec206afd 100644
--- a/src/corelib/thread/qfuture.qdoc
+++ b/src/corelib/thread/qfuture.qdoc
@@ -75,6 +75,15 @@
     If \c testFuture gets canceled, its state is propagated to the next then(),
     which will be also canceled. So in this case \c {Block 6} will be called.
 
+    The future can have only one continuation. Consider the following example:
+
+    \snippet code/src_corelib_thread_qfuture.cpp 31
+
+    In this case \c f1 and \c f2 are effectively the same QFuture object, as
+    they share the same internal state. As a result, calling
+    \l {QFuture::}{then} on \c f2 will overwrite the continuation specified for
+    \c {f1}. So, only \c {"second"} will be printed when this code is executed.
+
     QFuture also offers ways to interact with a running computation. For
     instance, the computation can be canceled with the cancel() function. To
     suspend or resume the computation, use the setSuspended() function or one of
diff --git a/src/corelib/thread/qfutureinterface.cpp b/src/corelib/thread/qfutureinterface.cpp
index ed46052fa7..e656527e04 100644
--- a/src/corelib/thread/qfutureinterface.cpp
+++ b/src/corelib/thread/qfutureinterface.cpp
@@ -834,6 +834,10 @@ void QFutureInterfaceBase::setContinuation(std::function<void(const QFutureInter
     // store the move-only continuation, to guarantee that the associated
     // future's data stays alive.
     if (d->continuationState != QFutureInterfaceBasePrivate::Cleaned) {
+        if (d->continuation) {
+            qWarning() << "Adding a continuation to a future which already has a continuation. "
+                          "The existing continuation is overwritten.";
+        }
         d->continuation = std::move(func);
         d->continuationData = continuationFutureData;
     }