3 files changed, 65 insertions, 6 deletions

diff --git a/src/corelib/statemachine/qstatemachine.cpp b/src/corelib/statemachine/qstatemachine.cpp
index 1fe7f96b3c..b3e91c1ac0 100644
--- a/src/corelib/statemachine/qstatemachine.cpp
+++ b/src/corelib/statemachine/qstatemachine.cpp
@@ -1335,6 +1335,28 @@ void QStateMachinePrivate::clearHistory()
     }
 }
 
+/*!
+  \internal
+
+  Registers all signal transitions whose sender object lives in another thread.
+
+  Normally, signal transitions are lazily registered (when a state becomes
+  active). But if the sender is in a different thread, the transition must be
+  registered early to keep the state machine from "dropping" signals; e.g.,
+  a second (transition-bound) signal could be emitted on the sender thread
+  before the state machine gets to process the first signal.
+*/
+void QStateMachinePrivate::registerMultiThreadedSignalTransitions()
+{
+    Q_Q(QStateMachine);
+    QList<QSignalTransition*> transitions = rootState()->findChildren<QSignalTransition*>();
+    for (int i = 0; i < transitions.size(); ++i) {
+        QSignalTransition *t = transitions.at(i);
+        if ((t->machine() == q) && t->senderObject() && (t->senderObject()->thread() != q->thread()))
+            registerSignalTransition(t);
+    }
+}
+
 void QStateMachinePrivate::_q_start()
 {
     Q_Q(QStateMachine);
@@ -1346,6 +1368,8 @@ void QStateMachinePrivate::_q_start()
     externalEventQueue.clear();
     clearHistory();
 
+    registerMultiThreadedSignalTransitions();
+
 #ifdef QSTATEMACHINE_DEBUG
     qDebug() << q << ": starting";
 #endif
@@ -1703,8 +1727,8 @@ void QStateMachinePrivate::unregisterTransition(QAbstractTransition *transition)
 void QStateMachinePrivate::maybeRegisterSignalTransition(QSignalTransition *transition)
 {
     Q_Q(QStateMachine);
-    if (((state == Running) && configuration.contains(transition->sourceState()))
-            || (transition->senderObject() && (transition->senderObject()->thread() != q->thread()))) {
+    if ((state == Running) && (configuration.contains(transition->sourceState())
+            || (transition->senderObject() && (transition->senderObject()->thread() != q->thread())))) {
         registerSignalTransition(transition);
     }
 }
@@ -1771,14 +1795,10 @@ void QStateMachinePrivate::registerSignalTransition(QSignalTransition *transitio
 
 void QStateMachinePrivate::unregisterSignalTransition(QSignalTransition *transition)
 {
-    Q_Q(QStateMachine);
     int signalIndex = QSignalTransitionPrivate::get(transition)->signalIndex;
     if (signalIndex == -1)
         return; // not registered
     const QObject *sender = QSignalTransitionPrivate::get(transition)->sender;
-    // Don't unregister if the sender is on another thread
-    if (!sender || (sender->thread() != q->thread()))
-        return;
     QSignalTransitionPrivate::get(transition)->signalIndex = -1;
 
     connectionsMutex.lock();
diff --git a/src/corelib/statemachine/qstatemachine_p.h b/src/corelib/statemachine/qstatemachine_p.h
index cf20ee31ca..c8d09586b4 100644
--- a/src/corelib/statemachine/qstatemachine_p.h
+++ b/src/corelib/statemachine/qstatemachine_p.h
@@ -175,6 +175,7 @@ public:
     void maybeRegisterSignalTransition(QSignalTransition *transition);
     void registerSignalTransition(QSignalTransition *transition);
     void unregisterSignalTransition(QSignalTransition *transition);
+    void registerMultiThreadedSignalTransitions();
 #ifndef QT_NO_STATEMACHINE_EVENTFILTER
     void maybeRegisterEventTransition(QEventTransition *transition);
     void registerEventTransition(QEventTransition *transition);
diff --git a/tests/auto/corelib/statemachine/qstatemachine/tst_qstatemachine.cpp b/tests/auto/corelib/statemachine/qstatemachine/tst_qstatemachine.cpp
index dd036641b2..15f23e05cd 100644
--- a/tests/auto/corelib/statemachine/qstatemachine/tst_qstatemachine.cpp
+++ b/tests/auto/corelib/statemachine/qstatemachine/tst_qstatemachine.cpp
@@ -209,6 +209,7 @@ private slots:
     void createSignalTransitionWhenRunning();
     void createEventTransitionWhenRunning();
     void signalTransitionSenderInDifferentThread();
+    void signalTransitionSenderInDifferentThread2();
     void signalTransitionRegistrationThreadSafety();
     void childModeConstructor();
 };
@@ -4895,6 +4896,43 @@ void tst_QStateMachine::signalTransitionSenderInDifferentThread()
     QTRY_VERIFY(thread.wait());
 }
 
+void tst_QStateMachine::signalTransitionSenderInDifferentThread2()
+{
+    QStateMachine machine;
+    QState *s1 = new QState(&machine);
+    machine.setInitialState(s1);
+
+    QState *s2 = new QState(&machine);
+    SignalEmitter emitter;
+    // At the time of the transition creation, the machine and the emitter
+    // are both in the same thread.
+    s1->addTransition(&emitter, SIGNAL(signalWithNoArg()), s2);
+
+    QFinalState *s3 = new QFinalState(&machine);
+    s2->addTransition(&emitter, SIGNAL(signalWithDefaultArg()), s3);
+
+    QThread thread;
+    // Move the machine and its states to a secondary thread, but let the
+    // SignalEmitter stay in the main thread.
+    machine.moveToThread(&thread);
+
+    thread.start();
+    QTRY_VERIFY(thread.isRunning());
+
+    QSignalSpy startedSpy(&machine, SIGNAL(started()));
+    QSignalSpy finishedSpy(&machine, SIGNAL(finished()));
+    machine.start();
+    QTRY_COMPARE(startedSpy.count(), 1);
+
+    emitter.emitSignalWithNoArg();
+    // The second emission should not get "lost".
+    emitter.emitSignalWithDefaultArg();
+    QTRY_COMPARE(finishedSpy.count(), 1);
+
+    thread.quit();
+    QTRY_VERIFY(thread.wait());
+}
+
 class SignalTransitionMutatorThread : public QThread
 {
 public: