statemachine: Really fix signal transition handling in multi-threaded setup

Commit f9a17d7f0f02f7af849afdf653a763ffdaf78a1b fixed it for the case
where the sender object is in a different thread at transition setup
time. However, it still didn't work if either the sender object or the
state machine was moved to a different thread at some later time,
before the machine was started.

Therefore: Bite the sour grape and traverse all the machine's
transitions when the machine is being started, registering those
signal transitions whose sender objects are in other threads.

This will increase the machine's startup time (proportional to the
number of transitions), but at least it works in all known scenarios,
meaning we don't have to document weird restrictions regarding the
order in which the user's operations have to be done.

Task-number: QTBUG-19789
Change-Id: I5f1dd1321994e49635f52be65cf56d2678ed1253
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>

2 files changed, 27 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);