Johannes Wienke created SCXML-207:
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Summary: Race condition in transition notification for
SCXMLListeners
Key: SCXML-207
URL: https://issues.apache.org/jira/browse/SCXML-207
Project: Commons SCXML
Issue Type: Bug
Affects Versions: 2.0
Reporter: Johannes Wienke
When using a SimpleSCXMLListener we have seen impossible transitions occur in
the log output of that listener. After digging around the source code I can see
the issue that results in theses outputs.
In order to derive the event name passed as the last argument to
SCXMLListener#onTransition, the following code inside SCXMLSemanticsImpl is
used:
{code:java}
public void notifyOnTransition(final SCXMLExecutionContext exctx, final
Transition t,
final TransitionTarget target) {
EventVariable event =
(EventVariable)exctx.getScInstance().getSystemContext().getVars().get(SCXMLSystemContext.EVENT_KEY);
String eventName = event != null ? event.getName() : null;
exctx.getNotificationRegistry().fireOnTransition(t, t.getParent(), target,
t, eventName);
exctx.getNotificationRegistry().fireOnTransition(exctx.getStateMachine(),
t.getParent(), target, t, eventName);
}
{code}
When triggering events on the engine from multiple threads (which is ok
according to the online documentation), a different thread might have already
replaced the contents of the SCXMLSystemContext.EVENT_KEY context item and
hence a wrong event name might be used from this point on.
Actually, I don't even understand why the event name is looked up from the
context here at all, since is already available inside the transition.
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