NPE when calling Event.getProperty() - Race of two concurrent events with the same topic

[Cewin-91]

• I verified I can reproduce this issue against 2026-06

Problem

!MESSAGE java.lang.NullPointerException: Cannot invoke
"org.osgi.service.event.Event.getProperty(String)" because "event" is null

org.eclipse.e4.core.di.InjectionException: java.lang.NullPointerException: ...
Caused by: java.lang.NullPointerException
at org.eclipse.e4.ui.workbench.addons.perspectiveswitcher.PerspectiveSwitcher.handleLabelEvent(PerspectiveSwitcher.java:197)

EventObjectSupplier.get(...) reads its currentEvents map (keyed by topic only) via an unsynchronized check-then-act:

if (!currentEvents.containsKey(topic)) {
    return IInjector.NOT_A_VALUE;
}
return currentEvents.get(topic).getProperty(DATA);   // entry may have been removed meanwhile -> NPE

Steps to reproduce

DIEventHandler.handleEvent adds/removes that entry non-atomically around every delivery, so two concurrent deliveries on the same topic race. The attached JUnit test drives the production EventObjectSupplier directly (no OSGi, no injector) — one thread repeatedly does addCurrentEvent/removeCurrentEvent, the main thread repeatedly calls get(...) for a non-Event parameter type, which hits the branch that dereferences the (possibly just-removed) map entry:
EventObjectSupplierRaceTest.java

[iloveeclipse]

Seen similar errors in our regression tests

!ENTRY org.eclipse.ui 4 0 2026-09-18 18:54:17.408
!MESSAGE Unhandled event loop exception
!STACK 0
org.eclipse.e4.core.di.InjectionException: java.lang.NullPointerException: Cannot invoke "org.osgi.service.event.Event.getProperty(String)" because "event" is null
	at org.eclipse.e4.core.internal.di.MethodRequestor.execute(MethodRequestor.java:68)
	at org.eclipse.swt.widgets.Synchronizer.syncExec(Synchronizer.java:182)
	at org.eclipse.ui.internal.UISynchronizer.syncExec(UISynchronizer.java:136)
	at org.eclipse.swt.widgets.Display.syncExec(Display.java:5995)
	at org.eclipse.e4.ui.workbench.swt.DisplayUISynchronize.syncExec(DisplayUISynchronize.java:34)
	at org.eclipse.e4.ui.internal.di.UIEventObjectSupplier$UIEventHandler.handleEvent(UIEventObjectSupplier.java:65)
	at org.eclipse.equinox.internal.event.EventHandlerWrapper.handleEvent(EventHandlerWrapper.java:206)
	at org.eclipse.equinox.internal.event.EventHandlerTracker.dispatchEvent(EventHandlerTracker.java:201)
	at org.eclipse.equinox.internal.event.EventHandlerTracker.dispatchEvent(EventHandlerTracker.java:1)
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:230)
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151)
	at org.eclipse.equinox.internal.event.EventAdminImpl.dispatchEvent(EventAdminImpl.java:132)
	at org.eclipse.equinox.internal.event.EventAdminImpl.sendEvent(EventAdminImpl.java:73)
	at org.eclipse.equinox.internal.event.EventComponent.sendEvent(EventComponent.java:48)
	at org.eclipse.e4.ui.services.internal.events.EventBroker.send(EventBroker.java:55)
	at org.eclipse.e4.ui.internal.workbench.UIEventPublisher.notifyChanged(UIEventPublisher.java:61)
	at org.eclipse.emf.common.notify.impl.BasicNotifierImpl.eNotify(BasicNotifierImpl.java:424)
	at org.eclipse.emf.ecore.util.EcoreEList.dispatchNotification(EcoreEList.java:248)
	at org.eclipse.emf.common.notify.impl.NotifyingListImpl.addUnique(NotifyingListImpl.java:304)
	at org.eclipse.emf.common.util.AbstractEList.add(AbstractEList.java:304)
	at org.eclipse.ui.internal.progress.WorkbenchSiteProgressService.showBusy(WorkbenchSiteProgressService.java:361)
	at org.eclipse.ui.internal.progress.WorkbenchSiteProgressService$SiteUpdateJob.runInUIThread(WorkbenchSiteProgressService.java:118)
	at org.eclipse.ui.progress.UIJob.lambda$0(UIJob.java:148)
	at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:40)
	at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:131)
	at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:5080)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:4536)
	at org.eclipse.jface.operation.ModalContext$ModalContextThread.block(ModalContext.java:164)
	at org.eclipse.jface.operation.ModalContext.run(ModalContext.java:360)
	at org.eclipse.jface.dialogs.ProgressMonitorDialog.run(ProgressMonitorDialog.java:472)
	at org.eclipse.ui.internal.progress.ProgressMonitorJobsDialog.run(ProgressMonitorJobsDialog.java:231)
	at org.eclipse.ui.internal.progress.ProgressManager.lambda$26(ProgressManager.java:921)
	at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:69)
	at org.eclipse.ui.internal.progress.ProgressManager.busyCursorWhile(ProgressManager.java:952)
	at org.eclipse.ui.internal.progress.ProgressManager.busyCursorWhile(ProgressManager.java:930)
	at org.eclipse.debug.internal.ui.DebugUIPlugin.launchInForeground(DebugUIPlugin.java:1137)
	at org.eclipse.debug.ui.DebugUITools.launch(DebugUITools.java:967)
	at org.eclipse.debug.ui.DebugUITools.launch(DebugUITools.java:758)

... LOT of Unit 5 related code we don't care ...

Caused by: java.lang.NullPointerException: Cannot invoke "org.osgi.service.event.Event.getProperty(String)" because "event" is null
	at org.eclipse.e4.ui.workbench.swt.util.BindingProcessingAddon.subscribeAppElemTopicTags(BindingProcessingAddon.java:401)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
	at java.base/java.lang.reflect.Method.invoke(Method.java:565)
	at org.eclipse.e4.core.internal.di.MethodRequestor.execute(MethodRequestor.java:58)
	... 182 more

[iloveeclipse]

@Cewin-91 : I plan to provide a fix for this ticket. Your test has no file header, and I'm not sure whether I can use it & add to the regression tests. Is it OK for me to reuse it?

[iloveeclipse]

@Cewin-91 : I plan to provide a fix for this ticket. Your test has no file header, and I'm not sure whether I can use it & add to the regression tests. Is it OK for me to reuse it?

Ignore this. I've asked AI to generate the test, and it looks good, nothing in needed now from your end except you might want to review / test the fix in #2975.