ConcurrentModificationExceptions from UiInternals

[MSCCSS]

Description of the bug

When interacting with the UI over multiple threads, the UiInternals class sometimes throws ConcurrentModificationExceptions. These even occur within the access/accessSyncronously Methods (first and second stacktrace).

Stacktrace 1:
java.util.ConcurrentModificationException: null at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:1023) at java.util.LinkedHashMap$LinkedKeyIterator.next(LinkedHashMap.java:1046) at java.util.Spliterators$IteratorSpliterator.tryAdvance(Spliterators.java:1950) at java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:129) at java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:527) at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:513) at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) at java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:230) at java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:196) at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) at java.util.stream.ReferencePipeline.anyMatch(ReferencePipeline.java:632) at com.vaadin.flow.component.internal.UIInternals.isDirty(UIInternals.java:1268) at com.vaadin.flow.component.UI.push(UI.java:757) at com.vaadin.flow.server.VaadinSession.unlock(VaadinSession.java:769) at com.vaadin.flow.server.VaadinSession.accessSynchronously(VaadinSession.java:1020)

Stacktrace 2:
java.util.ConcurrentModificationException: null at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:1023) at java.util.LinkedHashMap$LinkedKeyIterator.next(LinkedHashMap.java:1046) at java.util.Spliterators$IteratorSpliterator.tryAdvance(Spliterators.java:1950) at java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:129) at java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:527) at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:513) at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) at java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:230) at java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:196) at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) at java.util.stream.ReferencePipeline.anyMatch(ReferencePipeline.java:632) at com.vaadin.flow.component.internal.UIInternals.isDirty(UIInternals.java:1268) at com.vaadin.flow.component.UI.push(UI.java:757) at com.vaadin.flow.server.VaadinSession.unlock(VaadinSession.java:769) at com.vaadin.flow.server.VaadinService.ensureAccessQueuePurged(VaadinService.java:2352) at com.vaadin.flow.server.VaadinService.accessSession(VaadinService.java:2319) at com.vaadin.flow.server.VaadinSession.access(VaadinSession.java:1065) at com.vaadin.flow.component.UI.access(UI.java:574) at com.vaadin.flow.component.UI.access(UI.java:557)

Stacktrace 3:
java.util.ConcurrentModificationException: null at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:1023) at java.util.LinkedHashMap$LinkedKeyIterator.next(LinkedHashMap.java:1046) at java.lang.Iterable.forEach(Iterable.java:74) at com.vaadin.flow.component.internal.UIInternals.dumpPendingJavaScriptInvocations(UIInternals.java:645) at com.vaadin.flow.server.communication.UidlWriter.createUidl(UidlWriter.java:187) at com.vaadin.flow.server.communication.UidlRequestHandler.createUidl(UidlRequestHandler.java:185) at com.vaadin.flow.server.communication.UidlRequestHandler.writeUidl(UidlRequestHandler.java:174) at com.vaadin.flow.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:137) at com.vaadin.flow.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:63) at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1879) at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:398) at com.vaadin.cdi.CdiVaadinServlet.service(CdiVaadinServlet.java:66) at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:614) at io.undertow.servlet.handlers.ServletHandler.handleRequest(ServletHandler.java:74)

Stacktrace 4:
java.util.ConcurrentModificationException: null at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:1023) at java.util.LinkedHashMap$LinkedKeyIterator.next(LinkedHashMap.java:1046) at java.util.Spliterators$IteratorSpliterator.tryAdvance(Spliterators.java:1950) at java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:129) at java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:527) at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:513) at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) at java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:230) at java.util.stream.MatchOps$MatchOp.evaluateSequential(MatchOps.java:196) at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) at java.util.stream.ReferencePipeline.anyMatch(ReferencePipeline.java:632) at com.vaadin.flow.component.internal.UIInternals.containsPendingJavascript(UIInternals.java:721) at com.vaadin.flow.component.UI.replaceStateIfDiffersAndNoReplacePending(UI.java:1926) at com.vaadin.flow.component.UI.browserNavigate(UI.java:1907) at com.vaadin.flow.component.ComponentEventBus.fireEventForListener(ComponentEventBus.java:244) at com.vaadin.flow.component.ComponentEventBus.handleDomEvent(ComponentEventBus.java:501) at com.vaadin.flow.component.ComponentEventBus.lambda$addDomTrigger$dd1b7957$1(ComponentEventBus.java:303) at com.vaadin.flow.internal.nodefeature.ElementListenerMap.lambda$fireEvent$2(ElementListenerMap.java:475) at java.util.ArrayList.forEach(ArrayList.java:1596) at com.vaadin.flow.internal.nodefeature.ElementListenerMap.fireEvent(ElementListenerMap.java:475) at com.vaadin.flow.server.communication.rpc.EventRpcHandler.handleNode(EventRpcHandler.java:62) at com.vaadin.flow.server.communication.rpc.AbstractRpcInvocationHandler.handle(AbstractRpcInvocationHandler.java:79) at com.vaadin.flow.server.communication.ServerRpcHandler.handleInvocationData(ServerRpcHandler.java:568) at com.vaadin.flow.server.communication.ServerRpcHandler.lambda$handleInvocations$6(ServerRpcHandler.java:549) at java.util.ArrayList.forEach(ArrayList.java:1596) at com.vaadin.flow.server.communication.ServerRpcHandler.handleInvocations(ServerRpcHandler.java:549) at com.vaadin.flow.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:376) at com.vaadin.flow.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:136) at com.vaadin.flow.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:63) at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1879) at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:398) at com.vaadin.cdi.CdiVaadinServlet.service(CdiVaadinServlet.java:66) at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:614)

Expected behavior

UiInternals should never throw ConcurrentModificationException.

Minimal reproducible example

Not consistently reproducible. Seems to occur when interacting with a page while a non-ui thread calls Ui.access/accessSynchronously.

Versions

• Vaadin / Flow version: 24.9.5
• Java version: 21

[Legioth]

These even occur within the access/accessSyncronously Methods (first and second stacktrace)

The issue in 99% of all cases is that there's one thread that doesn't do proper lock that triggers the issue and then the issue is detected by another thread that does proper locking.

There is probably nothing we can do without a way of reproducing this issue since the ConcurrentModificationException works like it does for very good reasons.

[subIT1]

But what could solve the issue would be using modern concurrent structures in the UiInternals -> Deque and so on

[Legioth]

It would further hide the underlying problem that application logic updates UI state from multiple threads at the same time. For that to work, all components, including all add-ons and all views in the application would also have to be built with concurrency in mind.

Something as simple as layout.add(child); would become a very complex matter since there's a brief moment in time when the parent pointer and the child pointer are not in sync with each other.

We could for instance no longer guarantee that a component is still attached at the time when an attach listener is invoked because there's no guarantee that another thread hasn't already detached the component.