Error when emptying the cart

[guikcd]

A 500 error is returned by the ui app when removing the last item of the cart.

Step to reproduce:

• Add one item to loadout
• Remove the item (trash icon)
• /cart/remove return a 500 error

Stack trace

2025-03-04T19:44:20.057Z ERROR 1 --- [ctor-http-nio-3] a.w.r.e.AbstractErrorWebExceptionHandler : [a3ca77fb-40]  500 Server Error for HTTP POST "/cart/remove"

java.util.ConcurrentModificationException: null
	at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1095) ~[?:?]
	Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
Error has been observed at the following site(s):
	*__checkpoint ⇢ com.amazon.sample.ui.web.util.SessionIDWebFilter [DefaultWebFilterChain]
	*__checkpoint ⇢ HTTP POST "/cart/remove" [ExceptionHandlingWebHandler]
Original Stack Trace:
		at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1095) ~[?:?]
		at java.base/java.util.ArrayList$Itr.next(ArrayList.java:1049) ~[?:?]
		at com.amazon.sample.ui.services.carts.model.Cart.removeItem(Cart.java:50) ~[!/:0.0.1-SNAPSHOT]
		at com.amazon.sample.ui.services.carts.MockCartsService.removeItem(MockCartsService.java:83) ~[!/:0.0.1-SNAPSHOT]
		at com.amazon.sample.ui.web.CartController.remove(CartController.java:78) ~[!/:0.0.1-SNAPSHOT]
		at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
		at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
		at org.springframework.web.reactive.result.method.InvocableHandlerMethod.lambda$invoke$0(InvocableHandlerMethod.java:200) ~[spring-webflux-6.2.2.jar!/:6.2.2]
		at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:132) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoZip$ZipCoordinator.signal(MonoZip.java:297) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoZip$ZipInner.onNext(MonoZip.java:478) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onNext(MonoPeekTerminal.java:180) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:122) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoFlatMap$FlatMapMain.secondComplete(MonoFlatMap.java:245) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoFlatMap$FlatMapInner.onNext(MonoFlatMap.java:305) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:294) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:188) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:237) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onComplete(MonoIgnoreThen.java:204) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onComplete(MonoPeekTerminal.java:299) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onComplete(MonoPeekTerminal.java:299) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreElements$IgnoreElementsSubscriber.onComplete(MonoIgnoreElements.java:89) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:260) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:260) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:144) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoZip$ZipCoordinator.signal(MonoZip.java:298) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoZip$ZipInner.onNext(MonoZip.java:478) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1865) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoCacheTime.subscribeOrReturn(MonoCacheTime.java:151) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:63) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoZip$ZipCoordinator.request(MonoZip.java:220) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxMap$MapSubscriber.request(FluxMap.java:164) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.request(FluxPeek.java:138) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.request(FluxPeek.java:138) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreElements$IgnoreElementsSubscriber.onSubscribe(MonoIgnoreElements.java:72) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onSubscribe(FluxPeek.java:171) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onSubscribe(FluxPeek.java:171) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxMap$MapSubscriber.onSubscribe(FluxMap.java:92) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoZip.subscribe(MonoZip.java:129) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.Mono.subscribe(Mono.java:4576) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:265) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:51) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:200) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:294) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:188) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.subscribeNext(MonoIgnoreThen.java:237) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onComplete(MonoIgnoreThen.java:204) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoIgnoreElements$IgnoreElementsSubscriber.onComplete(MonoIgnoreElements.java:89) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:260) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:260) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:144) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoZip$ZipCoordinator.signal(MonoZip.java:298) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoZip$ZipInner.onNext(MonoZip.java:478) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1865) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoCacheTime$CoordinatorSubscriber.signalCached(MonoCacheTime.java:337) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoCacheTime$CoordinatorSubscriber.onNext(MonoCacheTime.java:354) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:74) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:129) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onNext(FluxContextWrite.java:107) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:299) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxFilterFuseable$FilterFuseableConditionalSubscriber.onNext(FluxFilterFuseable.java:337) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.Operators$BaseFluxToMonoOperator.completePossiblyEmpty(Operators.java:2097) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.MonoCollect$CollectSubscriber.onComplete(MonoCollect.java:145) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:144) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:260) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:144) ~[reactor-core-3.7.2.jar!/:3.7.2]
		at reactor.netty.channel.FluxReceive.onInboundComplete(FluxReceive.java:413) ~[reactor-netty-core-1.2.2.jar!/:1.2.2]
		at reactor.netty.channel.ChannelOperations.onInboundComplete(ChannelOperations.java:455) ~[reactor-netty-core-1.2.2.jar!/:1.2.2]
		at reactor.netty.http.server.HttpServerOperations.handleLastHttpContent(HttpServerOperations.java:883) ~[reactor-netty-http-1.2.2.jar!/:1.2.2]
		at reactor.netty.http.server.HttpServerOperations.onInboundNext(HttpServerOperations.java:792) ~[reactor-netty-http-1.2.2.jar!/:1.2.2]
		at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:115) ~[reactor-netty-core-1.2.2.jar!/:1.2.2]
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at reactor.netty.http.server.HttpTrafficHandler.channelRead(HttpTrafficHandler.java:316) ~[reactor-netty-http-1.2.2.jar!/:1.2.2]
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:346) ~[netty-codec-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:318) ~[netty-codec-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1357) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:868) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:166) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:788) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562) ~[netty-transport-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997) ~[netty-common-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.117.Final.jar!/:4.1.117.Final]
		at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.1.117.Final.jar!/:4.1.117.Final]
		at java.base/java.lang.Thread.run(Thread.java:1583) [?:?]

This is not urgent, I'll take a look at it and try to propose a change.