spring-cloud-gateway: Errors in gateway during high load

spring-cloud-dependencies: Finchley.M5 spring-boot-starter-parent: 2.0.0.M7

I configured a jmeter test which creates 200 concurrent threads and it hits my gateway. Under this load some request fail with a NullPointerException.

I created two services, the first one using spring cloud gateway and the other one using spring web react. The latter one exposes a /hello GET endpoint and with a delay of 200 ms with the following bean:

    @Bean
    public RouterFunction<ServerResponse> routes() {
        return route(GET("/hello"),
                (req)-> ok()
                        .body(BodyInserters.fromObject("Hello")).delayElement(Duration.ofMillis(200))
        );
    }

The first service has a routing to the second service. After debugging, I noticed what causes the issue is the line below in NettyRoutingFilter, as responseHeaders() returns null:

res.responseHeaders().forEach(entry -> headers.add(entry.getKey(), entry.getValue()));

Full Stacktrace:

java.lang.NullPointerException: null
	at org.springframework.cloud.gateway.filter.NettyRoutingFilter.lambda$filter$4(NettyRoutingFilter.java:105) ~[spring-cloud-gateway-core-2.0.0.M5.jar:2.0.0.M5]
	at reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:177) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
	at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:450) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
	at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:115) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
	at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:450) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
	at reactor.core.publisher.FluxRetryPredicate$RetryPredicateSubscriber.onNext(FluxRetryPredicate.java:81) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
	at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:450) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
	at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:450) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
	at reactor.core.publisher.MonoCreate$DefaultMonoSink.success(MonoCreate.java:139) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
	at reactor.ipc.netty.channel.PooledClientContextHandler.fireContextActive(PooledClientContextHandler.java:84) ~[reactor-netty-0.7.2.RELEASE.jar:0.7.2.RELEASE]
	at reactor.ipc.netty.channel.FluxReceive.drainReceiver(FluxReceive.java:176) ~[reactor-netty-0.7.2.RELEASE.jar:0.7.2.RELEASE]
	at reactor.ipc.netty.channel.FluxReceive.onInboundComplete(FluxReceive.java:342) ~[reactor-netty-0.7.2.RELEASE.jar:0.7.2.RELEASE]
	at reactor.ipc.netty.channel.ChannelOperations.onHandlerTerminate(ChannelOperations.java:420) ~[reactor-netty-0.7.2.RELEASE.jar:0.7.2.RELEASE]
	at reactor.ipc.netty.channel.ChannelOperationsHandler.channelInactive(ChannelOperationsHandler.java:108) ~[reactor-netty-0.7.2.RELEASE.jar:0.7.2.RELEASE]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:245) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:231) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:224) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelInactive(CombinedChannelDuplexHandler.java:420) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.handler.codec.ByteToMessageDecoder.channelInputClosed(ByteToMessageDecoder.java:377) ~[netty-codec-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.handler.codec.ByteToMessageDecoder.channelInactive(ByteToMessageDecoder.java:342) ~[netty-codec-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.handler.codec.http.HttpClientCodec$Decoder.channelInactive(HttpClientCodec.java:282) ~[netty-codec-http-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.CombinedChannelDuplexHandler.channelInactive(CombinedChannelDuplexHandler.java:223) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:245) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:231) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:224) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelInactive(DefaultChannelPipeline.java:1354) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:245) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:231) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.DefaultChannelPipeline.fireChannelInactive(DefaultChannelPipeline.java:917) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.AbstractChannel$AbstractUnsafe$8.run(AbstractChannel.java:822) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163) ~[netty-common-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:403) ~[netty-common-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:463) ~[netty-transport-4.1.17.Final.jar:4.1.17.Final]
	at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858) ~[netty-common-4.1.17.Final.jar:4.1.17.Final]
	at java.lang.Thread.run(Thread.java:748) ~[na:1.8.0_152]
	Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Assembly trace from producer [reactor.core.publisher.MonoPeek] :
	reactor.core.publisher.Mono.doOnSignal(Mono.java:3622)
	reactor.core.publisher.Mono.doOnNext(Mono.java:1646)
	org.springframework.cloud.gateway.filter.NettyRoutingFilter.filter(NettyRoutingFilter.java:101)
	org.springframework.cloud.gateway.handler.FilteringWebHandler$GatewayFilterAdapter.filter(FilteringWebHandler.java:121)
	org.springframework.cloud.gateway.filter.OrderedGatewayFilter.filter(OrderedGatewayFilter.java:40)
	org.springframework.cloud.gateway.handler.FilteringWebHandler$DefaultGatewayFilterChain.filter(FilteringWebHandler.java:103)
	org.springframework.cloud.gateway.filter.RouteToRequestUrlFilter.filter(RouteToRequestUrlFilter.java:59)
	org.springframework.cloud.gateway.handler.FilteringWebHandler$GatewayFilterAdapter.filter(FilteringWebHandler.java:121)
	org.springframework.cloud.gateway.filter.OrderedGatewayFilter.filter(OrderedGatewayFilter.java:40)
	org.springframework.cloud.gateway.handler.FilteringWebHandler$DefaultGatewayFilterChain.filter(FilteringWebHandler.java:103)
	org.springframework.cloud.gateway.filter.factory.RemoveNonProxyHeadersGatewayFilterFactory.lambda$apply$1(RemoveNonProxyHeadersGatewayFilterFactory.java:79)
	org.springframework.cloud.gateway.filter.OrderedGatewayFilter.filter(OrderedGatewayFilter.java:40)
	org.springframework.cloud.gateway.handler.FilteringWebHandler$DefaultGatewayFilterChain.filter(FilteringWebHandler.java:103)
	org.springframework.cloud.gateway.filter.NettyWriteResponseFilter.filter(NettyWriteResponseFilter.java:52)
	org.springframework.cloud.gateway.handler.FilteringWebHandler$GatewayFilterAdapter.filter(FilteringWebHandler.java:121)
	org.springframework.cloud.gateway.filter.OrderedGatewayFilter.filter(OrderedGatewayFilter.java:40)
	org.springframework.cloud.gateway.handler.FilteringWebHandler$DefaultGatewayFilterChain.filter(FilteringWebHandler.java:103)
	org.springframework.cloud.gateway.handler.FilteringWebHandler.handle(FilteringWebHandler.java:87)
	org.springframework.web.reactive.result.SimpleHandlerAdapter.handle(SimpleHandlerAdapter.java:62)
	org.springframework.web.reactive.DispatcherHandler.invokeHandler(DispatcherHandler.java:168)
	org.springframework.web.reactive.DispatcherHandler.lambda$handle$1(DispatcherHandler.java:160)
	reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:118)
	reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:67)
	reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:76)
	reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.innerNext(FluxConcatMap.java:271)
	reactor.core.publisher.FluxConcatMap$ConcatMapInner.onNext(FluxConcatMap.java:803)
	reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:115)
	reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:67)
	reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1092)
	reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:144)
	reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:115)
	reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:76)
	reactor.core.publisher.FluxFilterFuseable$FilterFuseableSubscriber.tryOnNext(FluxFilterFuseable.java:129)
	reactor.core.publisher.FluxIterable$IterableSubscriptionConditional.fastPath(FluxIterable.java:574)
	reactor.core.publisher.FluxIterable$IterableSubscriptionConditional.request(FluxIterable.java:459)
	reactor.core.publisher.FluxFilterFuseable$FilterFuseableSubscriber.request(FluxFilterFuseable.java:170)
	reactor.core.publisher.MonoNext$NextSubscriber.request(MonoNext.java:102)
	reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.request(FluxMapFuseable.java:156)
	reactor.core.publisher.MonoFlatMap$FlatMapMain.onSubscribe(MonoFlatMap.java:103)
	reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onSubscribe(FluxMapFuseable.java:90)
	reactor.core.publisher.MonoNext$NextSubscriber.onSubscribe(MonoNext.java:64)
	reactor.core.publisher.FluxFilterFuseable$FilterFuseableSubscriber.onSubscribe(FluxFilterFuseable.java:79)
	reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:124)
	reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:61)
	reactor.core.publisher.FluxFilterFuseable.subscribe(FluxFilterFuseable.java:51)
	reactor.core.publisher.MonoNext.subscribe(MonoNext.java:40)
	reactor.core.publisher.MonoMapFuseable.subscribe(MonoMapFuseable.java:59)
	reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:60)
	reactor.core.publisher.MonoSwitchIfEmpty.subscribe(MonoSwitchIfEmpty.java:44)
	reactor.core.publisher.MonoMapFuseable.subscribe(MonoMapFuseable.java:59)
	reactor.core.publisher.Mono.subscribe(Mono.java:3008)
	reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:418)
	reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:210)
	reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:128)
	reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:61)
	reactor.core.publisher.FluxConcatMap.subscribe(FluxConcatMap.java:121)
	reactor.core.publisher.MonoNext.subscribe(MonoNext.java:40)
	reactor.core.publisher.MonoSwitchIfEmpty.subscribe(MonoSwitchIfEmpty.java:44)
	reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:60)
	reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:60)
	reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
	reactor.core.publisher.MonoPeekTerminal.subscribe(MonoPeekTerminal.java:61)
	reactor.core.publisher.MonoPeekFuseable.subscribe(MonoPeekFuseable.java:74)
	reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
	reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
	reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44)
	reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44)
	reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44)
	reactor.core.publisher.Mono.subscribe(Mono.java:3008)
	reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.drain(MonoIgnoreThen.java:167)
	reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:56)
	reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44)
	reactor.core.publisher.MonoPeekTerminal.subscribe(MonoPeekTerminal.java:61)
	reactor.ipc.netty.channel.ChannelOperations.applyHandler(ChannelOperations.java:383)
	reactor.ipc.netty.http.server.HttpServerOperations.onHandlerStart(HttpServerOperations.java:359)
	io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163)
	io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:403)
	io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:463)
	io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858)
Error has been observed by the following operator(s):
	|_	Mono.doOnNext(NettyRoutingFilter.java:101)
	|_	Mono.then(NettyRoutingFilter.java:113)
	|_	Mono.then(NettyWriteResponseFilter.java:52)
	|_	Mono.then(SimpleHandlerAdapter.java:63)
	|_	Mono.flatMap(DispatcherHandler.java:160)
	|_	Mono.flatMap(DispatcherHandler.java:161)
	|_	Mono.defer(DefaultWebFilterChain.java:71)
	|_	Mono.doOnSuccess(MetricsWebFilter.java:59)
	|_	Mono.doOnError(MetricsWebFilter.java:60)
	|_	Mono.compose(MetricsWebFilter.java:54)
	|_	Mono.defer(DefaultWebFilterChain.java:71)

And below how the gateway is configured:

server.port: 8080
spring.reactor.stacktrace-mode.enabled: true
spring:
  cloud:   
    gateway:
      routes:
      - id: hello
        uri: http://localhost:8081
        predicates:
        - Path=/hello