[Question] Need opinion on whether to allow a blocking call in spring-core

[philsttr]

@bsideup I'd like to get your opinion on the blocking call encountered in the stacktrace below.

I'm trying to figure out:

1. if it should be whitelisted,
2. what should be whitelisted, and
3. where it should be whitelisted.

My current thought is that

1. yes, it should be whitelisted
2. org.springframework.util.ConcurrentReferenceHashMap.doTask
3. a blockhound integration provided by spring-core.

What are your thoughts?

java.lang.Error: Blocking call! jdk.internal.misc.Unsafe#park
	at reactor.blockhound.BlockHound$Builder.lambda$new$0(BlockHound.java:196)
	at reactor.blockhound.BlockHound$Builder.lambda$install$6(BlockHound.java:318)
	at reactor.blockhound.BlockHoundRuntime.checkBlocking(BlockHoundRuntime.java:46)
	at java.base/jdk.internal.misc.Unsafe.park(Unsafe.java)
	at java.base/java.util.concurrent.locks.LockSupport.park(LockSupport.java:194)
	at java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:885)
	at java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:917)
	at java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1240)
	at java.base/java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:267)
	at org.springframework.util.ConcurrentReferenceHashMap$Segment.doTask(ConcurrentReferenceHashMap.java:524)
	at org.springframework.util.ConcurrentReferenceHashMap.doTask(ConcurrentReferenceHashMap.java:419)
	at org.springframework.util.ConcurrentReferenceHashMap.put(ConcurrentReferenceHashMap.java:282)
	at org.springframework.util.ConcurrentReferenceHashMap.put(ConcurrentReferenceHashMap.java:271)
	at org.springframework.core.ResolvableType.forType(ResolvableType.java:1420)
	at org.springframework.core.ResolvableType.forType(ResolvableType.java:1344)
	at org.springframework.web.reactive.function.BodyExtractors.toMono(BodyExtractors.java:80)
	at org.springframework.security.oauth2.core.web.reactive.function.OAuth2AccessTokenResponseBodyExtractor.extract(OAuth2AccessTokenResponseBodyExtractor.java:59)
	at org.springframework.security.oauth2.core.web.reactive.function.OAuth2AccessTokenResponseBodyExtractor.extract(OAuth2AccessTokenResponseBodyExtractor.java:48)
	at org.springframework.web.reactive.function.client.DefaultClientResponse.body(DefaultClientResponse.java:110)
	at org.springframework.security.oauth2.client.endpoint.WebClientReactiveClientCredentialsTokenResponseClient.lambda$null$0(WebClientReactiveClientCredentialsTokenResponseClient.java:85)
	at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:118)
	at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:67)
	at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:121)
	at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onNext(FluxPeekFuseable.java:203)
	at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onNext(FluxPeekFuseable.java:203)
	at reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:76)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onNext(MonoFlatMapMany.java:242)
	at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2186)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:143)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onSubscribe(MonoFlatMapMany.java:237)
	at reactor.core.publisher.MonoJust.subscribe(MonoJust.java:54)
	at reactor.core.publisher.Flux.subscribe(Flux.java:8143)
	at reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:188)
	at reactor.core.publisher.FluxRetryPredicate$RetryPredicateSubscriber.onNext(FluxRetryPredicate.java:82)
	at reactor.core.publisher.MonoCreate$DefaultMonoSink.success(MonoCreate.java:156)
	at reactor.netty.http.client.HttpClientConnect$HttpObserver.onStateChange(HttpClientConnect.java:397)
	at reactor.netty.ReactorNetty$CompositeConnectionObserver.onStateChange(ReactorNetty.java:494)
	at reactor.netty.resources.PooledConnectionProvider$DisposableAcquire.onStateChange(PooledConnectionProvider.java:526)
	at reactor.netty.resources.PooledConnectionProvider$PooledConnection.onStateChange(PooledConnectionProvider.java:435)
	at reactor.netty.http.client.HttpClientOperations.onInboundNext(HttpClientOperations.java:521)
	at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:89)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:352)
	at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:102)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:352)
	at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:438)
	at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:326)
	at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:313)
	at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:427)
	at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:281)
	at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:253)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:352)
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1422)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360)
	at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:931)
	at io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:792)
	at io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:502)
	at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:407)
	at io.netty.util.concurrent.SingleThreadEventExecutor$6.run(SingleThreadEventExecutor.java:1050)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.base/java.lang.Thread.run(Thread.java:834)

[bsideup]

Hi @philsttr,

I would say whitelisting org.springframework.web.reactive.function.BodyExtractors.toMono is a good choice:

1. it does not run any user provided code
2. it is a method from Spring Framework, and it is their responsibility to keep it non-blocking
3. I expect it to be fixed soon (did you check if there is a report of this blocking behaviour in SF, btw?) since it returns Mono and the blocking put can be wrapped (or ignored if they consider it harmless)

[philsttr]

Created spring-projects/spring-framework#24241