WebFlux leaks buffers when server request is cancelled

[kzander91]

Spring WebFlux 7.0.5

With a controller like this:

@RestController
class TestController {

    @GetMapping
    Set<String> endpoint() {
        return Set.of("SET ELEMENT");
    }

}

And this config:

spring.netty.leak-detection=paranoid
logging.level.root=debug

We occasionally get leak reports from Netty, happening when the client cancelled the request before receiving the body. We are not creating/handling DataBuffer anywhere in our application, so this appears to be an issue with the framework.
Here's the logs associated with a request and the subsequent leak report:

{"@timestamp":"2026-03-10T12:34:02.308692871Z","message":"[ec2939a0, L:/172.21.10.15:8080 - R:/172.21.10.120:41258] New http connection, requesting read","logger_name":"reactor.netty.http.server.HttpServerOperations","thread_name":"reactor-http-epoll-7","level":"DEBUG"}
{"@timestamp":"2026-03-10T12:34:02.308783621Z","message":"[ec2939a0, L:/172.21.10.15:8080 - R:/172.21.10.120:41258] Initialized pipeline DefaultChannelPipeline{(reactor.left.httpCodec = io.netty.handler.codec.http.HttpServerCodec), (reactor.left.httpTrafficHandler = reactor.netty.http.server.HttpTrafficHandler), (reactor.right.reactiveBridge = reactor.netty.channel.ChannelOperationsHandler)}","logger_name":"reactor.netty.transport.TransportConfig","thread_name":"reactor-http-epoll-7","level":"DEBUG"}
{"@timestamp":"2026-03-10T12:34:02.308986932Z","message":"[ec2939a0, L:/172.21.10.15:8080 - R:/172.21.10.120:41258] Increasing pending responses count: 1","logger_name":"reactor.netty.http.server.HttpServerOperations","thread_name":"reactor-http-epoll-7","level":"DEBUG"}
{"@timestamp":"2026-03-10T12:34:02.309015114Z","message":"[ec2939a0-1, L:/172.21.10.15:8080 - R:/172.21.10.120:41258] Handler is being applied: org.springframework.http.server.reactive.ReactorHttpHandlerAdapter@1398a3c4","logger_name":"reactor.netty.http.server.HttpServer","thread_name":"reactor-http-epoll-7","level":"DEBUG"}
{"@timestamp":"2026-03-10T12:34:02.309044789Z","message":"[ec2939a0-28160] HTTP GET \"/my/request/path\"","logger_name":"org.springframework.web.server.adapter.HttpWebHandlerAdapter","thread_name":"reactor-http-epoll-7","level":"DEBUG","tags":["COMMONS-LOGGING"]}
{"@timestamp":"2026-03-10T12:34:02.312145169Z","message":"[ec2939a0-28160] Mapped to my.company.ControllerName#controllerMethod()","logger_name":"org.springframework.web.reactive.result.method.annotation.RequestMappingHandlerMapping","thread_name":"parallel-1","level":"DEBUG","traceId":"edbef23c90b1f5d0e1eef76c6fea79d3","spanId":"621b56e814ce2053","tags":["COMMONS-LOGGING"]}
{"@timestamp":"2026-03-10T12:34:02.312448804Z","message":"[ec2939a0-28160] Using 'application/json' given [*/*] and supported [application/json]","logger_name":"org.springframework.web.reactive.result.method.annotation.ResponseBodyResultHandler","thread_name":"task-13623","level":"DEBUG","traceId":"edbef23c90b1f5d0e1eef76c6fea79d3","spanId":"621b56e814ce2053","tags":["COMMONS-LOGGING"]}
{"@timestamp":"2026-03-10T12:34:02.312510228Z","message":"[ec2939a0-28160] 0..1 [java.util.Set<java.lang.String>]","logger_name":"org.springframework.web.reactive.result.method.annotation.ResponseBodyResultHandler","thread_name":"task-13623","level":"DEBUG","traceId":"edbef23c90b1f5d0e1eef76c6fea79d3","spanId":"621b56e814ce2053","tags":["COMMONS-LOGGING"]}
{"@timestamp":"2026-03-10T12:34:02.312700699Z","message":"[ec2939a0-28160] Encoding [[SET ELEMENT]]","logger_name":"org.springframework.http.codec.json.JacksonJsonEncoder","thread_name":"task-13623","level":"DEBUG","traceId":"edbef23c90b1f5d0e1eef76c6fea79d3","spanId":"621b56e814ce2053","tags":["COMMONS-LOGGING"]}
{"@timestamp":"2026-03-10T12:34:02.313039046Z","message":"[ec2939a0-1, L:/172.21.10.15:8080 ! R:/172.21.10.120:41258] [HttpServer] Channel inbound receiver cancelled (channel disconnected).","logger_name":"reactor.netty.channel.ChannelOperations","thread_name":"reactor-http-epoll-7","level":"DEBUG"}
{"@timestamp":"2026-03-10T12:34:02.313089093Z","message":"[ec2939a0-28160] Completed 200 OK","logger_name":"org.springframework.web.server.adapter.HttpWebHandlerAdapter","thread_name":"task-13623","level":"DEBUG","traceId":"edbef23c90b1f5d0e1eef76c6fea79d3","spanId":"1c5dac71e20cfa03","tags":["COMMONS-LOGGING"]}
{"@timestamp":"2026-03-10T12:35:04.378519737Z","message":"LEAK: ByteBuf.release() was not called before it's garbage-collected. 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Source)\n#3:\n\tHint: [ec2939a0-28160] \n\torg.springframework.core.io.buffer.NettyDataBuffer.touch(NettyDataBuffer.java:360)\n\torg.springframework.core.io.buffer.NettyDataBuffer.touch(NettyDataBuffer.java:40)\n\torg.springframework.core.io.buffer.DataBufferUtils.touch(DataBufferUtils.java:583)\n\torg.springframework.core.codec.Hints.touchDataBuffer(Hints.java:165)\n\torg.springframework.http.codec.AbstractJacksonEncoder.encodeValue(AbstractJacksonEncoder.java:246)\n\torg.springframework.http.codec.AbstractJacksonEncoder.lambda$encode$1(AbstractJacksonEncoder.java:151)\n\treactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:284)\n\treactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2564)\n\treactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:361)\n\treactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.request(FluxContextWriteRestoringThreadLocals.java:164)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:151)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onSubscribe(MonoFlatMapMany.java:246)\n\treactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onSubscribe(FluxContextWriteRestoringThreadLocals.java:105)\n\treactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:265)\n\treactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)\n\treactor.core.publisher.FluxFromMonoOperator.subscribe(FluxFromMonoOperator.java:83)\n\treactor.core.publisher.FluxDeferContextual.subscribe(FluxDeferContextual.java:58)\n\treactor.core.publisher.FluxContextWriteRestoringThreadLocals.subscribe(FluxContextWriteRestoringThreadLocals.java:45)\n\treactor.core.publisher.Flux.subscribe(Flux.java:8888)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:197)\n\treactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:122)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)\n\treactor.core.publisher.FluxFilter$FilterSubscriber.onNext(FluxFilter.java:114)\n\treactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:224)\n\treactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2564)\n\treactor.core.publisher.FluxMap$MapConditionalSubscriber.request(FluxMap.java:294)\n\treactor.core.publisher.FluxFilter$FilterSubscriber.request(FluxFilter.java:186)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.request(MonoFlatMap.java:194)\n\treactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.request(FluxDefaultIfEmpty.java:98)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribe(MonoFlatMapMany.java:142)\n\treactor.core.publisher.Operators$BaseFluxToMonoOperator.onSubscribe(Operators.java:2047)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onSubscribe(MonoFlatMap.java:117)\n\treactor.core.publisher.FluxFilter$FilterSubscriber.onSubscribe(FluxFilter.java:86)\n\treactor.core.publisher.FluxMap$MapConditionalSubscriber.onSubscribe(FluxMap.java:194)\n\treactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)\n\treactor.core.publisher.MonoDeferContextual.subscribe(MonoDeferContextual.java:56)\n\treactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:75)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165)\n\treactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)\n\treactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onNext(MonoSubscribeOn.java:147)\n\treactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)\n\treactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:203)\n\treactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:297)\n\treactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:191)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)\n\treactor.core.publisher.FluxSubscribeOnValue$ScheduledScalar.run(FluxSubscribeOnValue.java:181)\n\tio.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)\n\treactor.core.scheduler.ExecutorScheduler$ExecutorPlainRunnable.run(ExecutorScheduler.java:135)\n\tio.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)\n\tjava.base/java.lang.VirtualThread.run(Unknown Source)\n#4:\n\tio.netty.buffer.AdvancedLeakAwareByteBuf.writeBytes(AdvancedLeakAwareByteBuf.java:617)\n\torg.springframework.core.io.buffer.NettyDataBuffer.write(NettyDataBuffer.java:180)\n\torg.springframework.core.io.buffer.NettyDataBuffer.write(NettyDataBuffer.java:40)\n\torg.springframework.http.codec.AbstractJacksonEncoder.encodeValue(AbstractJacksonEncoder.java:245)\n\torg.springframework.http.codec.AbstractJacksonEncoder.lambda$encode$1(AbstractJacksonEncoder.java:151)\n\treactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:284)\n\treactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2564)\n\treactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:361)\n\treactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.request(FluxContextWriteRestoringThreadLocals.java:164)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:151)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onSubscribe(MonoFlatMapMany.java:246)\n\treactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onSubscribe(FluxContextWriteRestoringThreadLocals.java:105)\n\treactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:265)\n\treactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)\n\treactor.core.publisher.FluxFromMonoOperator.subscribe(FluxFromMonoOperator.java:83)\n\treactor.core.publisher.FluxDeferContextual.subscribe(FluxDeferContextual.java:58)\n\treactor.core.publisher.FluxContextWriteRestoringThreadLocals.subscribe(FluxContextWriteRestoringThreadLocals.java:45)\n\treactor.core.publisher.Flux.subscribe(Flux.java:8888)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:197)\n\treactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:122)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)\n\treactor.core.publisher.FluxFilter$FilterSubscriber.onNext(FluxFilter.java:114)\n\treactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:224)\n\treactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2564)\n\treactor.core.publisher.FluxMap$MapConditionalSubscriber.request(FluxMap.java:294)\n\treactor.core.publisher.FluxFilter$FilterSubscriber.request(FluxFilter.java:186)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.request(MonoFlatMap.java:194)\n\treactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.request(FluxDefaultIfEmpty.java:98)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribe(MonoFlatMapMany.java:142)\n\treactor.core.publisher.Operators$BaseFluxToMonoOperator.onSubscribe(Operators.java:2047)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onSubscribe(MonoFlatMap.java:117)\n\treactor.core.publisher.FluxFilter$FilterSubscriber.onSubscribe(FluxFilter.java:86)\n\treactor.core.publisher.FluxMap$MapConditionalSubscriber.onSubscribe(FluxMap.java:194)\n\treactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)\n\treactor.core.publisher.MonoDeferContextual.subscribe(MonoDeferContextual.java:56)\n\treactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:75)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165)\n\treactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)\n\treactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onNext(MonoSubscribeOn.java:147)\n\treactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)\n\treactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:203)\n\treactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:297)\n\treactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:191)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)\n\treactor.core.publisher.FluxSubscribeOnValue$ScheduledScalar.run(FluxSubscribeOnValue.java:181)\n\tio.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)\n\treactor.core.scheduler.ExecutorScheduler$ExecutorPlainRunnable.run(ExecutorScheduler.java:135)\n\tio.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)\n\tjava.base/java.lang.VirtualThread.run(Unknown Source)\nCreated at:\n\tio.netty.buffer.AdaptiveByteBufAllocator.newDirectBuffer(AdaptiveByteBufAllocator.java:67)\n\tio.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:168)\n\tio.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:159)\n\tio.netty.buffer.AbstractByteBufAllocator.buffer(AbstractByteBufAllocator.java:96)\n\torg.springframework.core.io.buffer.NettyDataBufferFactory.allocateBuffer(NettyDataBufferFactory.java:72)\n\torg.springframework.core.io.buffer.NettyDataBufferFactory.allocateBuffer(NettyDataBufferFactory.java:39)\n\torg.springframework.http.codec.AbstractJacksonEncoder.encodeValue(AbstractJacksonEncoder.java:244)\n\torg.springframework.http.codec.AbstractJacksonEncoder.lambda$encode$1(AbstractJacksonEncoder.java:151)\n\treactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:284)\n\treactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2564)\n\treactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:361)\n\treactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.request(FluxContextWriteRestoringThreadLocals.java:164)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:151)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onSubscribe(MonoFlatMapMany.java:246)\n\treactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onSubscribe(FluxContextWriteRestoringThreadLocals.java:105)\n\treactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:265)\n\treactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)\n\treactor.core.publisher.FluxFromMonoOperator.subscribe(FluxFromMonoOperator.java:83)\n\treactor.core.publisher.FluxDeferContextual.subscribe(FluxDeferContextual.java:58)\n\treactor.core.publisher.FluxContextWriteRestoringThreadLocals.subscribe(FluxContextWriteRestoringThreadLocals.java:45)\n\treactor.core.publisher.Flux.subscribe(Flux.java:8888)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:197)\n\treactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:122)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)\n\treactor.core.publisher.FluxFilter$FilterSubscriber.onNext(FluxFilter.java:114)\n\treactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:224)\n\treactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2564)\n\treactor.core.publisher.FluxMap$MapConditionalSubscriber.request(FluxMap.java:294)\n\treactor.core.publisher.FluxFilter$FilterSubscriber.request(FluxFilter.java:186)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.request(MonoFlatMap.java:194)\n\treactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.request(FluxDefaultIfEmpty.java:98)\n\treactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribe(MonoFlatMapMany.java:142)\n\treactor.core.publisher.Operators$BaseFluxToMonoOperator.onSubscribe(Operators.java:2047)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onSubscribe(MonoFlatMap.java:117)\n\treactor.core.publisher.FluxFilter$FilterSubscriber.onSubscribe(FluxFilter.java:86)\n\treactor.core.publisher.FluxMap$MapConditionalSubscriber.onSubscribe(FluxMap.java:194)\n\treactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)\n\treactor.core.publisher.MonoDeferContextual.subscribe(MonoDeferContextual.java:56)\n\treactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:75)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165)\n\treactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)\n\treactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onNext(MonoSubscribeOn.java:147)\n\treactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)\n\treactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:203)\n\treactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:297)\n\treactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:191)\n\treactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)\n\treactor.core.publisher.FluxSubscribeOnValue$ScheduledScalar.run(FluxSubscribeOnValue.java:181)\n\tio.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)\n\treactor.core.scheduler.ExecutorScheduler$ExecutorPlainRunnable.run(ExecutorScheduler.java:135)\n\tio.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)\n\tjava.base/java.lang.VirtualThread.run(Unknown Source)","logger_name":"io.netty.util.ResourceLeakDetector","thread_name":"reactor-http-epoll-8","level":"ERROR"}

The most recent place that touched the leaked buffer (see leak report above) is touchDataBuffer(buffer) here:

spring-framework/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractServerHttpResponse.java

Lines 172 to 196 in 16ecbfc

	if (body instanceof Mono) {
	return ((Mono<? extends DataBuffer>) body)
	.flatMap(buffer -> {
	touchDataBuffer(buffer);
	AtomicBoolean subscribed = new AtomicBoolean();
	return doCommit(
	() -> {
	try {
	return writeWithInternal(Mono.fromCallable(() -> buffer)
	.doOnSubscribe(s -> subscribed.set(true))
	.doOnDiscard(DataBuffer.class, DataBufferUtils::release));
	}
	catch (Throwable ex) {
	return Mono.error(ex);
	}
	})
	.doOnError(ex -> DataBufferUtils.release(buffer))
	.doOnCancel(() -> {
	if (!subscribed.get()) {
	DataBufferUtils.release(buffer);
	}
	});
	})
	.doOnError(t -> getHeaders().clearContentHeaders())
	.doOnDiscard(DataBuffer.class, DataBufferUtils::release);

Commit 25101fb (see #26232) added the release in doOnCancel(), but only if no subscription took place 🤔 Perhaps that's the culprit?

[bclozel]

Extracted buffer leak report:

LEAK: ByteBuf.release() was not called before it's garbage-collected. See https://netty.io/wiki/reference-counted-objects.html for more information.
Recent access records: 
#1:
  Hint: Channel id: ec2939a0-1, L:/172.21.10.15:8080 - R:/172.21.10.120:41258
  org.springframework.core.io.buffer.NettyDataBuffer.touch(NettyDataBuffer.java:360)
  org.springframework.core.io.buffer.NettyDataBuffer.touch(NettyDataBuffer.java:40)
  org.springframework.core.io.buffer.DataBufferUtils.touch(DataBufferUtils.java:583)
  org.springframework.http.server.reactive.ReactorServerHttpResponse.touchDataBuffer(ReactorServerHttpResponse.java:140)
  org.springframework.http.server.reactive.AbstractServerHttpResponse.lambda$writeWith$0(AbstractServerHttpResponse.java:175)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:132)
  reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onNext(FluxContextWrite.java:109)
  reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2564)
  reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.request(FluxContextWrite.java:138)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.request(MonoFlatMap.java:194)
  reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.request(MonoPeekTerminal.java:142)
  reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.request(FluxContextWrite.java:138)
  reactor.core.publisher.MonoFlatMap$FlatMapInner.onSubscribe(MonoFlatMap.java:292)
  reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onSubscribe(FluxContextWrite.java:103)
  reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onSubscribe(MonoPeekTerminal.java:155)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onSubscribe(MonoFlatMap.java:117)
  reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onSubscribe(FluxContextWrite.java:103)
  reactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)
  reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:75)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165)
  reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:75)
  reactor.core.publisher.Operators$MonoInnerProducerBase.complete(Operators.java:2858)
  reactor.core.publisher.MonoSingle$SingleSubscriber.onComplete(MonoSingle.java:181)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onComplete(MonoFlatMapMany.java:261)
  reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onComplete(FluxContextWriteRestoringThreadLocals.java:150)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onComplete(FluxMapFuseable.java:351)
  reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2566)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:361)
  reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.request(FluxContextWriteRestoringThreadLocals.java:164)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:151)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onSubscribe(MonoFlatMapMany.java:246)
  reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onSubscribe(FluxContextWriteRestoringThreadLocals.java:105)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:265)
  reactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)
  reactor.core.publisher.FluxFromMonoOperator.subscribe(FluxFromMonoOperator.java:83)
  reactor.core.publisher.FluxDeferContextual.subscribe(FluxDeferContextual.java:58)
  reactor.core.publisher.FluxContextWriteRestoringThreadLocals.subscribe(FluxContextWriteRestoringThreadLocals.java:45)
  reactor.core.publisher.Flux.subscribe(Flux.java:8888)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:197)
  reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:122)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)
  reactor.core.publisher.FluxFilter$FilterSubscriber.onNext(FluxFilter.java:114)
  reactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:224)
  reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2564)
  reactor.core.publisher.FluxMap$MapConditionalSubscriber.request(FluxMap.java:294)
  reactor.core.publisher.FluxFilter$FilterSubscriber.request(FluxFilter.java:186)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.request(MonoFlatMap.java:194)
  reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.request(FluxDefaultIfEmpty.java:98)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribe(MonoFlatMapMany.java:142)
  reactor.core.publisher.Operators$BaseFluxToMonoOperator.onSubscribe(Operators.java:2047)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onSubscribe(MonoFlatMap.java:117)
  reactor.core.publisher.FluxFilter$FilterSubscriber.onSubscribe(FluxFilter.java:86)
  reactor.core.publisher.FluxMap$MapConditionalSubscriber.onSubscribe(FluxMap.java:194)
  reactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)
  reactor.core.publisher.MonoDeferContextual.subscribe(MonoDeferContextual.java:56)
  reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:75)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165)
  reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)
  reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onNext(MonoSubscribeOn.java:147)
  reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)
  reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:203)
  reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:297)
  reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:191)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)
  reactor.core.publisher.FluxSubscribeOnValue$ScheduledScalar.run(FluxSubscribeOnValue.java:181)
  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
  reactor.core.scheduler.ExecutorScheduler$ExecutorPlainRunnable.run(ExecutorScheduler.java:135)
  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
  java.base/java.lang.VirtualThread.run(Unknown Source)
#2:
  Hint: [ec2939a0-28160] 
  org.springframework.core.io.buffer.NettyDataBuffer.touch(NettyDataBuffer.java:360)
  org.springframework.core.io.buffer.NettyDataBuffer.touch(NettyDataBuffer.java:40)
  org.springframework.core.io.buffer.DataBufferUtils.touch(DataBufferUtils.java:583)
  org.springframework.core.codec.Hints.touchDataBuffer(Hints.java:165)
  org.springframework.http.codec.EncoderHttpMessageWriter.lambda$write$1(EncoderHttpMessageWriter.java:133)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:132)
  reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:75)
  reactor.core.publisher.Operators$MonoInnerProducerBase.complete(Operators.java:2858)
  reactor.core.publisher.MonoSingle$SingleSubscriber.onComplete(MonoSingle.java:181)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onComplete(MonoFlatMapMany.java:261)
  reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onComplete(FluxContextWriteRestoringThreadLocals.java:150)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onComplete(FluxMapFuseable.java:351)
  reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2566)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:361)
  reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.request(FluxContextWriteRestoringThreadLocals.java:164)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:151)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyInner.onSubscribe(MonoFlatMapMany.java:246)
  reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onSubscribe(FluxContextWriteRestoringThreadLocals.java:105)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:265)
  reactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)
  reactor.core.publisher.FluxFromMonoOperator.subscribe(FluxFromMonoOperator.java:83)
  reactor.core.publisher.FluxDeferContextual.subscribe(FluxDeferContextual.java:58)
  reactor.core.publisher.FluxContextWriteRestoringThreadLocals.subscribe(FluxContextWriteRestoringThreadLocals.java:45)
  reactor.core.publisher.Flux.subscribe(Flux.java:8888)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:197)
  reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:122)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)
  reactor.core.publisher.FluxFilter$FilterSubscriber.onNext(FluxFilter.java:114)
  reactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:224)
  reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2564)
  reactor.core.publisher.FluxMap$MapConditionalSubscriber.request(FluxMap.java:294)
  reactor.core.publisher.FluxFilter$FilterSubscriber.request(FluxFilter.java:186)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.request(MonoFlatMap.java:194)
  reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.request(FluxDefaultIfEmpty.java:98)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribe(MonoFlatMapMany.java:142)
  reactor.core.publisher.Operators$BaseFluxToMonoOperator.onSubscribe(Operators.java:2047)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onSubscribe(MonoFlatMap.java:117)
  reactor.core.publisher.FluxFilter$FilterSubscriber.onSubscribe(FluxFilter.java:86)
  reactor.core.publisher.FluxMap$MapConditionalSubscriber.onSubscribe(FluxMap.java:194)
  reactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)
  reactor.core.publisher.MonoDeferContextual.subscribe(MonoDeferContextual.java:56)
  reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:75)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165)
  reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)
  reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onNext(MonoSubscribeOn.java:147)
  reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)
  reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:203)
  reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:297)
  reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:191)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)
  reactor.core.publisher.FluxSubscribeOnValue$ScheduledScalar.run(FluxSubscribeOnValue.java:181)
  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
  reactor.core.scheduler.ExecutorScheduler$ExecutorPlainRunnable.run(ExecutorScheduler.java:135)
  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
  java.base/java.lang.VirtualThread.run(Unknown Source)
#3:
  Hint: [ec2939a0-28160] 
  org.springframework.core.io.buffer.NettyDataBuffer.touch(NettyDataBuffer.java:360)
  org.springframework.core.io.buffer.NettyDataBuffer.touch(NettyDataBuffer.java:40)
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  org.springframework.core.codec.Hints.touchDataBuffer(Hints.java:165)
  org.springframework.http.codec.AbstractJacksonEncoder.encodeValue(AbstractJacksonEncoder.java:246)
  org.springframework.http.codec.AbstractJacksonEncoder.lambda$encode$1(AbstractJacksonEncoder.java:151)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:284)
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  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:151)
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  reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onSubscribe(FluxContextWriteRestoringThreadLocals.java:105)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:265)
  reactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)
  reactor.core.publisher.FluxFromMonoOperator.subscribe(FluxFromMonoOperator.java:83)
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  reactor.core.publisher.FluxContextWriteRestoringThreadLocals.subscribe(FluxContextWriteRestoringThreadLocals.java:45)
  reactor.core.publisher.Flux.subscribe(Flux.java:8888)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:197)
  reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:122)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)
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  reactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)
  reactor.core.publisher.MonoDeferContextual.subscribe(MonoDeferContextual.java:56)
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  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165)
  reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)
  reactor.core.publisher.MonoSubscribeOn$SubscribeOnSubscriber.onNext(MonoSubscribeOn.java:147)
  reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:80)
  reactor.core.publisher.FluxPeek$PeekSubscriber.onNext(FluxPeek.java:203)
  reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:297)
  reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onNext(MonoIgnoreThen.java:191)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)
  reactor.core.publisher.FluxSubscribeOnValue$ScheduledScalar.run(FluxSubscribeOnValue.java:181)
  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
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  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
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#4:
  io.netty.buffer.AdvancedLeakAwareByteBuf.writeBytes(AdvancedLeakAwareByteBuf.java:617)
  org.springframework.core.io.buffer.NettyDataBuffer.write(NettyDataBuffer.java:180)
  org.springframework.core.io.buffer.NettyDataBuffer.write(NettyDataBuffer.java:40)
  org.springframework.http.codec.AbstractJacksonEncoder.encodeValue(AbstractJacksonEncoder.java:245)
  org.springframework.http.codec.AbstractJacksonEncoder.lambda$encode$1(AbstractJacksonEncoder.java:151)
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  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:361)
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  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onSubscribeInner(MonoFlatMapMany.java:151)
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  reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onSubscribe(FluxContextWriteRestoringThreadLocals.java:105)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:265)
  reactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)
  reactor.core.publisher.FluxFromMonoOperator.subscribe(FluxFromMonoOperator.java:83)
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  reactor.core.publisher.FluxContextWriteRestoringThreadLocals.subscribe(FluxContextWriteRestoringThreadLocals.java:45)
  reactor.core.publisher.Flux.subscribe(Flux.java:8888)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:197)
  reactor.core.publisher.FluxDefaultIfEmpty$DefaultIfEmptySubscriber.onNext(FluxDefaultIfEmpty.java:122)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)
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  reactor.core.publisher.FluxMap$MapConditionalSubscriber.onSubscribe(FluxMap.java:194)
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  reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.complete(MonoIgnoreThen.java:297)
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  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
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  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
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Created at:
  io.netty.buffer.AdaptiveByteBufAllocator.newDirectBuffer(AdaptiveByteBufAllocator.java:67)
  io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:168)
  io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:159)
  io.netty.buffer.AbstractByteBufAllocator.buffer(AbstractByteBufAllocator.java:96)
  org.springframework.core.io.buffer.NettyDataBufferFactory.allocateBuffer(NettyDataBufferFactory.java:72)
  org.springframework.core.io.buffer.NettyDataBufferFactory.allocateBuffer(NettyDataBufferFactory.java:39)
  org.springframework.http.codec.AbstractJacksonEncoder.encodeValue(AbstractJacksonEncoder.java:244)
  org.springframework.http.codec.AbstractJacksonEncoder.lambda$encode$1(AbstractJacksonEncoder.java:151)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:284)
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  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:361)
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  reactor.core.publisher.FluxContextWriteRestoringThreadLocals$ContextWriteRestoringThreadLocalsSubscriber.onSubscribe(FluxContextWriteRestoringThreadLocals.java:105)
  reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:265)
  reactor.core.publisher.MonoJust.subscribe(MonoJust.java:56)
  reactor.core.publisher.FluxFromMonoOperator.subscribe(FluxFromMonoOperator.java:83)
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  reactor.core.publisher.FluxContextWriteRestoringThreadLocals.subscribe(FluxContextWriteRestoringThreadLocals.java:45)
  reactor.core.publisher.Flux.subscribe(Flux.java:8888)
  reactor.core.publisher.MonoFlatMapMany$FlatMapManyMain.onNext(MonoFlatMapMany.java:197)
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  reactor.core.publisher.MonoFlatMap$FlatMapMain.onSubscribe(MonoFlatMap.java:117)
  reactor.core.publisher.FluxFilter$FilterSubscriber.onSubscribe(FluxFilter.java:86)
  reactor.core.publisher.FluxMap$MapConditionalSubscriber.onSubscribe(FluxMap.java:194)
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  reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:75)
  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:165)
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  reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:158)
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  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
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  io.micrometer.context.ContextSnapshot.lambda$wrap$0(ContextSnapshot.java:91)
  java.base/java.lang.VirtualThread.run(Unknown Source)

[bclozel]

@kzander91 something is off in your issue description.

The leak report seems to show that Framework schedules the response handling on a virtual thread, which it should never do for reactive application. I guess critical information is missing from your issue description. Right now, my best guess is that hopping threads manually in a middle of a reactive pipeline is breaking expectations and might be very well responsible for the "onDiscard" operators to not act properly.

Does your application enables virtual threads and schedules controller methods on task scheduler threads in a reactive application? If so, I think this is an invalid scenario. Can you fix that and report back? Can you share a minimal sample application?

[kzander91]

@bclozel yes, I'm running on virtual threads with spring.threads.virtual.enabled=true. Since my controller method is not returning a Publisher, WebFlux invokes it on the (virtual thread-enabled) SimpleAsyncTaskExecutor auto-configured by Spring Boot.

See https://github.com/spring-projects/spring-boot/blob/1ab1436051b69288adb802afffd73aef94460ff1/module/spring-boot-webflux/src/main/java/org/springframework/boot/webflux/autoconfigure/WebFluxAutoConfiguration.java#L223-L233

Sorry for omitting this, I wasn't aware that this would be a relevant detail.

[bclozel]

Thanks for the update, I remember now that we did that on purpose but I'm wondering if this might be a factor here. Can you share a minimal sample application? You can simulate such connections being closed prematurely by using a raw Java Socket or with Apache HttpClient by aborting the request.

[kzander91]

I finally managed to reproduce this using a raw Socket: demo24.zip

1. Run main to start the server.
2. Run the unit test in Demo24ApplicationTests.

You may need to play with the number of test invocations or the socket timeout. On my Windows laptop, this results in at least one leak report in most attempts.
(Note that our main app runs on Linux, so I don't suspect the OS to be involved here).