Rest-client-mutiny issue, exhausting thread pool

Describe the bug

When using rest-client-mutiny, returning a Uni, it starts failing after configured pool size is exhausted, default appeared to be 50. quarkus.rest-client.config-key.connection-pool-size=5

connect-timeout,read-timeout,connection-ttl variables don’t affect the result. Sometimes, it manages to do “yet another round” of requests, so if pool-size is N, it appear to get 2*N working.

My workaround for now is set scope=RequestScoped, this issue is reproducible with Singleton, Dependent and ApplicationScoped scopes.

Expected behavior

I was expecting to manage infinite sequencial requests with the pool.

Actual behavior

It does not manage infinite requests, they are silently hanging.

When stopped the service, this is the error we get on each of the hanging threads

{
  "class": "java.util.concurrent.ThreadPoolExecutor$AbortPolicy",
  "method": "rejectedExecution",
  "line": 2065
},
{
  "class": "java.util.concurrent.ThreadPoolExecutor",
  "method": "reject",
  "line": 833
},
{
  "class": "java.util.concurrent.ScheduledThreadPoolExecutor",
  "method": "delayedExecute",
  "line": 340
},
{
  "class": "java.util.concurrent.ScheduledThreadPoolExecutor",
  "method": "schedule",
  "line": 562
},
{
  "class": "io.quarkus.mutiny.runtime.MutinyInfrastructure$2",
  "method": "schedule",
  "line": 54
},
{
  "class": "io.smallrye.mutiny.operators.uni.UniDelayOnItem$UniDelayOnItemProcessor",
  "method": "onItem",
  "line": 54
},
{
  "class": "io.smallrye.mutiny.operators.uni.builders.UniCreateFromKnownItem$KnownItemSubscription",
  "method": "forward",
  "line": 38
},
{
  "class": "io.smallrye.mutiny.operators.uni.builders.UniCreateFromKnownItem$KnownItemSubscription",
  "method": "access$100",
  "line": 26
},
{
  "class": "io.smallrye.mutiny.operators.uni.builders.UniCreateFromKnownItem",
  "method": "subscribe",
  "line": 23
},
{
  "class": "io.smallrye.mutiny.operators.AbstractUni",
  "method": "subscribe",
  "line": 36
},
{
  "class": "io.smallrye.mutiny.operators.uni.UniDelayOnItem",
  "method": "subscribe",
  "line": 28
},
{
  "class": "io.smallrye.mutiny.operators.AbstractUni",
  "method": "subscribe",
  "line": 36
},
{
  "class": "io.smallrye.mutiny.converters.uni.UniToMultiPublisher$UniToMultiSubscription",
  "method": "request",
  "line": 74
},
{
  "class": "io.smallrye.mutiny.operators.multi.MultiFlatMapOp$FlatMapInner",
  "method": "onSubscribe",
  "line": 601
},
{
  "class": "io.smallrye.mutiny.converters.uni.UniToMultiPublisher",
  "method": "subscribe",
  "line": 26
},
{
  "class": "io.smallrye.mutiny.groups.MultiCreate$1",
  "method": "subscribe",
  "line": 163
},
{
  "class": "io.smallrye.mutiny.operators.multi.MultiFlatMapOp$FlatMapMainSubscriber",
  "method": "onItem",
  "line": 193
},
{
  "class": "io.smallrye.mutiny.subscription.MultiSubscriber",
  "method": "onNext",
  "line": 61
},
{
  "class": "io.smallrye.mutiny.operators.multi.processors.UnicastProcessor",
  "method": "drainWithDownstream",
  "line": 103
},
{
  "class": "io.smallrye.mutiny.operators.multi.processors.UnicastProcessor",
  "method": "drain",
  "line": 134
},
{
  "class": "io.smallrye.mutiny.operators.multi.processors.UnicastProcessor",
  "method": "onNext",
  "line": 200
},
{
  "class": "io.smallrye.mutiny.operators.multi.processors.SerializedProcessor",
  "method": "onNext",
  "line": 105
},
{
  "class": "io.smallrye.mutiny.subscription.SerializedSubscriber",
  "method": "onItem",
  "line": 75
},
{
  "class": "io.smallrye.mutiny.subscription.MultiSubscriber",
  "method": "onNext",
  "line": 61
},
{
  "class": "io.smallrye.mutiny.operators.multi.MultiRetryWhenOp$RetryWhenOperator",
  "method": "onFailure",
  "line": 129
},
{
  "class": "io.smallrye.mutiny.subscription.MultiSubscriber",
  "method": "onError",
  "line": 73
},
{
  "class": "io.smallrye.mutiny.converters.uni.UniToMultiPublisher$UniToMultiSubscription",
  "method": "onFailure",
  "line": 103
},
{
  "class": "io.smallrye.mutiny.operators.uni.UniOperatorProcessor",
  "method": "onFailure",
  "line": 54
},
{
  "class": "io.smallrye.mutiny.operators.uni.UniOperatorProcessor",
  "method": "onFailure",
  "line": 54
},
{
  "class": "io.smallrye.mutiny.operators.uni.UniOperatorProcessor",
  "method": "onFailure",
  "line": 54
},
{
  "class": "io.smallrye.mutiny.operators.uni.UniOnItemConsume$UniOnItemComsumeProcessor",
  "method": "onFailure",
  "line": 65
},
{
  "class": "io.smallrye.mutiny.operators.uni.UniOperatorProcessor",
  "method": "onFailure",
  "line": 54
},
{
  "class": "io.smallrye.mutiny.operators.uni.builders.UniCreateFromCompletionStage$CompletionStageUniSubscription",
  "method": "forwardResult",
  "line": 58
},
{
  "class": "java.util.concurrent.CompletableFuture",
  "method": "uniWhenComplete",
  "line": 863
},
{
  "class": "java.util.concurrent.CompletableFuture$UniWhenComplete",
  "method": "tryFire",
  "line": 841
},
{
  "class": "java.util.concurrent.CompletableFuture",
  "method": "postComplete",
  "line": 510
},
{
  "class": "java.util.concurrent.CompletableFuture$AsyncSupply",
  "method": "run",
  "line": 1773
},
{
  "class": "org.jboss.resteasy.microprofile.client.impl.MpClientInvocation$Decorator",
  "method": "lambda$decorate$0",
  "line": 93
},
{
  "class": "io.smallrye.context.impl.wrappers.SlowContextualRunnable",
  "method": "run",
  "line": 19
},
{
  "class": "io.quarkus..core.runtime.VertxCoreRecorder$14",
  "method": "runWith",
  "line": 555
},
{
  "class": "org.jboss.threads.EnhancedQueueExecutor$Task",
  "method": "run",
  "line": 2449
},
{
  "class": "org.jboss.threads.EnhancedQueueExecutor$ThreadBody",
  "method": "run",
  "line": 1478
},
{
  "class": "org.jboss.threads.DelegatingRunnable",
  "method": "run",
  "line": 29
},
{
  "class": "org.jboss.threads.ThreadLocalResettingRunnable",
  "method": "run",
  "line": 29
},
{
  "class": "io.netty.util.concurrent.FastThreadLocalRunnable",
  "method": "run",
  "line": 30
},
{
  "class": "java.lang.Thread",
  "method": "run",
  "line": 833
},
{
  "class": "com.oracle.svm.core.thread.PlatformThreads",
  "method": "threadStartRoutine",
  "line": 704
},
{
  "class": "com.oracle.svm.core.posix.thread.PosixPlatformThreads",
  "method": "pthreadStartRoutine",
  "line": 202
}```

### How to Reproduce?

Library is deprecated, if you think there is any chance you fix it, I will create a reproducer.
I am not expecting a fix, just reporting it and maybe letting me know if I am doing something wrong...

My code is returning Uni<Void>:

 @RestClient
private final SomeClient client; 
 .....
Uni<Void> .... {
....
   client.sendResult(getAuthorizationHeader(), parameter,  entity)
                    .replaceWithVoid()
                    .invoke(logWebhookSent(entity));

}

### Output of `uname -a` or `ver`

_No response_

### Output of `java -version`

17.0.1

### GraalVM version (if different from Java)

22.2

### Quarkus version or git rev

2.14.0.Final

### Build tool (ie. output of `mvnw --version` or `gradlew --version`)

gradle 7.5.1

### Additional information

We started using mutiny instead of reactive because we needed this PR merged to use it https://github.com/quarkusio/quarkus/issues/27901

[Chexpir]

@cescoffier I will create one as soon as I have a short-term fix for my project and put it here.

[cescoffier]

If you use the reactive REST client, the issue does not happen. So, it’s a bug in the classic REST client. Unfortunately, I won’t be able to help on this one.