InvalidObservationException: Invalid start: Observation 'http.client.requests' has already been started

[viktorgunnarson]

Running the test class below with spring-boot 3.3.10 all cases pass, but with spring-boot 3.4.4 the test retryWithObservationRegistryFails() fails with the following error:

Expecting actual throwable to be an instance of:
  org.springframework.web.reactive.function.client.WebClientResponseException
but was:
  io.micrometer.observation.tck.InvalidObservationException: Invalid start: Observation 'http.client.requests' has already been started
START: org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:458)
STOP: org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$10(DefaultWebClient.java:484)
START: org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:458)
START: org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:458)
START: org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:458)
	at io.micrometer.observation.tck.ObservationValidator.throwInvalidObservationException(ObservationValidator.java:149)
	at io.micrometer.observation.tck.ObservationValidator.onStart(ObservationValidator.java:64)
	at io.micrometer.observation.SimpleObservation.notifyOnObservationStarted(SimpleObservation.java:214)
	...(107 remaining lines not displayed - this can be changed with Assertions.setMaxStackTraceElementsDisplayed)

	at com.example.demo.DemoApplicationTests.retryWithObservationRegistryFails(DemoApplicationTests.java:44)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)

Reproduction example

package com.example.demo;

import static org.assertj.core.api.Assertions.assertThatExceptionOfType;

import org.junit.jupiter.api.Test;
import org.springframework.web.reactive.function.client.WebClient;
import org.springframework.web.reactive.function.client.WebClientResponseException;

import io.micrometer.observation.tck.TestObservationRegistry;

class DemoApplicationTests {

    private final WebClient.Builder webclientBuilder = WebClient.builder()
        .baseUrl("https://httpstat.us/500");

    private final WebClient webClientNoObservationRegistry = webclientBuilder
        .build();

    private final WebClient webClient = webclientBuilder
        .observationRegistry(TestObservationRegistry.create())
        .build();

    @Test
    void noRetryWorks() {
        assertThatExceptionOfType(WebClientResponseException.class).isThrownBy(() ->
            webClient.get().retrieve()
                .bodyToMono(String.class)
                .block()
        );
    }

    @Test
    void retryWithoutObservationRegistryWorks() {
        assertThatExceptionOfType(WebClientResponseException.class).isThrownBy(() ->
            webClientNoObservationRegistry.get().retrieve()
                .bodyToMono(String.class)
                .retry(3)
                .block()
        );
    }

    @Test
    void retryWithObservationRegistryFails() {
        assertThatExceptionOfType(WebClientResponseException.class).isThrownBy(() ->
            webClient.get().retrieve()
                .bodyToMono(String.class)
                .retry(3)
                .block()
        );
    }

}

[viktorgunnarson]

It's the combination of TestObservationRegistry and .retry() that is incompatible since the introduction of ObservationValidator in micrometer 1.14.0 see specifically micrometer-metrics/micrometer#5239

[bclozel]

Thanks @viktorgunnarson , this is now fixed and will be released with the next 6.2.x maintenance release in 2 weeks.