Returning Mono.empty causes controller to halt

[bazusaladra]

The following code used to work on SpringBoot 2.0.2

    @GetMapping(path = "search")
    public Mono<String> search(@RequestParam(value = "query") String query) {
        if (Objects.isNull(query) || query.isEmpty()) {
            return Mono.empty();
        }
        return Mono.just("result: " + query);
    }

After upgrading to SpringBoot 2.1.0 calling

String actualEmptyResult = restTemplate.getForObject("/api/search?query=", String.class);

causes any subsequent requests to this endpoint to be stalled.
Please check out test org.springtest.EmptyMonoFailureTest#testMono in repository https://github.com/bazusaladra/spring-mono-test.
SpringBoot team advised this issue may be related to Reactor Netty: spring-projects/spring-boot#15214

[rstoyanchev]

This looks like the same issue as https://jira.spring.io/browse/SPR-17506. The fix on the WebFlux side, was to call response.setComplete() which completes without writing. Nevertheless there is a likely issue in Reactor Netty where writing with Mono.empty() never completes.

[bclozel]

FYI I can't reproduce this issue anymore with Spring Boot 2.1.1.RELEASE, i.e. reactor netty 0.8.3.RELEASE (I can reproduce it with 2.1.0.RELEASE, i.e. 0.8.2.RELEASE).

[violetagg]

@bclozel try with spring framework 5.1.2.RELEASE and Spring Boot 2.1.1.RELEASE

[bclozel]

You're right @violetagg, this was fixed in spring-projects/spring-framework@abf9ce8a34

[rstoyanchev]

This has been fixed in the Spring Framework. See https://jira.spring.io/browse/SPR-17609.

[giriganapathy]

Hello Team,

I am new to Flux, and i am streaming the file using Flux, however, my subsequent requests from browser is getting stuck, and here is my code:

@RequestMapping (value="/downloadReactive", produces=MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux downloadReactiveWay(ServerHttpRequest request, ServerHttpResponse response) throws Exception {

	Flux<DataBuffer> flux = null;		
	UrlResource urlResource = new UrlResource(FILE1);
	long totalSize = urlResource.contentLength();
	
	response.getHeaders().add("Content-Type", MediaType.APPLICATION_OCTET_STREAM_VALUE);
	response.getHeaders().add("Content-Disposition", "attachment; filename=\"app\"");		
	response.getHeaders().add("Content-Length", String.valueOf(totalSize));
	
	
	flux = DataBufferUtils.read(urlResource, new DefaultDataBufferFactory(), 1024);
	//https://github.com/spring-projects/spring-framework/commit/abf9ce8a34
	flux.switchIfEmpty(Flux.defer(()->{
		response.getHeaders().setContentLength(0);
		return response.setComplete().then(Mono.empty());
	}));
	
	
	return flux;		
}

POM.xml:

4.0.0

org.springframework.boot
spring-boot-starter-parent
2.1.5.RELEASE


com.reactive
download_server
0.0.1-SNAPSHOT
download_server
Demo project for Spring Boot

<properties>
	<java.version>1.8</java.version>
</properties>

<dependencies>
	<dependency>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-webflux</artifactId>
	</dependency>

	<dependency>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-test</artifactId>
		<scope>test</scope>
	</dependency>
	<dependency>
		<groupId>io.projectreactor</groupId>
		<artifactId>reactor-test</artifactId>
		<scope>test</scope>
	</dependency>
</dependencies>

<build>
	<plugins>
		<plugin>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-maven-plugin</artifactId>
		</plugin>
	</plugins>
</build>

I am stuck here, It would be great, if someone shares the solution.
Thanks.