Reading POST body: java.lang.IllegalStateException: Only one connection receive subscriber allowed - spring-webflux

Question

In Spring Webflux, when I read a POST body, and try to work with it, it always results in the IllegalStateException from the title.
Here the code:
#Bean
public RouterFunction<ServerResponse> selectByPost(SasoSecurityFacade solrHandler) {
return RouterFunctions.route(RequestPredicates.POST("/v1/{collection}/select")
.and(RequestPredicates.accept(MediaType.APPLICATION_JSON)), request -> request.bodyToMono(String.class)
.flatMap(s -> {
System.out.println(s);
return ServerResponse.ok()
.syncBody(s);
}));
}
Also important (as it turns out), the request TO the server:
curl 'https://<myserver>:9443/v1/banana-int/select' -H 'Pragma: no-cache' -H 'Origin: https://<myserver>:9443' -H 'Accept-Encoding: gzip, deflate, br' -H 'Accept-Language: de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7' -H 'User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36' -H 'Content-type: application/x-www-form-urlencoded' -H 'Accept: application/json, text/plain, */*' -H 'Cache-Control: no-cache' -H 'Referer: https://<myserver>:9443/banana/src/index.html' -H 'Connection: keep-alive' -H 'DNT: 1' --data 'q=*:*&rows=20&wt=json' --compressed
What am I doing wrong? How can I debug this? Who else is reading my post body, so that I get this error message?

Answer 1

The request you're sending is a POST Form request, which is automatically read by HiddenHttpMethodFilter. This filter is consuming the form data to mutate the HTTP method in case an alternate method is specified through a "_method" parameter.
In this case, consuming the form data with the dedicated request.formData() API in WebFlux annotation/functional is the best choice, this it is cached for future use and does not trigger a new subscription.
If you wish to completely disable this behavior, you'll be able to set a property in Spring Boot 2.1.