Spring Zombie socket file descriptors issue

After uprading from spring boot 2.3.0 to 2.3.1, our application in staging environment quickly reached linux maximum open files limit. Using lsof -i 6 -p <pid> to check open file descriptors of the spring boot webflux application, there are lots this kind of open file descriptors:

java      898        ce-devel 2540u     sock                0,8       0t0     27988 protocol: TCPv6
java      898        ce-devel 2541u     sock                0,8       0t0     27989 protocol: TCPv6
java      898        ce-devel 2542u     sock                0,8       0t0     27990 protocol: TCPv6
java      898        ce-devel 2543u     sock                0,8       0t0     27991 protocol: TCPv6
java      898        ce-devel 2544u     sock                0,8       0t0     27992 protocol: TCPv6
java      898        ce-devel 2545u     sock                0,8       0t0     27993 protocol: TCPv6
java      898        ce-devel 2546u     sock                0,8       0t0     27994 protocol: TCPv6
java      898        ce-devel 2547u     sock                0,8       0t0     27995 protocol: TCPv6
java      898        ce-devel 2548u     sock                0,8       0t0     27996 protocol: TCPv6
java      898        ce-devel 2549u     sock                0,8       0t0     27997 protocol: TCPv6
java      898        ce-devel 2550u     sock                0,8       0t0     27998 protocol: TCPv6
java      898        ce-devel 2551u     sock                0,8       0t0     27999 protocol: TCPv6
java      898        ce-devel 2552u     sock                0,8       0t0     28000 protocol: TCPv6
java      898        ce-devel 2553u     sock                0,8       0t0     28001 protocol: TCPv6
java      898        ce-devel 2554u     sock                0,8       0t0     28002 protocol: TCPv6
java      898        ce-devel 2555u     sock                0,8       0t0     28003 protocol: TCPv6
java      898        ce-devel 2556u     sock                0,8       0t0     28004 protocol: TCPv6
java      898        ce-devel 2557u     sock                0,8       0t0     28005 protocol: TCPv6
java      898        ce-devel 2558u     sock                0,8       0t0     28006 protocol: TCPv6
java      898        ce-devel 2559u     sock                0,8       0t0     28007 protocol: TCPv6
java      898        ce-devel 2560u     sock                0,8       0t0     28008 protocol: TCPv6
java      898        ce-devel 2561u     sock                0,8       0t0     28009 protocol: TCPv6
java      898        ce-devel 2562u     sock                0,8       0t0     28010 protocol: TCPv6

and the number keep increasing. Downgrading to spring boot 2.3.0 solved the problem.

This problem only occurs resides in applications where reactor netty based webclients are used with no special configuration:

    @LoadBalanced
    @Bean
    public WebClient.Builder loadBalancedWebClient() {
        return WebClient.builder();
    }

    @Primary
    @Bean
    public WebClient.Builder webClient() {
        return WebClient.builder();
    }

So I think the problem exists in WebClient.Builder or reactor netty. I think this is really a serious issue and stopped us from upgrading to newer version.

For your reference : https://serverfault.com/questions/1000338/in-lsof-output-what-are-those-sock-lines

Comment From: bclozel

This is a duplicate of #25326 Thanks for your report!