[9.4] (backport #19017) fix forced-shutdown (double SIGINT) behaviour for JRuby 10

[mergify]

Release notes

[rn:skip]

What does this PR do?

Fixes an issue introduced with the upgrade to JRuby 10 in which a doubled-SIGINT does not force an immediate shutdown because the SystemExit exception is caught and squashed by the default thread exception handler.

Why is it important/What is the impact to the user?

Sometimes our users need to force Logstash to shut down before it has finished processing the events in-flight. The upgrade to JRuby10 inadvertently removed this functionality, and we restore it here.

Checklist

• My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

How to test this PR locally

1. Invoke Logstash with intentional back-pressure:

   bin/logstash -e 'input { generator {} } filter { ruby { code => "sleep 1" } } output { sink {} }'
   

2. Wait until processing, then send TWO SIGINT signals (CTRL+C in most shells)
3. Observe that the process correctly bails immediately after the second SIGINT.

(without this patch, Logstash keeps processing until the in-flight events are done)

Related issues

Caused by: #18755

Logs

Without (note how it keeps executing after logging "Terminating Immediately"):

[2026-04-17T15:29:58,450][INFO ][logstash.agent           ] Pipelines running {count: 1, running_pipelines: [:main], non_running_pipelines: []}
^C[2026-04-17T15:30:02,168][WARN ][logstash.runner          ] SIGINT received. Shutting down.
^C[2026-04-17T15:30:02,946][FATAL][logstash.runner          ] SIGINT received. Terminating immediately..
[2026-04-17T15:30:02,947][ERROR][org.logstash.Logstash    ] uncaught exception (in thread SIGINT handler)
org.jruby.exceptions.SystemExit: (SystemExit) exit
        at org.jruby.RubyKernel.exit(org/jruby/RubyKernel.java:974)
        at org.jruby.RubyKernel.exit(org/jruby/RubyKernel.java:933)
        at org.jruby.RubyThread.kill(org/jruby/RubyThread.java:1955)
[2026-04-17T15:30:07,184][WARN ][logstash.runner          ] Received shutdown signal, but pipeline is still waiting for in-flight events
to be processed. Sending another ^C will force quit Logstash, but this may cause
data loss.
[2026-04-17T15:30:07,283][ERROR][org.logstash.execution.ShutdownWatcherExt] The shutdown process appears to be stalled due to busy or blocked plugins. Check the logs for more information.
[2026-04-17T15:30:07,283][INFO ][org.logstash.execution.ShutdownWatcherExt] The queue for pipeline main is draining before shutdown.
^C[2026-04-17T15:30:20,063][FATAL][logstash.runner          ] SIGINT received. Terminating immediately..
[2026-04-17T15:30:20,064][ERROR][org.logstash.Logstash    ] uncaught exception (in thread SIGINT handler)
org.jruby.exceptions.SystemExit: (SystemExit) exit
        at org.jruby.RubyKernel.exit(org/jruby/RubyKernel.java:974)
        at org.jruby.RubyKernel.exit(org/jruby/RubyKernel.java:933)
        at org.jruby.RubyThread.kill(org/jruby/RubyThread.java:1955)

With (note how it actually terminates immediately after logging "Terminating Immediately"):

[2026-04-17T16:06:52,247][INFO ][logstash.javapipeline    ][main] Pipeline started {"pipeline.id" => "main"}
[2026-04-17T16:06:52,256][INFO ][logstash.agent           ] Pipelines running {count: 1, running_pipelines: [:main], non_running_pipelines: []}
^C[2026-04-17T16:06:57,660][WARN ][logstash.runner          ] SIGINT received. Shutting down.
[2026-04-17T16:07:02,674][WARN ][logstash.runner          ] Received shutdown signal, but pipeline is still waiting for in-flight events
to be processed. Sending another ^C will force quit Logstash, but this may cause
data loss.
[2026-04-17T16:07:02,791][ERROR][org.logstash.execution.ShutdownWatcherExt] The shutdown process appears to be stalled due to busy or blocked plugins. Check the logs for more information.
[2026-04-17T16:07:02,792][INFO ][org.logstash.execution.ShutdownWatcherExt] The queue for pipeline main is draining before shutdown.
^C[2026-04-17T16:07:04,711][FATAL][logstash.runner          ] SIGINT received. Terminating immediately..
[error: 1 (00:00:19)] 

---

This is an automatic backport of pull request #19017 done by [Mergify](https://mergify.com).

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