[BUG] Opensearch process crashes when running a painless script

[shiv0408]

Describe the bug
OpenSearch process dies when running a painless script where a def type value is explicitly returned where the context is void type with following error.

» fatal error in thread [opensearch[runTask-0][write][T#5]], exiting
»  java.lang.NoClassDefFoundError: V
»       at org.opensearch.painless.PainlessScript$Script.execute(def x=1;return x;:16)
»       at org.opensearch.action.update.UpdateHelper.executeScript(UpdateHelper.java:337)
»       at org.opensearch.action.update.UpdateHelper.prepareUpdateScriptRequest(UpdateHelper.java:277)
»       at org.opensearch.action.update.UpdateHelper.prepare(UpdateHelper.java:107)
»       at org.opensearch.action.update.UpdateHelper.prepare(UpdateHelper.java:88)
»       at org.opensearch.action.update.TransportUpdateAction.shardOperation(TransportUpdateAction.java:224)
»       at org.opensearch.action.update.TransportUpdateAction.shardOperation(TransportUpdateAction.java:217)
»       at org.opensearch.action.update.TransportUpdateAction.shardOperation(TransportUpdateAction.java:89)
»       at org.opensearch.action.support.single.instance.TransportInstanceSingleOperationAction$ShardTransportHandler$1.doRun(TransportInstanceSingleOperationAction.java:321)
»       at org.opensearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:806)
»       at org.opensearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:52)
»       at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
»       at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
»       at java.base/java.lang.Thread.run(Thread.java:829)
»  Caused by: java.lang.ClassNotFoundException: V
»       at java.base/java.lang.ClassLoader.findClass(ClassLoader.java:719)
»       at org.opensearch.painless.Compiler$Loader.findClass(Compiler.java:115)
»       at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:589)
»       at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
»       ... 14 more

To Reproduce
Steps to reproduce the behavior:

1. Index some documents

curl -X PUT "localhost:9200/hockey/_bulk?refresh&pretty" -H 'Content-Type: application/json' -d'
{"index":{"_id":1}}
{"first":"johnny","last":"gaudreau","goals":[9,27,1],"assists":[17,46,0],"gp":[26,82,1],"born":"1993/08/13"}
{"index":{"_id":2}}
{"first":"sean","last":"monohan","goals":[7,54,26],"assists":[11,26,13],"gp":[26,82,82],"born":"1994/10/12"}
{"index":{"_id":3}}
{"first":"jiri","last":"hudler","goals":[5,34,36],"assists":[11,62,42],"gp":[24,80,79],"born":"1984/01/04"}
{"index":{"_id":4}}
{"first":"micheal","last":"frolik","goals":[4,6,15],"assists":[8,23,15],"gp":[26,82,82],"born":"1988/02/17"}
{"index":{"_id":5}}
{"first":"sam","last":"bennett","goals":[5,0,0],"assists":[8,1,0],"gp":[26,1,0],"born":"1996/06/20"}
{"index":{"_id":6}}
{"first":"dennis","last":"wideman","goals":[0,26,15],"assists":[11,30,24],"gp":[26,81,82],"born":"1983/03/20"}
{"index":{"_id":7}}
{"first":"david","last":"jones","goals":[7,19,5],"assists":[3,17,4],"gp":[26,45,34],"born":"1984/08/10"}
{"index":{"_id":8}}
{"first":"tj","last":"brodie","goals":[2,14,7],"assists":[8,42,30],"gp":[26,82,82],"born":"1990/06/07"}
{"index":{"_id":39}}
{"first":"mark","last":"giordano","goals":[6,30,15],"assists":[3,30,24],"gp":[26,60,63],"born":"1983/10/03"}
{"index":{"_id":10}}
{"first":"mikael","last":"backlund","goals":[3,15,13],"assists":[6,24,18],"gp":[26,82,82],"born":"1989/03/17"}
{"index":{"_id":11}}
{"first":"joe","last":"colborne","goals":[3,18,13],"assists":[6,20,24],"gp":[26,67,82],"born":"1990/01/30"}
'

2. Using a simple painless script in update context to replicate the error

curl -X POST "localhost:9200/hockey/_update/3?pretty" -H 'Content-Type: application/json' -d'
{
  "script": {
    "source": "def x=1;return x;",
    "lang": "painless"
  }
}
'

Expected behavior
OpenSearch process should not die and run the script as expected.

Plugins
N/A

Host/Environment (please complete the following information):

• OS: MacOS
• Version 13.2.1

[dblock]

Looks like a bug. Want to try and turn it into a failing unit test, @shiv0408?

[shiv0408]

Sure, I will take a stab at it.

[shiv0408]

@dblock I have created a failing UT. But to create this unit test, I had to make change supported scripts in _scripts/painless/_execute API by adding the update context. How can we keep this unit test in this code without modifying the supported scripts?

Any ideas about how we can keep the UT without modifying other functionalities?

[dblock]

Let's discuss this in the PR! Thank you for doing the work.