Groovy assert misses StringBuffer

[SweetNSourPavement]

Elasticsearch version:
5.0.0-alpha4

JVM version:
Oracle Corporation/Java HotSpot(TM) 64-Bit Server VM/1.8.0_101/25.101-b13

OS version:
Linux 107r01pc 4.2.0-42-generic #49~14.04.1-Ubuntu SMP Wed Jun 29 20:22:11 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

Description of the problem including expected versus actual behavior:
In a groovy scripted update asserting while referencing variables and specifying a message will cause a class not found exception.

Steps to reproduce:

1. on a clean elasticsearch enable scripting
2. run some curl commands, see below
3. observe the results

Various commands:

1. curl -XPOST 'http://localhost:9200/a/a/a/_update' -d '{"scripted_upsert": true, "script": {"inline": "assert false, \"foo\";"}, "upsert": {}}'
   • {"error":{"root_cause":[{"type":"remote_transport_exception","reason":"[Vertigo][127.0.0.1:9300][indices:data/write/update[s]]"}],"type":"illegal_argument_exception","reason":"failed to execute script","caused_by":{"type":"script_exception","reason":"Error evaluating assert false, \"foo\";","caused_by":{"type":"assertion_error","reason":"foo. Expression: false"},"script_stack":[],"script":"","lang":"groovy"}},"status":400}
2. curl -XPOST 'http://localhost:9200/a/a/b/_update' -d '{"scripted_upsert": true, "script": {"inline": "def bar=false; assert bar, \"foo\";"}, "upsert": {}}'
   • {"error":{"root_cause":[{"type":"remote_transport_exception","reason":"[Vertigo][127.0.0.1:9300][indices:data/write/update[s]]"}],"type":"illegal_argument_exception","reason":"failed to execute script","caused_by":{"type":"script_exception","reason":"Error evaluating def bar=false; assert bar, \"foo\";","caused_by":{"type":"no_class_def_found_error","reason":"java/lang/StringBuffer"},"script_stack":[],"script":"","lang":"groovy"}},"status":400}
3. curl -XPOST 'http://localhost:9200/a/a/b/_update' -d '{"scripted_upsert": true, "script": {"inline": "def bar=false; assert bar;"}, "upsert": {}}'
   • {"error":{"root_cause":[{"type":"remote_transport_exception","reason":"[Vertigo][127.0.0.1:9300][indices:data/write/update[s]]"}],"type":"illegal_argument_exception","reason":"failed to execute script","caused_by":{"type":"script_exception","reason":"Error evaluating def bar=false; assert bar;","caused_by":{"type":"power_assertion_error","reason":"assert bar\n |\n false"},"script_stack":[],"script":"","lang":"groovy"}},"status":400}

Somewhat more clearly:

1. using assert false, "message" works as expected
2. using assert var, "message" throws a class not found exception somewhere down the stack
3. using assert var works as expected

I've tried this on a more recent snapshot (34bb150) and the assert shows up in the log, but curl never prints anything nor exists, as if no response is generated.
This is the log:

[2016-08-04 18:07:53,886][ERROR][bootstrap                ] [] fatal error in thread [elasticsearch[0ZFv9t-][index][T#1]], exiting
java.lang.AssertionError: foo. Expression: false
    at org.codehaus.groovy.runtime.InvokerHelper.assertFailed(InvokerHelper.java:404)
    at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.assertFailed(ScriptBytecodeAdapter.java:650)
    at 20a61a940854553b62db2cfb00cb1511e91aac90.run(20a61a940854553b62db2cfb00cb1511e91aac90:1)
    at java.security.AccessController.doPrivileged(Native Method)
    at org.elasticsearch.script.groovy.GroovyScriptEngineService$GroovyScript.run(GroovyScriptEngineService.java:295)
    at org.elasticsearch.action.update.UpdateHelper.executeScript(UpdateHelper.java:253)
    at org.elasticsearch.action.update.UpdateHelper.prepare(UpdateHelper.java:103)
    at org.elasticsearch.action.update.UpdateHelper.prepare(UpdateHelper.java:80)
    at org.elasticsearch.action.update.TransportUpdateAction.shardOperation(TransportUpdateAction.java:179)
    at org.elasticsearch.action.update.TransportUpdateAction.shardOperation(TransportUpdateAction.java:172)
    at org.elasticsearch.action.update.TransportUpdateAction.shardOperation(TransportUpdateAction.java:69)
    at org.elasticsearch.action.support.single.instance.TransportInstanceSingleOperationAction$ShardTransportHandler.messageReceived(TransportInstanceSingleOperationAction.java:247)
    at org.elasticsearch.action.support.single.instance.TransportInstanceSingleOperationAction$ShardTransportHandler.messageReceived(TransportInstanceSingleOperationAction.java:243)
    at org.elasticsearch.transport.TransportRequestHandler.messageReceived(TransportRequestHandler.java:33)
    at org.elasticsearch.transport.RequestHandlerRegistry.processMessageReceived(RequestHandlerRegistry.java:69)
    at org.elasticsearch.transport.TransportService$5.doRun(TransportService.java:517)
    at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:510)
    at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

Exception: java.security.AccessControlException thrown from the UncaughtExceptionHandler in thread "elasticsearch[0ZFv9t-][index][T#1]"

[dakrone]

@SweetNSourPavement I was able to reproduce this, this looks pretty serious, it causes a thread to essentially hang forever on master. I'll work on a fix.