Why doesn't ReentrantLock lock in a Jenkins Pipeline?

Question

I cannot seem to get this concurrence pattern to work in my Jenkins Pipeline script as intended. I have simplified the scenario as much as possible and the result still makes no sense. Here is the entire Jenkinsfile:

import java.util.concurrent.locks.ReentrantLock

// create lock and index vars to make sure concurrent threads write different output files
shellLock = new ReentrantLock()
shellIndex = 0

def doIt() {
    shellLock.lock()
    def threadShellIndex = ++shellIndex
    if (threadShellIndex == 1) {
        sh("rm -rf shell")
        sh("mkdir shell")
    }
    shellLock.unlock()

    sh "touch shell/${threadShellIndex}"
}

node {
    checkout scm
    runs = [
        "1": { doIt() },
        "2": { doIt() },
    ]
    parallel runs
}

This does not, as I expect, remove and replace the "shell" directory before both runs proceed to create a file in it. Instead, the console output is:

[Pipeline] parallel
[Pipeline] { (Branch: 1)
[Pipeline] { (Branch: 2)
[Pipeline] sh
[Pipeline] sh
[1] + rm -rf shell
[Pipeline] sh
[2] + touch shell/2
touch: cannot touch ‘shell/2’: No such file or directory
[Pipeline] }
Failed in branch 2
[1] + mkdir shell
[Pipeline] }
Failed in branch 1
[Pipeline] // parallel
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
Also:   hudson.AbortException: script returned exit code 1
        at org.jenkinsci.plugins.workflow.steps.durable_task.DurableTaskStep$Execution.handleExit(DurableTaskStep.java:658)
        at org.jenkinsci.plugins.workflow.steps.durable_task.DurableTaskStep$Execution.check(DurableTaskStep.java:604)
        at org.jenkinsci.plugins.workflow.steps.durable_task.DurableTaskStep$Execution.run(DurableTaskStep.java:548)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
java.lang.IllegalMonitorStateException
    at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:151)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1261)
    at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:457)
    at sun.reflect.GeneratedMethodAccessor1111.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:93)
    at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:325)
    at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1213)
    at groovy.lang.ExpandoMetaClass.invokeMethod(ExpandoMetaClass.java:1125)
    at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1022)
    at org.codehaus.groovy.runtime.callsite.PojoMetaClassSite.call(PojoMetaClassSite.java:47)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
    at org.kohsuke.groovy.sandbox.impl.Checker$1.call(Checker.java:163)
    at org.kohsuke.groovy.sandbox.GroovyInterceptor.onMethodCall(GroovyInterceptor.java:23)
    at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onMethodCall(SandboxInterceptor.java:157)
    at org.kohsuke.groovy.sandbox.impl.Checker$1.call(Checker.java:161)
    at org.kohsuke.groovy.sandbox.impl.Checker.checkedCall(Checker.java:165)
    at com.cloudbees.groovy.cps.sandbox.SandboxInvoker.methodCall(SandboxInvoker.java:17)
    at WorkflowScript.doIt(WorkflowScript:14)
    at WorkflowScript.run(WorkflowScript:22)
    at ___cps.transform___(Native Method)
    at com.cloudbees.groovy.cps.impl.ContinuationGroup.methodCall(ContinuationGroup.java:86)
    at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.dispatchOrArg(FunctionCallBlock.java:113)
    at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.fixName(FunctionCallBlock.java:78)
    at sun.reflect.GeneratedMethodAccessor109.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72)
    at com.cloudbees.groovy.cps.impl.ConstantBlock.eval(ConstantBlock.java:21)
    at com.cloudbees.groovy.cps.Next.step(Next.java:83)
    at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:174)
    at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:163)
    at org.codehaus.groovy.runtime.GroovyCategorySupport$ThreadCategoryInfo.use(GroovyCategorySupport.java:129)
    at org.codehaus.groovy.runtime.GroovyCategorySupport.use(GroovyCategorySupport.java:268)
    at com.cloudbees.groovy.cps.Continuable.run0(Continuable.java:163)
    at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.access$001(SandboxContinuable.java:18)
    at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.run0(SandboxContinuable.java:51)
    at org.jenkinsci.plugins.workflow.cps.CpsThread.runNextChunk(CpsThread.java:185)
    at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.run(CpsThreadGroup.java:400)
    at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.access$400(CpsThreadGroup.java:96)
    at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:312)
    at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:276)
    at org.jenkinsci.plugins.workflow.cps.CpsVmExecutorService$2.call(CpsVmExecutorService.java:67)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at hudson.remoting.SingleLaneExecutorService$1.run(SingleLaneExecutorService.java:131)
    at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
    at jenkins.security.ImpersonatingExecutorService$1.run(ImpersonatingExecutorService.java:59)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
Finished: FAILURE

How do I get run 2 to wait until run 1 has recreated the directory?

Answer 1

It seems that the reason ReentrantLock has no effect is that pipeline groovy doesn't actually multithread. It uses a single thread to host multiple "green" threads.

From https://github.com/jenkinsci/workflow-cps-plugin :

All program logic is run inside a “CPS VM thread”, which is just a Java thread pool that can run binary methods and figure out which continuation to do next. The parallel step uses “green threads” (also known as coöperative multitasking): it records logical thread (~ branch) names for various actions, but does not literally run them simultaneously.

Since all calls are in the same thread, ReentrantLock re-enters.

In my case, I was able to get around the need to lock with the use of AtomicInteger and some symlink cleverness around re-creating the directory.

import java.util.concurrent.atomic.AtomicInteger
shellCounter = new AtomicInteger()

@NonCPS
def getNextShellCounter() {
    return shellCounter.incrementAndGet()
}

...

    def threadShellIndex = getNextShellCounter()

    // concurrency-safe creation of output dir and removal of previous
    def shellDir = "shell.${currentBuild.number}"
    sh """#!/bin/bash +x
        mkdir -p ${shellDir}
        ln -sfn ${shellDir} shell
        rm -rf shell.foo `ls -d shell.* | sed -e '/^${shellDir}\$/d' `
    """

With the way threading works here, a simple ++ might have functioned the same, but better safe than sorry.