Q:

Security sandbox stacktrace debugging.  How do? 

How in the world are folks doing debugging of script security sandbox stack traces?  For instance, in the stacktrace below, it sure seems as though there's not enough info to determine where I might have gone off the rails in my Jenkinsfile. 
org.jenkinsci.plugins.scriptsecurity.sandbox. RejectedAccessException: unclassified method java.lang. String call
at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy. SandboxInterceptor.onMethodCall(SandboxInterceptor.java:113)
at org.kohsuke.groovy.sandbox.impl. Checker$1.call(Checker.java:149)
at org.kohsuke.groovy.sandbox.impl. Checker.checkedCall(Checker.java:146)
at com.cloudbees.groovy.cps.sandbox. SandboxInvoker.methodCall(SandboxInvoker.java:16)
at WorkflowScript.run(WorkflowScript:136)
at ___cps.transform___(Native Method)
at com.cloudbees.groovy.cps.impl. ContinuationGroup.methodCall(ContinuationGroup.java:57)
at com.cloudbees.groovy.cps.impl. FunctionCallBlock$ContinuationImpl.dispatchOrArg(FunctionCallBlock.java:109)
at com.cloudbees.groovy.cps.impl. FunctionCallBlock$ContinuationImpl.fixName(FunctionCallBlock.java:77)
at sun.reflect. GeneratedMethodAccessor381.invoke(Unknown Source)
at sun.reflect. DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect. Method.invoke(Method.java:497)
at com.cloudbees.groovy.cps.impl. ContinuationPtr$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:58)
at com.cloudbees.groovy.cps. Continuable.run0(Continuable.java:154)
at org.jenkinsci.plugins.workflow.cps. SandboxContinuable.access$001(SandboxContinuable.java:18)
at org.jenkinsci.plugins.workflow.cps. SandboxContinuable$1.call(SandboxContinuable.java:33)
at org.jenkinsci.plugins.workflow.cps. SandboxContinuable$1.call(SandboxContinuable.java:30)
at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy. GroovySandbox.runInSandbox(GroovySandbox.java:108)
at org.jenkinsci.plugins.workflow.cps. SandboxContinuable.run0(SandboxContinuable.java:30)
at org.jenkinsci.plugins.workflow.cps. CpsThread.runNextChunk(CpsThread.java:163)
at org.jenkinsci.plugins.workflow.cps. CpsThreadGroup.run(CpsThreadGroup.java:324)
at org.jenkinsci.plugins.workflow.cps. CpsThreadGroup.access$100(CpsThreadGroup.java:78)
at org.jenkinsci.plugins.workflow.cps. CpsThreadGroup$2.call(CpsThreadGroup.java:236)
at org.jenkinsci.plugins.workflow.cps. CpsThreadGroup$2.call(CpsThreadGroup.java:224)
at org.jenkinsci.plugins.workflow.cps. CpsVmExecutorService$2.call(CpsVmExecutorService.java:63)
at java.util.concurrent. FutureTask.run(FutureTask.java:266)
at hudson.remoting. SingleLaneExecutorService$1.run(SingleLaneExecutorService.java:112)
at jenkins.util. ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
at java.util.concurrent. Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent. FutureTask.run(FutureTask.java:266)
at java.util.concurrent. ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent. ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang. Thread.run(Thread.java:745)
Finished: FAILURE

A:

The relevant line for you here is mainly:
at WorkflowScript.run(WorkflowScript:136)

That tells you where in your Jenkinsfile it is failing. 
org.jenkinsci.plugins.scriptsecurity.sandbox. RejectedAccessException:unclassified method java.lang. String call

This indicates that you're trying a method call that is not whitelisted for pipeline executions. 
A couple of options:

Uncheck "User Groovy Sandbox" in your job configuration to reduce the security threshold
Whitelist the method call yourself in the "In-process Script Approval" section of "Manage Jenkins"
Rewrite the offending line to use an approved method call, which is incredibly non-obvious and makes it much harder for a Groovy programmer to make good use of the Jenkinsfile

