Avoid String concatenation in compareTo

[AlexanderGunnarssonMW]

We have generative tests that generate 10,000 test cases. These have recently started to sometimes hang for one hour before completing. We are not yet sure why (we have some hypotheses), but by taking thread dumps we can see that the coordinating node is stuck on populating a TreeMap which causes expensive string concatenations. This PR optimizes this to avoid concatenations while keeping the same behavior.

Thread dump where the coordinating node apparently gets stuck:

at [email protected]/java.lang.StringConcatHelper.prepend(StringConcatHelper.java:326)
at [email protected]/java.lang.StringConcatHelper.prepend(StringConcatHelper.java:343)
at [email protected]/java.lang.invoke.DirectMethodHandle$Holder.invokeStatic(DirectMethodHandle$Holder)
at [email protected]/java.lang.invoke.LambdaForm$MH/0x00007fbb34008400.invoke(LambdaForm$MH)
at [email protected]/java.lang.invoke.LambdaForm$MH/0x00007fbb340b9400.invoke(LambdaForm$MH)
at [email protected]/java.lang.invoke.LambdaForm$MH/0x00007fbb340bb000.invoke(LambdaForm$MH)
at [email protected]/java.lang.invoke.Invokers$Holder.linkToTargetMethod(Invokers$Holder)
at PluginClassLoader for junit//hudson.tasks.junit.CaseResult.getFullName(CaseResult.java:616)
at PluginClassLoader for junit//hudson.tasks.junit.CaseResult.compareTo(CaseResult.java:958)
at PluginClassLoader for junit//hudson.tasks.junit.CaseResult.compareTo(CaseResult.java:62)
at [email protected]/java.util.TreeMap.put(TreeMap.java:849)
at [email protected]/java.util.TreeMap.put(TreeMap.java:569)
at [email protected]/java.util.TreeSet.add(TreeSet.java:259)
at PluginClassLoader for junit//hudson.tasks.junit.ClassResult.add(ClassResult.java:198)
at PluginClassLoader for junit//hudson.tasks.junit.PackageResult.add(PackageResult.java:304)
at PluginClassLoader for junit//hudson.tasks.junit.TestResult.freeze(TestResult.java:1156)
at PluginClassLoader for junit//hudson.tasks.junit.TestResultAction.loadCached(TestResultAction.java:326)
at PluginClassLoader for junit//hudson.tasks.junit.TestResultAction.load(TestResultAction.java:279)
at PluginClassLoader for junit//hudson.tasks.junit.TestResultAction.getResult(TestResultAction.java:173)
-  locked hudson.tasks.junit.TestResultAction@25ac2d6e
at PluginClassLoader for junit//hudson.tasks.junit.TestResultAction.getResult(TestResultAction.java:70)
at PluginClassLoader for junit//hudson.tasks.test.AbstractTestResultAction.findCorrespondingResult(AbstractTestResultAction.java:286)
at PluginClassLoader for junit//hudson.tasks.test.TestResult.getPreviousResult(TestResult.java:146)
at PluginClassLoader for junit//hudson.tasks.junit.TestResult.getPreviousResult(TestResult.java:467)
at PluginClassLoader for junit//hudson.tasks.junit.CaseResult.getPreviousResult(CaseResult.java:768)
at PluginClassLoader for junit//hudson.tasks.junit.CaseResult.getPreviousResult(CaseResult.java:62)
at PluginClassLoader for test-stability//de.esailors.jenkins.teststability.StabilityTestDataPublisher.getPreviousResultSafely(StabilityTestDataPublisher.java:171)
at PluginClassLoader for test-stability//de.esailors.jenkins.teststability.StabilityTestDataPublisher.getPreviousHistory(StabilityTestDataPublisher.java:130)
at PluginClassLoader for test-stability//de.esailors.jenkins.teststability.StabilityTestDataPublisher.contributeTestData(StabilityTestDataPublisher.java:82)
at PluginClassLoader for junit//hudson.tasks.junit.JUnitResultArchiver.parseAndSummarize(JUnitResultArchiver.java:325)
-  locked hudson.model.FreeStyleBuild@6bb44616
at PluginClassLoader for junit//hudson.tasks.junit.JUnitResultArchiver.perform(JUnitResultArchiver.java:194)
at jenkins.tasks.SimpleBuildStep.perform(SimpleBuildStep.java:123)
at hudson.tasks.BuildStepCompatibilityLayer.perform(BuildStepCompatibilityLayer.java:80)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:818)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:767)
at hudson.model.Build$BuildExecution.post2(Build.java:179)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:711)
at hudson.model.Run.execute(Run.java:1917)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:44)

Testing done

Ran mvn hpi:run and could reach web interface locally. I'm not sure how to trigger this code path from a blank jenkins setup, however judging from the changed lines the behavior should be identical.

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[dwnusbaum]

Seems ok, but I wonder if it will really help, or if you will just see the comparator methods instead in thread dumps. I guess the main issue is that #625 switched ClassResult.cases from ArrayList to TreeSet and so adding results is just generally more expensive now. The reason for the change I think is explained here, so if I understand correctly, it was intended to improve performance when reading/parsing test results.

[timja]

Lets see if it helps.
Otherwise if you can raise an issue with a way to generate data in a similar manner that demonstrates the slowness that would be useful too.