[JIRA] [prioritysorter-plugin] (JENKINS-27770) AdvancedQueueSorter call to sort violates the comparison contract

Mon, 06 Apr 2015 18:12:06 -0700

Issue Type: Bug Bug
Assignee: Magnus Sandberg
Components: prioritysorter-plugin
Created: 07/Apr/15 1:10 AM
Description:

At random times, jobs are unable to start builds and several exceptions are thrown in the log:

Apr 06, 2015 6:32:21 AM hudson.triggers.SafeTimerTask run
SEVERE: Timer task hudson.model.Queue$MaintainTask@5116dc28 failed
java.lang.IllegalArgumentException: Comparison method violates its general contract!
        at java.util.TimSort.mergeHi(TimSort.java:868)
        at java.util.TimSort.mergeAt(TimSort.java:485)
        at java.util.TimSort.mergeForceCollapse(TimSort.java:426)
        at java.util.TimSort.sort(TimSort.java:223)
        at java.util.TimSort.sort(TimSort.java:173)
        at java.util.Arrays.sort(Arrays.java:659)
        at java.util.Collections.sort(Collections.java:217)
        at jenkins.advancedqueue.sorter.AdvancedQueueSorter.sortBuildableItems(AdvancedQueueSorter.java:81)
        at hudson.model.Queue.maintain(Queue.java:1021)
        at hudson.model.Queue$MaintainTask.doRun(Queue.java:1994)
        at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
        at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:304)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.j
ava:178)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293
)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)



The call to sortBuildableItems() calls the java.utils.Collections.sort() which had a change semantics from Java 6 to 7.  Specifically there is a contract constraint in compare() that requires that sgn(compare(x, y)) == -sgn(compare(y, x)) for all x and y.



Since any two calls to getInQueueSince() will return different values for any BuildableItem object, it's possible that while the signs maybe the same, the absolute values are not and the exception is thrown.





// Sort
Collections.sort(items, new Comparator<BuildableItem>() {
	public int compare(BuildableItem o1, BuildableItem o2) {
		float o1weight = getCalculatedWeight(o1);
		float o2weight = getCalculatedWeight(o2);
		if (o1weight > o2weight) {
			return 1;
		}
		if (o1weight < o2weight) {
			return -1;
		}
		return (int) (o1.getInQueueSince() - o2.getInQueueSince());
	}
});





There is a workaround by disabling timsort and reverting to the Java 6's mergesort using -Djava.util.Arrays.useLegacyMergeSort=true.  The problem longterm is that the legacy sorting algorithm might go away in a future release of Java.



Recommend changing the subtraction operation to a set of comparisons instead.
Environment: java 1.7.0_65, core 1.565.3, prioritysorter-plugin 2.6
Project: Jenkins
Priority: Minor Minor
Reporter: Charles Stephens
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