LabelRegions cannot be used concurrently.

[tinevez]

I found out serependititly that the LabelRegions class, which allows iterating through the labels of a Labeling cannot be used concurrently.

When trying the access a region with a specific label, the cache is update if needed. In multithreaded applications, the cache updated might get called concurrently and this is probably the reason for the error.

• Maybe it could be specified in the javadocs?
• Maybe it can be fixed by declaring the update() method synchronized?

The failing method is the private method update https://github.com/imglib/imglib2-roi/blob/master/src/main/java/net/imglib2/roi/labeling/LabelRegions.java#L329

The error stack when trying to access regions concurrently looks like the following:

Exception in thread "[NucleiSplitter] thread 0" java.lang.IndexOutOfBoundsException: Index: 459, Size: 459 at java.util.ArrayList.RangeCheck(ArrayList.java:547) at java.util.ArrayList.get(ArrayList.java:322) at net.imglib2.roi.labeling.LabelRegions.update(LabelRegions.java:361) at net.imglib2.roi.labeling.LabelRegions.getLabelRegion(LabelRegions.java:116) at fiji.plugin.cwnt.segmentation.NucleiSplitter$1.run(NucleiSplitter.java:124)

[tinevez]

My bad. The issue was in another class calling this one.