GeoTiffRasterSource Thread Safety

[jbouffard]

Overview

This PR makes reading from GeoTiffRasterSources thread safe by synchronizing the tiff objects within these instances. That will ensure that the state of the data doesn't change as it's being accessed by different threads.

Checklist

• Add entry to CHANGELOG.md
• Tests

Closes locationtech/geotrellis#2954

[pomadchin]

LGTM, though specs can be improved

[pomadchin]

Along with the comments I want to add, that a couple of times GeoTiffReprojectedRasterSource should be threadsafe failed with the following error:

java.lang.StackOverflowError
	at org.locationtech.proj4j.datum.AxisOrder.toENU(AxisOrder.java:122)
	at org.locationtech.proj4j.BasicCoordinateTransform.transform(BasicCoordinateTransform.java:137)
	at geotrellis.proj4.Proj4Transform$$anonfun$apply$2.apply(Transform.scala:39)
	at geotrellis.proj4.Proj4Transform$$anonfun$apply$2.apply(Transform.scala:36)
	at geotrellis.vector.reproject.Reproject$.refine$1(Reproject.scala:88)
	at geotrellis.vector.reproject.Reproject$.refine$1(Reproject.scala:97)
	... (tons of lines, it is a recursive stack)
	at geotrellis.vector.reproject.Reproject$.refine$1(Reproject.scala:97)
        ...

This actually can be related to locationtech/proj4j#29
If it is so we can workaround it by Transform.synchronized or by ReprojectRasterExtent.synchronized

It can also happen on the readBounds line targetRasterExtent.extent.reprojectAsPolygon(backTransform, 0.001) (i'm pretty sure that that is it):

val sourceExtent = closestTiffOverview.synchronized { targetRasterExtent.extent.reprojectAsPolygon(backTransform, 0.001).envelope }

[pomadchin]

Merging once travis is happy