Bug in VersionSpecificWorkerContextWrapper.allStructureDefinitions()

[Boereck]

Describe the bug
The following code on the current (at the time of writing) master:

hapi-fhir/hapi-fhir-validation/src/main/java/org/hl7/fhir/common/hapi/validation/validator/VersionSpecificWorkerContextWrapper.java

Lines 232 to 246 in e412b38

	retVal = allStructureDefinitions.stream()
	.map(myVersionCanonicalizer::structureDefinitionToCanonical)
	.collect(Collectors.toList());
	myAllStructures = retVal;
	try {
	for (IBaseResource next : allStructureDefinitions) {
	Resource converted = convertToCanonicalVersionAndGenerateSnapshot(next, false);
	retVal.add((StructureDefinition) converted);
	}
	myAllStructures = retVal;
	} catch (Exception e) {
	ourLog.error("Failure during snapshot generation", e);
	myAllStructures = null;
	}

Starting from the try block, the cached StructureDefinitions are snapshotted. However, instead of creating a new List for the resulting snapshotted StructureDefinitions, the resulting Resources are added to the list of previously fetched and converted StructureDefinitions. I am sure this is not correct, because both the non-snapshotted and the snapshotted version are stored to the same list.

In addition to that, the issue causes ConcurrentModificationExceptions, since the List being modified in the try block was stored to the myAllStructures field before. When a second thread calls allStructureDefinitions, before the first one enters the try block, it will get the list and can read it, while the first thread adds elements to that list.

However, this issue is easy to fix: Before entering the try, add the line

retVal = new ArrayList<>();`

@fil512 it seems this was introduced in commit 362dc09 by you. At least you are attributed as the author. Maybe you want to fix the issue, since this is a pretty trivial fix, I can open a PR as well, if this is preferred.

To Reproduce
Steps to reproduce the behavior:
Validate several resources in parallel.