SnapshotGenerator: Keep snapshot when differential is missing

[mharthoorn]

Problem:
If the snapshot generator encounters a StructureDefinition (of type LogicalModel) that already has a snapshot, but not a differential, it will remove that snapshot without replacing it (only when SnapshotGeneratorSettings.ForceRegenerateSnapshots is set to true)

Solution
The SnapshotGenerator should not remove a snapshot when there is no differential present in a StructureDefinition.

Alternatives

• Create an extra setting SnapshotGeneratorSettings.SkipRegenerateOnMissingDifferential
• Throw an Exception
• Add a warning to the OperationOutcome

Additional context
In bulk snapshot operations this situation tends to go unnoticed and leads to loss of data / correctness in publications.

[Kasdejong]

[TestMethod]
        public async T.Task TestMissingDifferential()
        {
            // Create a profile without a differential
            var profile = ObservationTypeSliceProfile;
            profile.Differential = null;
            profile.Kind = StructureDefinition.StructureDefinitionKind.Logical;
            var settings = new SnapshotGeneratorSettings { ForceRegenerateSnapshots = true };

            var resolver = new InMemoryResourceResolver(profile);
            var multiResolver = new MultiResolver(_testResolver, resolver);
            _generator = new SnapshotGenerator(multiResolver, settings);

            var (_, expanded) = await generateSnapshotAndCompare(profile);
            Assert.IsNotNull(expanded);
            Assert.IsTrue(expanded.HasSnapshot);

            expanded.Snapshot.Element.Dump();
        }

We are having issues reproducing the issue with this unit test, though the unit test seems to meet all the conditions. Is there something missing?

@mharthoorn

[Rob5045]

Note that Simplifier uses version 5.1 of the SDK. Maybe the issue has been resolved in the mean time? I will check to see if I can reproduce the issue in the current Simplifier version.

[Rob5045]

Unfortunately Martijn does not have reproduction steps. However I did find a possible cause. According to the specification for logical models (https://www.hl7.org/fhir/R5/logical.html) the base type can either be Element or Base. When the base type is set to Base the snapshot generator tries to resolve "http://hl7.org/fhir/StructureDefinition/Base". I created a resolver using ZipSource.CreateValidationSource() but it does not find the resource. It does find "http://hl7.org/fhir/StructureDefinition/Element". Looking at specification.zip I cannot find a definition for Base either. The snapshot generator always returns null when it cannot find the base definition.

[mmsmits]

Thanks Rob, we will create a unit test for this, and check if the snapshot is overwritten.

[mmsmits]

@Rob5045 Thanks for the pointer, otherwise we never would have found this :)

[mmsmits]

This fix looks correct, and we still think it is.

However, a validator unit test crashes because of this. Because it results in different behavior of SnapshotSource, and we should take a look at the ensureSnapshot() function, to see if we need to copy this fix there too.

We should also check in the validator-api, why we don't crash on empty shapshots.

All reasons the revert this fix for and thoroughly investigate what's going on.