SnapshotSource does not reuse self-generated snapshot

[cknaap]

Describe the bug
SnapshotSource should return the same snapshot upon multiple retrievals. Instead, it regenerates the snapshot every time.

To Reproduce
Steps to reproduce the behavior:

1. Create a SnapshotSource, with regenerate: true, based on a CachedResolver
2. Get a StructureDefinition by canonical, e.g. from Element
3. Save a reference to the resulting StructureDefinition.Snapshot
4. Repeat step 2
5. Save a reference to the second resulting StructureDefinition.Snapshot
6. Observe that both references are not the same.

Expected behavior
When the SnapshotSource has generated (by the Generator) a snapshot, it is marked as IsCreatedBySnapshotGenerator(), and it should not be re-generated again upon subsequent retrievals.

Screenshots
n/a

Version used:

• FHIR Version: All
• Version: 5.8.1

Additional context

[ewoutkramer]

This is not a bug, but a feature. Set regenerate to false to get the desired behaviour. I must be missing something ;-)

[ewoutkramer]

Offline discussion added some new insights:

• The current Regenerate options will always regenerate a new snapshot, even though the documentation suggests it will not repeatedly do so.
• Setting this to true seems to be not really useful, since -except for some testing usecases- it's not clear why you'd like to snapshot again and again.
• Setting it to false is what most people want: generate the snapshot if it does not exist or re-generate it when it's not done by our tools. This is the default and many tools will depend on it.
• It is not possible to allow snapshots created by other tools (probably: the java tooling) at this moment.
• Since whether a snapshot is "ours" is done using an in-memory annotation, initially every snapshot will be regenerated, the flag only ensures we're not doing it twice.

If we'd apply #2803 we will certainly get regressions, since this PR will cause true to result in the old behaviour for false, and false (still the default) will cause 3rd-party snapshots to be used instead of being regenerated.

A first conclusion might be that the current regenerate option is not particularly useful (we'd be better of without it), is not documented clearly and does not allow for 3rd-party snapshots to be used. We should obsolete it and introduce a more explicit setting. Basically there are only two cases:

• Add a snapshot if there is none or use whatever is there (new behaviour)
• Regenerate snapshot once to ensure it is "ours" (and/or at least updated to the lastest version) (current default behaviour)
• Maybe: heuristically try to determine whether the snapshot is ours (there must be obvious differences) and then avoid regenerating it.