LF Provide function to reversion a contract

[remyhaemmerle-da]

No description provided.

[simonmaxen-da]

I've experimented with using this API directly, and as @matthiasS-da observed, the problem we have is that we may observe several Nodes that bind the contract to different target template ids. As I expect we want to use the one with the maximum package version. Then we also need a method an API that calls this converter with the

we only populate with view with a single contract instance although the transaction tree:

 def reversion(
      contract: FatContractInstance,
      maxVersionOfTmplId: Set[Ref.TypeConName],
  ): Result[FatContractInstance] 

[simonmaxen-da]

Previously that argument was consistent with the templateId so it is not nice this is no longer the case. It also means that (contract.templateId == dstTmplId) => ResultDone(contract) will not work for pre-converted contracts. An alternative could be to also template the templateId with the converted templateId. If this is to be considered Hash.assertHashContractInstance would need to be modified not to include the packageId.

[simonmaxen-da]

Maybe add an optional reversionedPackageId to FatContractInstance ?

[matthiasS-da]

@remyhaemmerle-da Please read @simonmaxen-da 's first comment and double-check there is no fundamental problem with the engine. That said, it is totally up to you how you handle things inside of the engine.

Maybe add an optional reversionedPackageId to FatContractInstance ?

Please keep in mind that such a change may be challenging to consume in the Canton repo. If you add the field, please take care of the fall-out, too.

[simonmaxen-da]

Would not work for pre-converted contracts.

[matthiasS-da]

@remyhaemmerle-da please double-check there is no problem with the engine. That said, up to you how you handle things.

[matthiasS-da]

@simonmaxen-da and myself have reorganized ourselves. As a result, I'll take over the review of this PR.

[matthiasS-da]

My main critique right now is that the interface here does not (yet) match what we have agreed on.

For convenience, let me copy over the signature:

def reVersionValue(
                      value: VersionedValue,
                      dstTmplIds: Set[Ref.TypeConName],
                      pkgs: Map[Ref.PackageId, Package]
                    ): Either[Error, VersionedValue]

With the following constraints:

1. pkgs needs to contain only the package ids from dstTmplIds.
2. It returns an error only in case of a missing package (cf. 1) or if reversioning is impossible.
3. Reversioning does not change the contract hash (cf. LfHash.assertHashContractInstance).

Additionally:
4. The returned versioned value needs to be parseable by any participant (possibly running a different Canton version), whose Engine is able to load the packages in pkgs.

Last but not least: please double-check that the pkgs in (1) are really sufficient for reversioning. I'm concerned about the case that a template uses data types defined in a different package.