How do we get block finalization for blockchains with a permanent gap?

[shamil-gadelshin]

We use Substrate framework for our blockchain and encounter a problem with block finalization. It seems Substrate doesn't support a use case with a permanent block gap created by some form of fast sync.

Take Client.apply_finality_with_block_hash from sc-service crate for example: it uses tree_route method several times which will fail when we try to finalize a recent block received by sync (it will try to obtain a path to the last finalized block which will be the genesis and fail on missing blocks between genesis and some recent block having a permanent gap created by some form of fast sync).
Substrate solves it by downloading the gap and finalizing blocks after that (all the block headers will be present in the db). But what if in our case we don't need to download even headers?

How do we get block finalization having a permanent block gap?

Here is the related issue: #5119 (edited)

[bkchr]

Sounds like this is a duplicate of your own issue: #4407?

[shamil-gadelshin]

Sounds like this is a duplicate of your own issue: #4407?

It's more like a subtask than a duplicate. We discovered it recently and the finalization problem was not mentioned in #4407

[shamil-gadelshin]

Substrate already has a way to deal with such cases. BlockImportParams.finalized resolves the issue!