Devise a way export state to sync a fresh sidechain to a preexisting rootchain contract

[colin-axner]

Summary

Offer a way to export state of a sidechain into a genesis file.

Problem

If plasmad unsafe-reset-all is run, it is likely a new contract needs to be deployed. It would be very useful if we could simply provide a genesis file which allows the chain to reprocess all txs that occured relative to the rootchain contract

Proposal

Initial thought: Post all txBytes processed in tm blocks into genesis file. Run all txBytes in genesis file at the beginning of the chain. Disable exit checking when doing this, otherwise it will run into sync issues. Fail if any block header != rootchain block header, otherwise process all txBytes

[hamdiallam]

Can TM create multiple blocks from genesis?

[hamdiallam]

I could see this working if we get around to #145

[colin-axner]

Can TM create multiple blocks from genesis?

I don't think it can

I could see this working if we get around to #145

I agree, if none of our code relies on the correct TM block number than the first question wouldn't matter. We could process blocks 1 to n in the genesis block and as long as the correct info is added into the stores then queries should return correctly. I suppose this wouldn't work because of TM consensus, ie I'm on block 2 and you are on block n + 1, but I think this could work under given conditions.

If we require this feature only to be used during software upgrades then I don't see an issue. So if all the validators/full nodes export into genesis and resync from the contract they would all be at n+1 plasma block and TM block 2. My initial thought is you could do breaking changes without redeploying rootchain contract. Hard code in if statement if plasmaBlock <= n then old_code else new_code