Validate consensus rules on root hash and change data modeling.

[hdevalence]

Tasks

There are various consensus rules noted in the comments of the RootHash enum that we never actually check. To check these, I think we should:

• Change Block::root_hash to return a Result rather than an Option and enforce the all-zeros rules;
• Change PreSaplingReserved([u8; 32]) to PreSaplingReserved;
• Change ChainHistoryActivationReserved([u8; 32]) to ChainHistoryActivationReserved;
• Update the function comments to describe when each variant is verified.

These changes might be impacted by the chain history implementation in #1567.

Variants

• 5 variants: Pre-Sapling, Sapling/Blossom, Heartwood activation, Heartwood post-activation, NU5
  • Are there separate NU5 activation and NU5 post-activation variants?
• reserved: all zeroes?
• final_sapling_root_hash: LEBS2OSP256(rt) where rt is the root of the Sapling note commitment tree for the final Sapling treestate of this block
  • only depends on transactions in this block?
• heartwood activation: all zeroes
• history_chain_hash: see https://zips.z.cash/zip-0221#block-header-semantics-and-consensus-rules
  • depends on all previous blocks for the current network upgrade
  • consider using the zcash_history crate, which is also used by zcashd
• the NU5 variant, see ZIP-244 and ZIP 244: Transaction Identifier Non-Malleability #1874

Previous PR was #856

[teor2345]

Let's also make sure that we document where each enum variant is validated - while the reserved variants are valid as soon as block::light_client_root_hash returns Ok(), the other variants will need further checks.

(In particular, the Sapling variant needs to be checked against the transactions in the block, and the Heartwood variant needs to be checked against the chain history.)

[vramana]

• Update the function comments to describe when each variant is verified.

Where are these function comments? I couldn't find them.

[teor2345]

• Update the function comments to describe when each variant is verified.

Where are these function comments? I couldn't find them.

We changed the name of the type to RootHash and the function to root_hash, but forgot to update this ticket:

• Block::root_hash: https://github.com/ZcashFoundation/zebra/blob/main/zebra-chain/src/block.rs#L58
• RootHash: https://github.com/ZcashFoundation/zebra/blob/main/zebra-chain/src/block/root_hash.rs

I've updated the ticket, and I'll update the code comments with a quick PR.

[teor2345]

Just adding this to an epic