Serialize Block ID in Bank into Snapshot

Summary

Currently the block ID (the merkle root of the last FEC set) of the snapshot slot is not included in the snapshot Bank Fields. This would enforce the snapshot producer to include the block ID in the serialized bank.

Motivation

It is convenient for multiple validator clients to have the block ID when booting from a snapshot, and allows the inter-slot equivocation block ID verification to not be special-cased on startup. In addition, this future-proofs for Alpenglow, as Alpenglow votes and related tracking is done on (Slot, Block ID), and repair & duplicate block resolution uses block ID. Having the block ID available in snapshots enables this.

New Terminology

N/A

Detailed Design

The block_id already lives in the bank. Snapshot producers for all clients must include the block ID of the snapshot slot in the serialized bank of the snapshot. This can be implemented by adding a block_id field to the snapshot serialization schema (eg. the ExtraFieldsToSerialize addendum to BankFieldsToSerialize). The block_id will be serialized at the end of the manifest, after accounts_lt_hash.

Snapshot consumers may optionally read the block ID in the deserialized bank, and use it to validate as seen fit.

In cases where an artificial bank is created (ex. via ledger tool during development or for a cluster restart), the block_id must also be populated. If the artificial bank with bank hash bank_hash builds on slot S with block ID parent_block_id, the block ID value for the serialized bank will be a SHA256 hash of the direct concatenation of 32-bytes of parent_block_id followed by 32-bytes of bank_hash, i.e. SHA256(parent_block_id || bank_hash)

Alternatives Considered

N/A

Impact

Clients will now need to include an extra field when generating a snapshot. Supporting deserializing the block_id will also need to be backported to the adjacent (previous) versions of Agave before we begin serializing the block_id in snapshots. Other clients will also update their corresponding snapshot serialize and deserialize structures.

Security Considerations

Security is improved because this now allows inter-slot verification between the snapshot slot and its child without repairing the block ID of the snapshot slot.

Backwards Compatibility

This feature is not backwards compatible.