Change ContextuallyVerifiedBlockWithTrees into an enum to correctly represent verification statuses

[teor2345]

Motivation

ContextuallyVerifiedBlockWithTrees actually represents two different types with two different verification statuses. This is a maintainability issue.

Design Background

Blocks from the checkpoint verifier are checkpoint-verified: they are semantically verified, but their transaction authorizing data isn't bound to their header.

Blocks from the non-finalized state are contextually verified: their transaction authorizing data is bound to their header. (And their trees have been calculated.)

Complex Code or Requirements

The consensus code is correct, but the names and types we're using could cause bugs during future changes.

Design

The current code is:

zebra/zebra-state/src/request.rs

Lines 277 to 282 in c058f77

	pub struct ContextuallyVerifiedBlockWithTrees {
	/// A block ready to be committed.
	pub checkpoint_verified: CheckpointVerifiedBlock,
	/// The tresstate associated with the block.
	pub treestate: Option<Treestate>,
	}

The different verification statuses can be represented using an enum:

pub enum FinalizableBlock {
    Checkpoint {
        checkpoint_verified: CheckpointVerifiedBlock
    },
    Contextual {
        contextually_verified: ContextuallyVerifiedBlock,
        treestate: Treestate,
    }
}

Here are some example code changes that we could do:

In the match statement in commit_finalized_direct():

• FinalizableBlock::Checkpoint { checkpoint_verified } replaces ContextuallyVerifiedBlockWithTrees { checkpoint_verified, treestate: None }
• FinalizableBlock::Contextual { contextually_verified, treestate } replaces ContextuallyVerifiedBlockWithTrees { checkpoint_verified, treestate: Some(treestate) }

The Checkpoint case can construct a ContextuallyVerifiedBlockWithTrees from the checkpoint block and the trees in commit_finalized_direct(). Then the rest of the finalized state can use ContextuallyVerifiedBlockWithTrees instead of passing separate finalized, history_tree, note_commitment_trees arguments.

Testing

Our existing tests should cover this change.

Related Work

This is part of:

• Renames to address confusion in zebra's handling of finalized state #6680

[mpguerra]

@upbqdn can you please add an estimate here?