168 lines
6.3 KiB
Markdown
168 lines
6.3 KiB
Markdown
# The Merge -- Fork Choice
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**Notice**: This document is a work-in-progress for researchers and implementers.
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## Table of contents
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<!-- TOC -->
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<!-- START doctoc generated TOC please keep comment here to allow auto update -->
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<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
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- [Introduction](#introduction)
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- [Protocols](#protocols)
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- [`ExecutionEngine`](#executionengine)
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- [`set_head`](#set_head)
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- [`finalize_block`](#finalize_block)
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- [Helpers](#helpers)
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- [`TransitionStore`](#transitionstore)
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- [`PowBlock`](#powblock)
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- [`get_pow_block`](#get_pow_block)
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- [`is_valid_terminal_pow_block`](#is_valid_terminal_pow_block)
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- [Updated fork-choice handlers](#updated-fork-choice-handlers)
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- [`on_block`](#on_block)
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<!-- END doctoc generated TOC please keep comment here to allow auto update -->
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<!-- /TOC -->
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## Introduction
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This is the modification of the fork choice according to the executable beacon chain proposal.
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*Note*: It introduces the process of transition from the last PoW block to the first PoS block.
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## Protocols
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### `ExecutionEngine`
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The following methods are added to the `ExecutionEngine` protocol for use in the fork choice:
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#### `set_head`
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Re-organizes the execution payload chain and corresponding state to make `block_hash` the head.
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The body of this function is implementation dependent.
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The Consensus API may be used to implement this with an external execution engine.
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```python
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def set_head(self: ExecutionEngine, block_hash: Hash32) -> bool:
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"""
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Returns True if the ``block_hash`` was successfully set as head of the execution payload chain.
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"""
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...
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```
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#### `finalize_block`
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Applies finality to the execution state: it irreversibly persists the chain of all execution payloads
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and corresponding state, up to and including `block_hash`.
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The body of this function is implementation dependent.
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The Consensus API may be used to implement this with an external execution engine.
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```python
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def finalize_block(self: ExecutionEngine, block_hash: Hash32) -> bool:
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"""
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Returns True if the data up to and including ``block_hash`` was successfully finalized.
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"""
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...
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```
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## Helpers
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### `TransitionStore`
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```python
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@dataclass
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class TransitionStore(object):
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terminal_total_difficulty: uint256
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```
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### `PowBlock`
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```python
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@dataclass
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class PowBlock(object):
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block_hash: Hash32
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parent_hash: Hash32
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total_difficulty: uint256
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difficulty: uint256
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```
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### `get_pow_block`
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Let `get_pow_block(block_hash: Hash32) -> PowBlock` be the function that given the hash of the PoW block returns its data.
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*Note*: The `eth_getBlockByHash` JSON-RPC method may be used to pull this information from an execution client.
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### `is_valid_terminal_pow_block`
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Used by fork-choice handler, `on_block`.
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```python
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def is_valid_terminal_pow_block(transition_store: TransitionStore, block: PowBlock, parent: PowBlock) -> bool:
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is_total_difficulty_reached = block.total_difficulty >= transition_store.terminal_total_difficulty
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is_parent_total_difficulty_valid = parent.total_difficulty < transition_store.terminal_total_difficulty
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return is_total_difficulty_reached and is_parent_total_difficulty_valid
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```
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## Updated fork-choice handlers
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### `on_block`
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*Note*: The only modification is the addition of the verification of transition block conditions.
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```python
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def on_block(store: Store, signed_block: SignedBeaconBlock, transition_store: TransitionStore=None) -> None:
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block = signed_block.message
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# Parent block must be known
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assert block.parent_root in store.block_states
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# Make a copy of the state to avoid mutability issues
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pre_state = copy(store.block_states[block.parent_root])
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# Blocks cannot be in the future. If they are, their consideration must be delayed until the are in the past.
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assert get_current_slot(store) >= block.slot
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# Check that block is later than the finalized epoch slot (optimization to reduce calls to get_ancestor)
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finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
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assert block.slot > finalized_slot
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# Check block is a descendant of the finalized block at the checkpoint finalized slot
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assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root
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# Check the block is valid and compute the post-state
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state = pre_state.copy()
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state_transition(state, signed_block, True)
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# [New in Merge]
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if (transition_store is not None) and is_merge_block(pre_state, block.body):
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pow_block = get_pow_block(block.body.execution_payload.parent_hash)
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pow_parent = get_pow_block(pow_block.parent_hash)
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assert is_valid_terminal_pow_block(transition_store, pow_block, pow_parent)
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# Add new block to the store
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store.blocks[hash_tree_root(block)] = block
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# Add new state for this block to the store
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store.block_states[hash_tree_root(block)] = state
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# Update justified checkpoint
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if state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch:
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if state.current_justified_checkpoint.epoch > store.best_justified_checkpoint.epoch:
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store.best_justified_checkpoint = state.current_justified_checkpoint
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if should_update_justified_checkpoint(store, state.current_justified_checkpoint):
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store.justified_checkpoint = state.current_justified_checkpoint
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# Update finalized checkpoint
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if state.finalized_checkpoint.epoch > store.finalized_checkpoint.epoch:
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store.finalized_checkpoint = state.finalized_checkpoint
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# Potentially update justified if different from store
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if store.justified_checkpoint != state.current_justified_checkpoint:
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# Update justified if new justified is later than store justified
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if state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch:
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store.justified_checkpoint = state.current_justified_checkpoint
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return
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# Update justified if store justified is not in chain with finalized checkpoint
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finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
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ancestor_at_finalized_slot = get_ancestor(store, store.justified_checkpoint.root, finalized_slot)
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if ancestor_at_finalized_slot != store.finalized_checkpoint.root:
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store.justified_checkpoint = state.current_justified_checkpoint
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```
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