203 lines
8.0 KiB
Markdown
203 lines
8.0 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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- [Custom types](#custom-types)
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- [Protocols](#protocols)
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- [`ExecutionEngine`](#executionengine)
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- [`notify_forkchoice_updated`](#notify_forkchoice_updated)
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- [Helpers](#helpers)
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- [`PayloadAttributes`](#payloadattributes)
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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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- [`validate_merge_block`](#validate_merge_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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## Custom types
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| Name | SSZ equivalent | Description |
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| - | - | - |
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| `PayloadId` | `Bytes8` | Identifier of a payload building process |
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## Protocols
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### `ExecutionEngine`
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*Note*: The `notify_forkchoice_updated` function is added to the `ExecutionEngine` protocol to signal the fork choice updates.
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The body of this function is implementation dependent.
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The Engine API may be used to implement it with an external execution engine.
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#### `notify_forkchoice_updated`
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This function performs two actions *atomically*:
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* Re-organizes the execution payload chain and corresponding state to make `head_block_hash` the head.
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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 `finalized_block_hash`.
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Additionally, if `payload_attributes` is provided, this function sets in motion a payload build process on top of
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`head_block_hash` and returns an identifier of initiated process.
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```python
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def notify_forkchoice_updated(self: ExecutionEngine,
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head_block_hash: Hash32,
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finalized_block_hash: Hash32,
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payload_attributes: Optional[PayloadAttributes]) -> Optional[PayloadId]:
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...
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```
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*Note*: The call of the `notify_forkchoice_updated` function maps on the `POS_FORKCHOICE_UPDATED` event defined in the [EIP-3675](https://eips.ethereum.org/EIPS/eip-3675#definitions).
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As per EIP-3675, before a post-transition block is finalized, `notify_forkchoice_updated` must be called with `finalized_block_hash = Hash32()`.
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## Helpers
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### `PayloadAttributes`
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Used to signal to initiate the payload build process via `notify_forkchoice_updated`.
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```python
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@dataclass
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class PayloadAttributes(object):
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timestamp: uint64
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random: Bytes32
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fee_recipient: ExecutionAddress
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```
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### `PowBlock`
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```python
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class PowBlock(Container):
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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) -> Optional[PowBlock]` be the function that given the hash of the PoW block returns its data.
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It may result in `None` if the requested block is not yet available.
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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(block: PowBlock, parent: PowBlock) -> bool:
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is_total_difficulty_reached = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
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is_parent_total_difficulty_valid = parent.total_difficulty < 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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### `validate_merge_block`
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```python
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def validate_merge_block(block: BeaconBlock) -> None:
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"""
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Check the parent PoW block of execution payload is a valid terminal PoW block.
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Note: Unavailable PoW block(s) may later become available,
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and a client software MAY delay a call to ``validate_merge_block``
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until the PoW block(s) become available.
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"""
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if TERMINAL_BLOCK_HASH != Hash32():
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# If `TERMINAL_BLOCK_HASH` is used as an override, the activation epoch must be reached.
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assert compute_epoch_at_slot(block.slot) >= TERMINAL_BLOCK_HASH_ACTIVATION_EPOCH
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assert block.body.execution_payload.parent_hash == TERMINAL_BLOCK_HASH
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return
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pow_block = get_pow_block(block.body.execution_payload.parent_hash)
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# Check if `pow_block` is available
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assert pow_block is not None
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pow_parent = get_pow_block(pow_block.parent_hash)
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# Check if `pow_parent` is available
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assert pow_parent is not None
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# Check if `pow_block` is a valid terminal PoW block
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assert is_valid_terminal_pow_block(pow_block, pow_parent)
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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) -> None:
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"""
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Run ``on_block`` upon receiving a new block.
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A block that is asserted as invalid due to unavailable PoW block may be valid at a later time,
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consider scheduling it for later processing in such case.
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"""
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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 is_merge_block(pre_state, block.body):
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validate_merge_block(block)
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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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