158 lines
6.2 KiB
Markdown
158 lines
6.2 KiB
Markdown
# The Merge -- Honest Validator
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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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- [Prerequisites](#prerequisites)
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- [Custom types](#custom-types)
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- [Protocols](#protocols)
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- [`ExecutionEngine`](#executionengine)
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- [`prepare_payload`](#prepare_payload)
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- [`get_payload`](#get_payload)
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- [Beacon chain responsibilities](#beacon-chain-responsibilities)
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- [Block proposal](#block-proposal)
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- [Constructing the `BeaconBlockBody`](#constructing-the-beaconblockbody)
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- [ExecutionPayload](#executionpayload)
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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 document represents the changes to be made in the code of an "honest validator" to implement executable beacon chain proposal.
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## Prerequisites
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This document is an extension of the [Altair -- Honest Validator](../altair/validator.md) guide.
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All behaviors and definitions defined in this document, and documents it extends, carry over unless explicitly noted or overridden.
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All terminology, constants, functions, and protocol mechanics defined in the updated Beacon Chain doc of [The Merge](./beacon-chain.md) are requisite for this document and used throughout.
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Please see related Beacon Chain doc before continuing and use them as a reference throughout.
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## Custom types
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| Name | SSZ equivalent | Description |
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| - | - | - |
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| `PayloadId` | `uint64` | Identifier of a payload building process |
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## Protocols
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### `ExecutionEngine`
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*Note*: `prepare_payload` and `get_payload` functions are added to the `ExecutionEngine` protocol for use as a validator.
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The body of each of these functions is implementation dependent.
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The Engine API may be used to implement them with an external execution engine.
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#### `prepare_payload`
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Given the set of execution payload attributes, `prepare_payload` initiates a process of building an execution payload
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on top of the execution chain tip identified by `parent_hash`.
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```python
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def prepare_payload(self: ExecutionEngine,
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parent_hash: Hash32,
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timestamp: uint64,
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random: Bytes32,
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fee_recipient: ExecutionAddress) -> PayloadId:
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"""
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Return ``payload_id`` that is used to obtain the execution payload in a subsequent ``get_payload`` call.
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"""
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...
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```
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#### `get_payload`
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Given the `payload_id`, `get_payload` returns the most recent version of the execution payload that
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has been built since the corresponding call to `prepare_payload` method.
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```python
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def get_payload(self: ExecutionEngine, payload_id: PayloadId) -> ExecutionPayload:
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"""
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Return ``execution_payload`` object.
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"""
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...
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```
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## Beacon chain responsibilities
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All validator responsibilities remain unchanged other than those noted below. Namely, the transition block handling and the addition of `ExecutionPayload`.
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### Block proposal
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#### Constructing the `BeaconBlockBody`
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##### ExecutionPayload
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To obtain an execution payload, a block proposer building a block on top of a `state` must take the following actions:
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1. Set `payload_id = prepare_execution_payload(state, pow_chain, fee_recipient, execution_engine)`, where:
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* `state` is the state object after applying `process_slots(state, slot)` transition to the resulting state of the parent block processing
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* `pow_chain` is a list that abstractly represents all blocks in the PoW chain
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* `fee_recipient` is the value suggested to be used for the `coinbase` field of the execution payload
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```python
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def get_pow_block_at_terminal_total_difficulty(pow_chain: Sequence[PowBlock]) -> Optional[PowBlock]:
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# `pow_chain` abstractly represents all blocks in the PoW chain
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for block in pow_chain:
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parent = get_pow_block(block.parent_hash)
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block_reached_ttd = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
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parent_reached_ttd = parent.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
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if block_reached_ttd and not parent_reached_ttd:
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return block
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return None
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def get_terminal_pow_block(pow_chain: Sequence[PowBlock]) -> Optional[PowBlock]:
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if TERMINAL_BLOCK_HASH != Hash32():
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# Terminal block hash override takes precedence over terminal total difficulty
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pow_block_overrides = [block for block in pow_chain if block.block_hash == TERMINAL_BLOCK_HASH]
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if not any(pow_block_overrides):
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return None
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return pow_block_overrides[0]
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return get_pow_block_at_terminal_total_difficulty(pow_chain)
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def prepare_execution_payload(state: BeaconState,
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pow_chain: Sequence[PowBlock],
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fee_recipient: ExecutionAddress,
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execution_engine: ExecutionEngine) -> Optional[PayloadId]:
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if not is_merge_complete(state):
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terminal_pow_block = get_terminal_pow_block(pow_chain)
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if terminal_pow_block is None:
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# Pre-merge, no prepare payload call is needed
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return None
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# Signify merge via producing on top of the terminal PoW block
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parent_hash = terminal_pow_block.block_hash
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else:
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# Post-merge, normal payload
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parent_hash = state.latest_execution_payload_header.block_hash
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timestamp = compute_timestamp_at_slot(state, state.slot)
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random = get_randao_mix(state, get_current_epoch(state))
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return execution_engine.prepare_payload(parent_hash, timestamp, random, fee_recipient)
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```
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2. Set `block.body.execution_payload = get_execution_payload(payload_id, execution_engine)`, where:
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```python
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def get_execution_payload(payload_id: Optional[PayloadId], execution_engine: ExecutionEngine) -> ExecutionPayload:
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if payload_id is None:
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# Pre-merge, empty payload
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return ExecutionPayload()
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else:
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return execution_engine.get_payload(payload_id)
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```
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*Note*: It is recommended for a validator to call `prepare_execution_payload` as soon as input parameters become known,
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and make subsequent calls to this function when any of these parameters gets updated.
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