180 lines
7.6 KiB
Markdown
180 lines
7.6 KiB
Markdown
# Bellatrix -- Honest Validator
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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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- [Helpers](#helpers)
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- [`GetPayloadResponse`](#getpayloadresponse)
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- [`get_pow_block_at_terminal_total_difficulty`](#get_pow_block_at_terminal_total_difficulty)
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- [`get_terminal_pow_block`](#get_terminal_pow_block)
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- [Protocols](#protocols)
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- [`ExecutionEngine`](#executionengine)
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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 [Bellatrix](./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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## Helpers
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### `GetPayloadResponse`
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```python
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@dataclass
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class GetPayloadResponse(object):
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execution_payload: ExecutionPayload
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```
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### `get_pow_block_at_terminal_total_difficulty`
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```python
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def get_pow_block_at_terminal_total_difficulty(pow_chain: Dict[Hash32, 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.values():
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block_reached_ttd = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
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if block_reached_ttd:
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# If genesis block, no parent exists so reaching TTD alone qualifies as valid terminal block
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if block.parent_hash == Hash32():
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return block
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parent = pow_chain[block.parent_hash]
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parent_reached_ttd = parent.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
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if not parent_reached_ttd:
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return block
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return None
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```
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### `get_terminal_pow_block`
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```python
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def get_terminal_pow_block(pow_chain: Dict[Hash32, 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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if TERMINAL_BLOCK_HASH in pow_chain:
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return pow_chain[TERMINAL_BLOCK_HASH]
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else:
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return None
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return get_pow_block_at_terminal_total_difficulty(pow_chain)
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```
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*Note*: This function does *not* use simple serialize `hash_tree_root` as to
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avoid requiring simple serialize hashing capabilities in the Execution Layer.
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## Protocols
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### `ExecutionEngine`
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*Note*: `get_payload` function is added to the `ExecutionEngine` protocol for use as a validator.
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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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#### `get_payload`
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Given the `payload_id`, `get_payload` returns `GetPayloadResponse` with the most recent version of
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the execution payload that has been built since the corresponding call to `notify_forkchoice_updated` method.
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```python
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def get_payload(self: ExecutionEngine, payload_id: PayloadId) -> GetPayloadResponse:
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"""
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Return ``GetPayloadResponse`` 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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*Note*: A validator must not propose on or attest to a block that isn't deemed valid, i.e. hasn't yet passed the beacon chain state transition and execution validations. In future upgrades, an "execution Proof-of-Custody" will be integrated to prevent outsourcing of execution payload validations.
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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, safe_block_hash, finalized_block_hash, suggested_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 `Dict[Hash32, PowBlock]` dictionary that abstractly represents all blocks in the PoW chain with block hash as the dictionary key
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* `safe_block_hash` is the return value of the `get_safe_execution_payload_hash(store: Store)` function call
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* `finalized_block_hash` is the hash of the latest finalized execution payload (`Hash32()` if none yet finalized)
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* `suggested_fee_recipient` is the value suggested to be used for the `fee_recipient` field of the execution payload
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```python
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def prepare_execution_payload(state: BeaconState,
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safe_block_hash: Hash32,
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finalized_block_hash: Hash32,
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suggested_fee_recipient: ExecutionAddress,
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execution_engine: ExecutionEngine,
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pow_chain: Optional[Dict[Hash32, PowBlock]]=None) -> Optional[PayloadId]:
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if not is_merge_transition_complete(state):
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assert pow_chain is not None
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is_terminal_block_hash_set = TERMINAL_BLOCK_HASH != Hash32()
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is_activation_epoch_reached = get_current_epoch(state) >= TERMINAL_BLOCK_HASH_ACTIVATION_EPOCH
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if is_terminal_block_hash_set and not is_activation_epoch_reached:
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# Terminal block hash is set but activation epoch is not yet reached, no prepare payload call is needed
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return None
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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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# Set the forkchoice head and initiate the payload build process
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payload_attributes = PayloadAttributes(
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timestamp=compute_timestamp_at_slot(state, state.slot),
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prev_randao=get_randao_mix(state, get_current_epoch(state)),
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suggested_fee_recipient=suggested_fee_recipient,
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)
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return execution_engine.notify_forkchoice_updated(
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head_block_hash=parent_hash,
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safe_block_hash=safe_block_hash,
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finalized_block_hash=finalized_block_hash,
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payload_attributes=payload_attributes,
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)
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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).execution_payload
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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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