eth2.0-specs/specs/capella/fork-choice.md

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# Capella -- Fork Choice
**Notice**: This document is a work-in-progress for researchers and implementers.
## Table of contents
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- [Introduction](#introduction)
- [Custom types](#custom-types)
- [Protocols](#protocols)
- [`ExecutionEngine`](#executionengine)
- [`notify_forkchoice_updated`](#notify_forkchoice_updated)
- [Helpers](#helpers)
- [Extended `PayloadAttributes`](#extended-payloadattributes)
- [Updated fork-choice handlers](#updated-fork-choice-handlers)
- [`on_block`](#on_block)
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## Introduction
This is the modification of the fork choice according to the Capella upgrade.
Unless stated explicitly, all prior functionality from [Bellatrix](../bellatrix/fork-choice.md) is inherited.
## Custom types
## Protocols
### `ExecutionEngine`
*Note*: The `notify_forkchoice_updated` function is modified in the `ExecutionEngine` protocol at the Capella upgrade.
#### `notify_forkchoice_updated`
The only change made is to the `PayloadAttributes` container through the addition of `withdrawals`.
Otherwise, `notify_forkchoice_updated` inherits all prior functionality.
```python
def notify_forkchoice_updated(self: ExecutionEngine,
head_block_hash: Hash32,
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safe_block_hash: Hash32,
finalized_block_hash: Hash32,
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payload_attributes: Optional[PayloadAttributes]) -> Optional[PayloadId]:
...
```
## Helpers
### Extended `PayloadAttributes`
`PayloadAttributes` is extended with the `withdrawals` field.
```python
@dataclass
class PayloadAttributes(object):
timestamp: uint64
prev_randao: Bytes32
suggested_fee_recipient: ExecutionAddress
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withdrawals: Sequence[Withdrawal] # [New in Capella]
```
## Updated fork-choice handlers
### `on_block`
*Note*: The only modification is the deletion of the verification of merge transition block conditions.
```python
def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
"""
Run ``on_block`` upon receiving a new block.
"""
block = signed_block.message
# Parent block must be known
assert block.parent_root in store.block_states
# Make a copy of the state to avoid mutability issues
pre_state = copy(store.block_states[block.parent_root])
# Blocks cannot be in the future. If they are, their consideration must be delayed until they are in the past.
assert get_current_slot(store) >= block.slot
# Check that block is later than the finalized epoch slot (optimization to reduce calls to get_ancestor)
finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
assert block.slot > finalized_slot
# Check block is a descendant of the finalized block at the checkpoint finalized slot
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finalized_checkpoint_block = get_checkpoint_block(
store,
block.parent_root,
store.finalized_checkpoint.epoch,
)
assert store.finalized_checkpoint.root == finalized_checkpoint_block
# Check the block is valid and compute the post-state
state = pre_state.copy()
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block_root = hash_tree_root(block)
state_transition(state, signed_block, True)
# Add new block to the store
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store.blocks[block_root] = block
# Add new state for this block to the store
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store.block_states[block_root] = state
# Add block timeliness to the store
time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT
is_before_attesting_interval = time_into_slot < SECONDS_PER_SLOT // INTERVALS_PER_SLOT
is_timely = get_current_slot(store) == block.slot and is_before_attesting_interval
store.block_timeliness[hash_tree_root(block)] = is_timely
# Add proposer score boost if the block is timely and not conflicting with an existing block
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is_first_block = store.proposer_boost_root == Root()
if is_timely and is_first_block:
store.proposer_boost_root = hash_tree_root(block)
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# Update checkpoints in store if necessary
update_checkpoints(store, state.current_justified_checkpoint, state.finalized_checkpoint)
# Eagerly compute unrealized justification and finality.
compute_pulled_up_tip(store, block_root)
```