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

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# Deneb -- Fork Choice
## Table of contents
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- [Introduction](#introduction)
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- [Containers](#containers)
- [Helpers](#helpers)
- [`validate_blobs`](#validate_blobs)
- [`is_data_available`](#is_data_available)
- [Updated fork-choice handlers](#updated-fork-choice-handlers)
- [`on_block`](#on_block)
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## Introduction
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This is the modification of the fork choice accompanying the Deneb upgrade.
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## Containers
## Helpers
#### `validate_blobs`
```python
def validate_blobs(expected_kzg_commitments: Sequence[KZGCommitment],
blobs: Sequence[Blob],
proofs: Sequence[KZGProof]) -> None:
assert len(expected_kzg_commitments) == len(blobs)
assert len(blobs) == len(proofs)
assert verify_blob_kzg_proof_batch(blobs, expected_kzg_commitments, proofs)
```
#### `is_data_available`
The implementation of `is_data_available` will become more sophisticated during later scaling upgrades.
Initially, verification requires every verifying actor to retrieve all matching `Blob`s and `KZGProof`s, and validate them with `validate_blobs`.
The block MUST NOT be considered valid until all valid `Blob`s have been downloaded. Blocks that have been previously validated as available SHOULD be considered available even if the associated `Blob`s have subsequently been pruned.
```python
def is_data_available(beacon_block_root: Root, blob_kzg_commitments: Sequence[KZGCommitment]) -> bool:
# `retrieve_blobs_and_proofs` is implementation and context dependent
# It returns all the blobs for the given block root, and raises an exception if not available
# Note: the p2p network does not guarantee sidecar retrieval outside of `MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS`
blobs, proofs = retrieve_blobs_and_proofs(beacon_block_root)
# For testing, `retrieve_blobs_and_proofs` returns ("TEST", "TEST").
# TODO: Remove it once we have a way to inject `BlobSidecar` into tests.
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if isinstance(blobs, str) or isinstance(proofs, str):
return True
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validate_blobs(blob_kzg_commitments, blobs, proofs)
return True
```
## Updated fork-choice handlers
### `on_block`
*Note*: The only modification is the addition of the blob data availability check.
```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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assert get_ancestor_at_epoch_boundary(store, block.parent_root, store.finalized_checkpoint.epoch) == store.finalized_checkpoint.root
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# [New in Deneb]
# Check if blob data is available
# If not, this block MAY be queued and subsequently considered when blob data becomes available
assert is_data_available(hash_tree_root(block), block.body.blob_kzg_commitments)
# 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)
# Check the merge transition
if is_merge_transition_block(pre_state, block.body):
validate_merge_block(block)
# 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 proposer score boost if the block is timely
time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT
is_before_attesting_interval = time_into_slot < SECONDS_PER_SLOT // INTERVALS_PER_SLOT
if get_current_slot(store) == block.slot and is_before_attesting_interval:
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)
```