Move is_data_available check to fork-choice on_block

specs/eip4844/beacon-chain.md

@@ -23,7 +23,6 @@
 - [Helper functions](#helper-functions)
   - [Misc](#misc)
     - [`validate_blobs_sidecar`](#validate_blobs_sidecar)
-    - [`is_data_available`](#is_data_available)
     - [`kzg_commitment_to_versioned_hash`](#kzg_commitment_to_versioned_hash)
     - [`tx_peek_blob_versioned_hashes`](#tx_peek_blob_versioned_hashes)
     - [`verify_kzg_commitments_against_transactions`](#verify_kzg_commitments_against_transactions)
@@ -162,30 +161,6 @@ def validate_blobs_sidecar(slot: Slot,
     assert verify_aggregate_kzg_proof(blobs, expected_kzg_commitments, kzg_aggregated_proof)
 ```
 
-#### `is_data_available`
-
-The implementation of `is_data_available` will become more sophisticated during later sharding upgrades.
-Initially, it requires every verifying actor to retrieve the matching `BlobsSidecar`,
-and validate the sidecar with `validate_blobs_sidecar`.
-
-The block MUST NOT be considered valid until a valid `BlobsSidecar` has been downloaded. Blocks that have been previously validated as available SHOULD be considered available even if the associated `BlobsSidecar` has subsequently been pruned.
-
-```python
-def is_data_available(slot: Slot, beacon_block_root: Root, blob_kzg_commitments: Sequence[KZGCommitment]) -> bool:
-    # `retrieve_blobs_sidecar` is implementation and context dependent, raises an exception if not available.
-    # Note: the p2p network does not guarantee sidecar retrieval outside of `MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS`
-    sidecar = retrieve_blobs_sidecar(slot, beacon_block_root)
-
-    # For testing, `retrieve_blobs_sidecar` returns "TEST".
-    # TODO: Remove it once we have a way to inject `BlobsSidecar` into tests.
-    if isinstance(sidecar, str):
-        return True
-
-    validate_blobs_sidecar(slot, beacon_block_root, blob_kzg_commitments, sidecar)
-    return True
-```
-
-
 #### `kzg_commitment_to_versioned_hash`
 
 ```python
@@ -241,9 +216,6 @@ def process_block(state: BeaconState, block: BeaconBlock) -> None:
     process_operations(state, block.body)
     process_sync_aggregate(state, block.body.sync_aggregate)
     process_blob_kzg_commitments(state, block.body)  # [New in EIP-4844]
-
-    # New in EIP-4844
-    assert is_data_available(block.slot, hash_tree_root(block), block.body.blob_kzg_commitments)
 ```
 
 #### Execution payload

specs/eip4844/fork-choice.md

@@ -0,0 +1,105 @@
+# EIP-4844 -- Fork Choice
+
+## Table of contents
+<!-- TOC -->
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
+- [Introduction](#introduction)
+- [Helpers](#helpers)
+    - [`is_data_available`](#is_data_available)
+- [Updated fork-choice handlers](#updated-fork-choice-handlers)
+  - [`on_block`](#on_block)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## Introduction
+
+This is the modification of the fork choice according to the executable beacon chain proposal.
+
+## Helpers
+
+#### `is_data_available`
+
+The implementation of `is_data_available` will become more sophisticated during later sharding upgrades.
+Initially, it requires every verifying actor to retrieve the matching `BlobsSidecar`,
+and validate the sidecar with `validate_blobs_sidecar`.
+
+The block MUST NOT be considered valid until a valid `BlobsSidecar` has been downloaded. Blocks that have been previously validated as available SHOULD be considered available even if the associated `BlobsSidecar` has subsequently been pruned.
+
+```python
+def is_data_available(slot: Slot, beacon_block_root: Root, blob_kzg_commitments: Sequence[KZGCommitment]) -> bool:
+    # `retrieve_blobs_sidecar` is implementation and context dependent, raises an exception if not available.
+    # Note: the p2p network does not guarantee sidecar retrieval outside of `MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS`
+    sidecar = retrieve_blobs_sidecar(slot, beacon_block_root)
+
+    # For testing, `retrieve_blobs_sidecar` returns "TEST".
+    # TODO: Remove it once we have a way to inject `BlobsSidecar` into tests.
+    if isinstance(sidecar, str):
+        return True
+
+    validate_blobs_sidecar(slot, beacon_block_root, blob_kzg_commitments, sidecar)
+    return True
+```
+
+## Updated fork-choice handlers
+
+### `on_block`
+
+*Note*: The only modification is the addition of the verification of 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
+    assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root
+
+    # [New in EIP-4844]
+    # Check if the block is available
+    assert is_data_available(block.slot, hash_tree_root(block), block.body.blob_kzg_commitments) 
+
+    # Check the block is valid and compute the post-state
+    state = pre_state.copy()
+    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
+    store.blocks[hash_tree_root(block)] = block
+    # Add new state for this block to the store
+    store.block_states[hash_tree_root(block)] = 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)
+
+    # Update justified checkpoint
+    if state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch:
+        if state.current_justified_checkpoint.epoch > store.best_justified_checkpoint.epoch:
+            store.best_justified_checkpoint = state.current_justified_checkpoint
+        if should_update_justified_checkpoint(store, state.current_justified_checkpoint):
+            store.justified_checkpoint = state.current_justified_checkpoint
+
+    # Update finalized checkpoint
+    if state.finalized_checkpoint.epoch > store.finalized_checkpoint.epoch:
+        store.finalized_checkpoint = state.finalized_checkpoint
+        store.justified_checkpoint = state.current_justified_checkpoint
+```