custody-based fork-choice

Author: fradamt

pysetup/spec_builders/eip7594.py

@@ -12,6 +12,14 @@ def imports(cls, preset_name: str):
         return f'''
 from eth2spec.deneb import {preset_name} as deneb
 '''
+    
+
+    @classmethod
+    def sundry_functions(cls) -> str:
+        return """
+def retrieve_column_sidecars(beacon_block_root: Root) -> Sequence[DataColumnSidecar]:
+    return []
+"""
 
     @classmethod
     def hardcoded_custom_type_dep_constants(cls, spec_object) -> str:

specs/_features/eip7594/fork-choice.md

@@ -0,0 +1,122 @@
+# EIP-7594 -- 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)
+    - [Modified `is_data_available`](#modified-is_data_available)
+    - [New `is_chain_available`](#new-is_chain_available)
+    - [Modified `get_head`](#modified-get_head)
+    - [New `is_peer_sampling_required`](#new-is_peer_sampling_required)
+- [Updated fork-choice handlers](#updated-fork-choice-handlers)
+  - [Modified `on_block`](#modified-on_block)
+    - [Pull-up tip helpers](#pull-up-tip-helpers)
+      - [Modified `compute_pulled_up_tip`](#modified-compute_pulled_up_tip)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## Introduction
+
+This is the modification of the fork choice accompanying EIP-7594.
+
+### Helpers
+
+#### Modified `is_data_available`
+
+```python
+def is_data_available(beacon_block_root: Root) -> bool:
+    # `retrieve_column_sidecars` is implementation and context dependent, replacing `retrieve_blobs_and_proofs`.
+    # For the given block root, it returns all column sidecars to custody, or raises an exception if they are not available. # The p2p network does not guarantee sidecar retrieval outside of `MIN_EPOCHS_FOR_DATA_COLUMN_SIDECARS_REQUESTS` epochs.  
+    column_sidecars = retrieve_column_sidecars(beacon_block_root)
+    return all(
+        verify_data_column_sidecar_kzg_proofs(column_sidecar)
+        for column_sidecar in column_sidecars
+    )
+```
+
+#### Modified `get_head`
+
+*Note*: children of the current `head` are required to be available in order to be considered by the fork-choice.
+
+```python
+def get_head(store: Store) -> Root:
+    # Get filtered block tree that only includes viable branches
+    blocks = get_filtered_block_tree(store)
+    # Execute the LMD-GHOST fork choice
+    head = store.justified_checkpoint.root
+    while True:
+        # Get available children for the current slot
+        children = [
+            root for (root, block) in blocks.items()
+            if (
+                block.parent_root == head
+                and is_data_available(root)
+            )
+        ]
+        if len(children) == 0:
+            return head
+        # Sort by latest attesting balance with ties broken lexicographically
+        # Ties broken by favoring block with lexicographically higher root
+        head = max(children, key=lambda root: (get_weight(store, root), root))
+```
+
+## Updated fork-choice handlers
+
+### Modified `on_block`
+
+*Note*: The blob data availability check is removed.
+
+```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
+    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
+    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
+    block_root = hash_tree_root(block)
+    state_transition(state, signed_block, True)
+
+    # Add new block to the store
+    store.blocks[block_root] = block
+    # Add new state for this block to the store
+    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
+    is_first_block = store.proposer_boost_root == Root()
+    if is_timely and is_first_block:
+        store.proposer_boost_root = hash_tree_root(block)
+
+    # 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)
+```