Add get_checkpoint_block to the fork choice spec

specs/bellatrix/fork-choice.md

@@ -170,7 +170,12 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     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
+    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()

specs/deneb/fork-choice.md

@@ -82,7 +82,12 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     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
+    finalized_checkpoint_block = get_checkpoint_block(
+        store,
+        block.parent_root,
+        store.finalized_checkpoint.epoch,
+    )
+    assert store.finalized_checkpoint.root == finalized_checkpoint_block
 
     # [New in Deneb]
     # Check if blob data is available

specs/phase0/fork-choice.md

@@ -18,6 +18,7 @@
     - [`get_current_slot`](#get_current_slot)
     - [`compute_slots_since_epoch_start`](#compute_slots_since_epoch_start)
     - [`get_ancestor`](#get_ancestor)
+    - [`get_checkpoint_block`](#get_checkpoint_block)
     - [`get_weight`](#get_weight)
     - [`get_voting_source`](#get_voting_source)
     - [`filter_block_tree`](#filter_block_tree)
@@ -192,6 +193,17 @@ def get_ancestor(store: Store, root: Root, slot: Slot) -> Root:
     return root
 ```
 
+#### `get_checkpoint_block`
+
+```python
+def get_checkpoint_block(store: Store, root: Root, epoch: Epoch) -> Root:
+    """
+    Compute the checkpoint block for epoch ``epoch`` in the chain of block ``root``
+    """
+    epoch_first_slot = compute_start_slot_at_epoch(epoch)
+    return get_ancestor(store, root, epoch_first_slot)
+```
+
 #### `get_weight`
 
 ```python
@@ -278,10 +290,15 @@ def filter_block_tree(store: Store, block_root: Root, blocks: Dict[Root, BeaconB
             voting_source.epoch + 2 >= current_epoch
         )
 
-    finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
+    finalized_checkpoint_block = get_checkpoint_block(
+        store,
+        block_root,
+        store.finalized_checkpoint.epoch,
+    )
+
     correct_finalized = (
         store.finalized_checkpoint.epoch == GENESIS_EPOCH
-        or store.finalized_checkpoint.root == get_ancestor(store, block_root, finalized_slot)
+        or store.finalized_checkpoint.root == finalized_checkpoint_block
     )
 
     # If expected finalized/justified, add to viable block-tree and signal viability to parent.
@@ -442,8 +459,7 @@ def validate_on_attestation(store: Store, attestation: Attestation, is_from_bloc
     assert store.blocks[attestation.data.beacon_block_root].slot <= attestation.data.slot
 
     # LMD vote must be consistent with FFG vote target
-    target_slot = compute_start_slot_at_epoch(target.epoch)
-    assert target.root == get_ancestor(store, attestation.data.beacon_block_root, target_slot)
+    assert target.root == get_checkpoint_block(store, attestation.data.beacon_block_root, target.epoch)
 
     # Attestations can only affect the fork choice of subsequent slots.
     # Delay consideration in the fork choice until their slot is in the past.
@@ -506,7 +522,12 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     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
+    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()

specs/phase0/p2p-interface.md

@@ -317,7 +317,7 @@ The following validations MUST pass before forwarding the `signed_beacon_block`
 - _[REJECT]_ The block's parent (defined by `block.parent_root`) passes validation.
 - _[REJECT]_ The block is from a higher slot than its parent.
 - _[REJECT]_ The current `finalized_checkpoint` is an ancestor of `block` -- i.e.
-  `get_ancestor(store, block.parent_root, compute_start_slot_at_epoch(store.finalized_checkpoint.epoch))
+  `get_checkpoint_block(store, block.parent_root, store.finalized_checkpoint.epoch)
   == store.finalized_checkpoint.root`
 - _[REJECT]_ The block is proposed by the expected `proposer_index` for the block's slot
   in the context of the current shuffling (defined by `parent_root`/`slot`).
@@ -356,7 +356,7 @@ The following validations MUST pass before forwarding the `signed_aggregate_and_
   (a client MAY queue aggregates for processing once block is retrieved).
 - _[REJECT]_ The block being voted for (`aggregate.data.beacon_block_root`) passes validation.
 - _[IGNORE]_ The current `finalized_checkpoint` is an ancestor of the `block` defined by `aggregate.data.beacon_block_root` -- i.e.
-  `get_ancestor(store, aggregate.data.beacon_block_root, compute_start_slot_at_epoch(store.finalized_checkpoint.epoch))
+  `get_checkpoint_block(store, aggregate.data.beacon_block_root, finalized_checkpoint.epoch)
   == store.finalized_checkpoint.root`
 
 
@@ -425,9 +425,9 @@ The following validations MUST pass before forwarding the `attestation` on the s
   (a client MAY queue attestations for processing once block is retrieved).
 - _[REJECT]_ The block being voted for (`attestation.data.beacon_block_root`) passes validation.
 - _[REJECT]_ The attestation's target block is an ancestor of the block named in the LMD vote -- i.e.
-  `get_ancestor(store, attestation.data.beacon_block_root, compute_start_slot_at_epoch(attestation.data.target.epoch)) == attestation.data.target.root`
+  `get_checkpoint_block(store, attestation.data.beacon_block_root, attestation.data.target.epoch) == attestation.data.target.root`
 - _[IGNORE]_ The current `finalized_checkpoint` is an ancestor of the `block` defined by `attestation.data.beacon_block_root` -- i.e.
-  `get_ancestor(store, attestation.data.beacon_block_root, compute_start_slot_at_epoch(store.finalized_checkpoint.epoch))
+  `get_checkpoint_block(store, attestation.data.beacon_block_root, store.finalized_checkpoint.epoch)
   == store.finalized_checkpoint.root`
 
 

tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py

@@ -479,7 +479,7 @@ def test_voting_source_within_two_epoch(spec, state):
     - store.voting_source[block_root].epoch != store.justified_checkpoint.epoch, and
     - store.unrealized_justifications[block_root].epoch >= store.justified_checkpoint.epoch, and
     - store.voting_source[block_root].epoch + 2 >= current_epoch, and
-    - store.finalized_checkpoint.root == get_ancestor(store, block_root, finalized_slot)
+    - store.finalized_checkpoint.root == get_checkpoint_block(store, block_root, store.finalized_checkpoint.epoch)
     """
     test_steps = []
     # Initialization
@@ -536,8 +536,11 @@ def test_voting_source_within_two_epoch(spec, state):
     assert store.unrealized_justifications[last_fork_block_root].epoch >= store.justified_checkpoint.epoch
     # assert store.voting_source[last_fork_block_root].epoch + 2 >= \
     #     spec.compute_epoch_at_slot(spec.get_current_slot(store))
-    finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    assert store.finalized_checkpoint.root == spec.get_ancestor(store, last_fork_block_root, finalized_slot)
+    assert store.finalized_checkpoint.root == spec.get_checkpoint_block(
+        store,
+        last_fork_block_root,
+        store.finalized_checkpoint.epoch
+    )
     assert spec.get_head(store) == last_fork_block_root
 
     yield 'steps', test_steps
@@ -552,7 +555,7 @@ def test_voting_source_beyond_two_epoch(spec, state):
     - store.voting_source[block_root].epoch != store.justified_checkpoint.epoch, and
     - store.unrealized_justifications[block_root].epoch >= store.justified_checkpoint.epoch, and
     - store.voting_source[block_root].epoch + 2 < current_epoch, and
-    - store.finalized_checkpoint.root == get_ancestor(store, block_root, finalized_slot)
+    - store.finalized_checkpoint.root == get_checkpoint_block(store, block_root, store.finalized_checkpoint.epoch)
     """
     test_steps = []
     # Initialization
@@ -617,8 +620,11 @@ def test_voting_source_beyond_two_epoch(spec, state):
     assert store.unrealized_justifications[last_fork_block_root].epoch >= store.justified_checkpoint.epoch
     # assert store.voting_source[last_fork_block_root].epoch + 2 < \
     #     spec.compute_epoch_at_slot(spec.get_current_slot(store))
-    finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    assert store.finalized_checkpoint.root == spec.get_ancestor(store, last_fork_block_root, finalized_slot)
+    assert store.finalized_checkpoint.root == spec.get_checkpoint_block(
+        store,
+        last_fork_block_root,
+        store.finalized_checkpoint.epoch
+    )
     assert spec.get_head(store) == correct_head
 
     yield 'steps', test_steps
@@ -641,7 +647,7 @@ def test_incorrect_finalized(spec, state):
     # Check that the store doesn't allow for a head block that has:
     # - store.voting_source[block_root].epoch == store.justified_checkpoint.epoch, and
     # - store.finalized_checkpoint.epoch != GENESIS_EPOCH, and
-    # - store.finalized_checkpoint.root != get_ancestor(store, block_root, finalized_slot)
+    # - store.finalized_checkpoint.root != get_checkpoint_block(store, block_root, store.finalized_checkpoint.epoch)
     test_steps = []
     # Initialization
     store, anchor_block = get_genesis_forkchoice_store_and_block(spec, state)
@@ -718,7 +724,11 @@ def test_incorrect_finalized(spec, state):
     assert store.voting_source[last_fork_block_root].epoch == store.justified_checkpoint.epoch
     assert store.finalized_checkpoint.epoch != spec.GENESIS_EPOCH
     finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    assert store.finalized_checkpoint.root != spec.get_ancestor(store, last_fork_block_root, finalized_slot)
+    assert store.finalized_checkpoint.root != spec.get_checkpoint_block(
+        store,
+        block_root,
+        store.finalized_checkpoint.epoch
+    )
     assert spec.get_head(store) != last_fork_block_root
     assert spec.get_head(store) == head_root
 

tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_on_block.py

@@ -352,8 +352,11 @@ def test_new_finalized_slot_is_not_justified_checkpoint_ancestor(spec, state):
         # NOTE: Do not call `on_tick` here
         yield from add_block(spec, store, block, test_steps)
 
-    finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    ancestor_at_finalized_slot = spec.get_ancestor(store, pre_store_justified_checkpoint_root, finalized_slot)
+    ancestor_at_finalized_slot = spec.get_checkpoint_block(
+        store,
+        pre_store_justified_checkpoint_root,
+        store.finalized_checkpoint.epoch
+    )
     assert ancestor_at_finalized_slot != store.finalized_checkpoint.root
 
     assert store.finalized_checkpoint == another_state.finalized_checkpoint
@@ -428,8 +431,11 @@ def test_new_finalized_slot_is_justified_checkpoint_ancestor(spec, state):
     for block in all_blocks:
         yield from tick_and_add_block(spec, store, block, test_steps)
 
-    finalized_slot = spec.compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
-    ancestor_at_finalized_slot = spec.get_ancestor(store, pre_store_justified_checkpoint_root, finalized_slot)
+    ancestor_at_finalized_slot = spec.get_checkpoint_block(
+        store,
+        pre_store_justified_checkpoint_root,
+        store.finalized_checkpoint.epoch
+    )
     assert ancestor_at_finalized_slot == store.finalized_checkpoint.root
 
     assert store.finalized_checkpoint == another_state.finalized_checkpoint
@@ -857,10 +863,18 @@ def test_incompatible_justification_update_start_of_epoch(spec, state):
     # Now add the blocks & check that justification update was triggered
     for signed_block in signed_blocks:
         yield from tick_and_add_block(spec, store, signed_block, test_steps)
-    finalized_slot = spec.compute_start_slot_at_epoch(state.finalized_checkpoint.epoch)
-    assert spec.get_ancestor(store, last_block_root, finalized_slot) == state.finalized_checkpoint.root
-    justified_slot = spec.compute_start_slot_at_epoch(state.current_justified_checkpoint.epoch)
-    assert spec.get_ancestor(store, last_block_root, justified_slot) != state.current_justified_checkpoint.root
+    finalized_checkpoint_block = spec.get_checkpoint_block(
+        store,
+        last_block_root,
+        state.finalized_checkpoint.epoch,
+    )
+    assert finalized_checkpoint_block == state.finalized_checkpoint.root
+    justified_checkpoint_block = spec.get_checkpoint_block(
+        store,
+        last_block_root,
+        state.current_justified_checkpoint.epoch,
+    )
+    assert justified_checkpoint_block != state.current_justified_checkpoint.root
     assert store.finalized_checkpoint.epoch == 4
     assert store.justified_checkpoint.epoch == 6
 
@@ -934,10 +948,18 @@ def test_incompatible_justification_update_end_of_epoch(spec, state):
     # Now add the blocks & check that justification update was triggered
     for signed_block in signed_blocks:
         yield from tick_and_add_block(spec, store, signed_block, test_steps)
-    finalized_slot = spec.compute_start_slot_at_epoch(state.finalized_checkpoint.epoch)
-    assert spec.get_ancestor(store, last_block_root, finalized_slot) == state.finalized_checkpoint.root
-    justified_slot = spec.compute_start_slot_at_epoch(state.current_justified_checkpoint.epoch)
-    assert spec.get_ancestor(store, last_block_root, justified_slot) != state.current_justified_checkpoint.root
+    finalized_checkpoint_block = spec.get_checkpoint_block(
+        store,
+        last_block_root,
+        state.finalized_checkpoint.epoch,
+    )
+    assert finalized_checkpoint_block == state.finalized_checkpoint.root
+    justified_checkpoint_block = spec.get_checkpoint_block(
+        store,
+        last_block_root,
+        state.current_justified_checkpoint.epoch,
+    )
+    assert justified_checkpoint_block != state.current_justified_checkpoint.root
     assert store.finalized_checkpoint.epoch == 4
     assert store.justified_checkpoint.epoch == 6