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forkchoice: seed anchor checkpoints from the anchor slot #677

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Merged
tcoratger merged 1 commit into from
Apr 23, 2026
+29 −16
Merged

forkchoice: seed anchor checkpoints from the anchor slot #677

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19 changes: 11 additions & 8 deletions src/lean_spec/subspecs/forkchoice/store.py
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Original file line number Diff line number Diff line change
Expand Up @@ -210,18 +210,21 @@ def from_anchor(
# Read the slot at which the anchor block was proposed.
anchor_slot = anchor_block.slot

# Initialize checkpoints from this state.
# Seed both checkpoints from the anchor block itself.
#
# We explicitly set the root to the anchor block root.
# The state internally might have zero-hash checkpoints (if genesis),
# but the Store must treat the anchor block as the justified/finalized point.
# The store treats the anchor as the new "genesis" for fork choice:
# all history below it is pruned. The justified and finalized checkpoints
# therefore point at the anchor block with the anchor's own slot,
# regardless of what the anchor state's embedded checkpoints say.
anchor_checkpoint = Checkpoint(root=anchor_root, slot=anchor_slot)

return cls(
time=Interval.from_slot(anchor_slot),
config=state.config,
head=anchor_root,
safe_target=anchor_root,
latest_justified=state.latest_justified.model_copy(update={"root": anchor_root}),
latest_finalized=state.latest_finalized.model_copy(update={"root": anchor_root}),
latest_justified=anchor_checkpoint,
latest_finalized=anchor_checkpoint,
blocks={anchor_root: anchor_block},
states={anchor_root: state},
validator_id=validator_id,
Expand Down Expand Up @@ -515,8 +518,8 @@ def on_block(
# Keep the checkpoint with the higher slot.
# On slot ties, prefer the store's own checkpoint.
#
# The store's checkpoint is pinned to the anchor block root at init.
# The anchor state may hold a pre-anchor root.
# The store's checkpoint is pinned to the anchor at init and only
# moves forward via real justification/finalization events.
# On ties the store's view is authoritative.
latest_justified = max(
self.latest_justified, post_state.latest_justified, key=lambda c: c.slot
Expand Down
26 changes: 18 additions & 8 deletions tests/consensus/devnet/fc/test_checkpoint_sync.py
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Expand Up @@ -47,7 +47,8 @@ def test_store_init_from_non_genesis_anchor(
-----------------------------

- Head points to the anchor block.
- Latest justified and latest finalized both reference the anchor root.
- Latest justified and latest finalized both reference the anchor root
at the anchor's own slot (beacon-chain seeding convention).
- Store clock is at the anchor slot (no pre-anchor intervals tracked).
- The anchor block is the only entry in the store's block map.

Expand Down Expand Up @@ -78,9 +79,12 @@ def test_store_init_from_non_genesis_anchor(
time=anchor_time_intervals,
head_slot=ANCHOR_SLOT,
head_root_label="genesis",
latest_justified_slot=anchor_state.latest_justified.slot,
# Both checkpoints are seeded from the anchor itself:
# slot = anchor.slot, root = anchor_root. The anchor
# state's embedded checkpoint slots are intentionally ignored.
latest_justified_slot=ANCHOR_SLOT,
latest_justified_root_label="genesis",
latest_finalized_slot=anchor_state.latest_finalized.slot,
latest_finalized_slot=ANCHOR_SLOT,
latest_finalized_root_label="genesis",
safe_target_root_label="genesis",
labels_in_store=["genesis"],
Expand Down Expand Up @@ -135,9 +139,11 @@ def test_extend_chain_from_non_genesis_anchor(
checks=StoreChecks(
head_slot=Slot(11),
head_root_label="block_11",
latest_justified_slot=anchor_state.latest_justified.slot,
# Empty blocks carry no attestations, so neither checkpoint
# advances past the anchor seeding.
latest_justified_slot=ANCHOR_SLOT,
latest_justified_root_label="genesis",
latest_finalized_slot=anchor_state.latest_finalized.slot,
latest_finalized_slot=ANCHOR_SLOT,
latest_finalized_root_label="genesis",
labels_in_store=["genesis", "block_11"],
),
Expand All @@ -151,7 +157,9 @@ def test_extend_chain_from_non_genesis_anchor(
checks=StoreChecks(
head_slot=Slot(12),
head_root_label="block_12",
latest_justified_slot=ANCHOR_SLOT,
latest_justified_root_label="genesis",
latest_finalized_slot=ANCHOR_SLOT,
latest_finalized_root_label="genesis",
labels_in_store=["genesis", "block_11", "block_12"],
),
Expand All @@ -165,7 +173,9 @@ def test_extend_chain_from_non_genesis_anchor(
checks=StoreChecks(
head_slot=Slot(13),
head_root_label="block_13",
latest_justified_slot=ANCHOR_SLOT,
latest_justified_root_label="genesis",
latest_finalized_slot=ANCHOR_SLOT,
latest_finalized_root_label="genesis",
labels_in_store=["genesis", "block_11", "block_12", "block_13"],
),
Expand Down Expand Up @@ -238,9 +248,9 @@ def test_fork_off_non_genesis_anchor(
# unambiguously heavier.
#
# Source is pinned to the anchor block because the store
# only knows about blocks at or after the anchor. The
# anchor state's internal justified root still points to
# pre-anchor history that was discarded on checkpoint sync.
# only knows about blocks at or after the anchor, and the
# store's latest_justified checkpoint is seeded at
# (anchor.slot, anchor_root).
BlockStep(
block=BlockSpec(
slot=Slot(12),
Expand Down
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