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Use parent_root for finalized chain check #1884

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Jun 13, 2020
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Use parent_root for finalized chain check #1884

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29d968b
Use parent_root for finalized chain check
paulhauner Jun 10, 2020
fdb6f15
unhandled exceptions do not modify forkchoice store
djrtwo Jun 13, 2020
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14 changes: 8 additions & 6 deletions specs/phase0/fork-choice.md
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Expand Up @@ -44,9 +44,11 @@ This document is the beacon chain fork choice spec, part of Ethereum 2.0 Phase 0

The head block root associated with a `store` is defined as `get_head(store)`. At genesis, let `store = get_forkchoice_store(genesis_state)` and update `store` by running:

- `on_tick(time)` whenever `time > store.time` where `time` is the current Unix time
- `on_block(block)` whenever a block `block: SignedBeaconBlock` is received
- `on_attestation(attestation)` whenever an attestation `attestation` is received
- `on_tick(store, time)` whenever `time > store.time` where `time` is the current Unix time
- `on_block(store, block)` whenever a block `block: SignedBeaconBlock` is received
- `on_attestation(store, attestation)` whenever an attestation `attestation` is received

Any of the above handlers that trigger an unhandled exception (e.g. a failed assert or an out-of-range list access) are considered invalid. Invalid calls to handlers must not modify `store`.

*Notes*:

Expand Down Expand Up @@ -347,17 +349,17 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
pre_state = store.block_states[block.parent_root].copy()
# Blocks cannot be in the future. If they are, their consideration must be delayed until the are in the past.
assert get_current_slot(store) >= block.slot
# Add new block to the store
store.blocks[hash_tree_root(block)] = block

# 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, hash_tree_root(block), finalized_slot) == store.finalized_checkpoint.root
assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root

# Check the block is valid and compute the post-state
state = state_transition(pre_state, signed_block, True)
# 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

Expand Down