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Rework data structures (#2747) 

1. Replace `header` and `finality_header` with `attested_header` (always the header signed by the committee) and `finailzed_header` (always the header verified by the Merkle branch)
2. Remove `LightClientSnapshot`, fold its fields into `LightClientStore` for simplicity
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vbuterin 2021-11-27 07:25:27 -06:00 committed by GitHub
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115
specs/altair/sync-protocol.md
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@ -13,7 +13,6 @@
- [Preset](#preset)
- [Misc](#misc)
- [Containers](#containers)
- [`LightClientSnapshot`](#lightclientsnapshot)
- [`LightClientUpdate`](#lightclientupdate)
- [`LightClientStore`](#lightclientstore)
- [Helper functions](#helper-functions)
@ -22,6 +21,7 @@
- [`get_safety_threshold`](#get_safety_threshold)
- [Light client state updates](#light-client-state-updates)
- [`process_slot`](#process_slot)
- [`get_active_header`](#get_active_header)
- [`validate_light_client_update`](#validate_light_client_update)
- [`apply_light_client_update`](#apply_light_client_update)
- [`process_light_client_update`](#process_light_client_update)
@ -57,28 +57,17 @@ uses sync committees introduced in [this beacon chain extension](./beacon-chain.
## Containers
### `LightClientSnapshot`
```python
class LightClientSnapshot(Container):
# Beacon block header
header: BeaconBlockHeader
# Sync committees corresponding to the header
current_sync_committee: SyncCommittee
next_sync_committee: SyncCommittee
```
### `LightClientUpdate`
```python
class LightClientUpdate(Container):
# Update beacon block header
header: BeaconBlockHeader
# The beacon block header that is attested to by the sync committee
attested_header: BeaconBlockHeader
# Next sync committee corresponding to the header
next_sync_committee: SyncCommittee
next_sync_committee_branch: Vector[Bytes32, floorlog2(NEXT_SYNC_COMMITTEE_INDEX)]
# Finality proof for the update header
finality_header: BeaconBlockHeader
# The finalized beacon block header attested to by Merkle branch
finalized_header: BeaconBlockHeader
finality_branch: Vector[Bytes32, floorlog2(FINALIZED_ROOT_INDEX)]
# Sync committee aggregate signature
sync_committee_bits: Bitvector[SYNC_COMMITTEE_SIZE]
@ -91,9 +80,16 @@ class LightClientUpdate(Container):
```python
class LightClientStore(object):
snapshot: LightClientSnapshot
# Beacon block header that is finalized
finalized_header: BeaconBlockHeader
# Sync committees corresponding to the header
current_sync_committee: SyncCommittee
next_sync_committee: SyncCommittee
# Best available header to switch finalized head to if we see nothing else
best_valid_update: Optional[LightClientUpdate]
# Most recent available reasonably-safe header
optimistic_header: BeaconBlockHeader
# Max number of participants in a sync committee (used to calculate safety threshold)
previous_period_max_attendance: uint64
current_period_max_attendance: uint64
```
@ -107,16 +103,6 @@ def get_subtree_index(generalized_index: GeneralizedIndex) -> uint64:
return uint64(generalized_index % 2**(floorlog2(generalized_index)))
```
### `get_signed_header`
```python
def get_signed_header(update: LightClientUpdate):
if update.finality_header is None:
return update.header
else:
return update.finality_header
```
### `get_safety_threshold`
```python
@ -140,44 +126,57 @@ def process_slot(store: LightClientStore, current_slot: Slot):
store.current_period_max_attendance = 0
```
### `get_active_header`
```python
def get_active_header(update: LightClientUpdate) -> BeaconBlockHeader:
# Is the update trying to convince us of a finalized header or an optimistic header?
if update.finalized_header BeaconBlockHeader():
return update.finalized_header
else:
return update.attested_header
```
#### `validate_light_client_update`
```python
def validate_light_client_update(snapshot: LightClientSnapshot,
def validate_light_client_update(store: LightClientStore,
update: LightClientUpdate,
genesis_validators_root: Root) -> None:
# Verify update slot is larger than snapshot slot
assert update.header.slot > snapshot.header.slot
# Verify update slot is larger than slot of current best finalized header
active_header = get_active_header(update)
assert active_header.slot > store.finalized_header.slot
# Verify update does not skip a sync committee period
snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
assert update_period in (snapshot_period, snapshot_period + 1)
finalized_period = compute_epoch_at_slot(store.finalized_header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
update_period = compute_epoch_at_slot(active_header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
assert update_period in (finalized_period, finalized_period + 1)
# Verify update header root is the finalized root of the finality header, if specified
if update.finality_header == BeaconBlockHeader():
if update.finalized_header == BeaconBlockHeader():
assert update.finality_branch == [Bytes32() for _ in range(floorlog2(FINALIZED_ROOT_INDEX))]
else:
assert is_valid_merkle_branch(
leaf=hash_tree_root(update.header),
leaf=hash_tree_root(update.finalized_header),
branch=update.finality_branch,
depth=floorlog2(FINALIZED_ROOT_INDEX),
index=get_subtree_index(FINALIZED_ROOT_INDEX),
root=update.finality_header.state_root,
root=update.attested_header.state_root,
)
# Verify update next sync committee if the update period incremented
if update_period == snapshot_period:
sync_committee = snapshot.current_sync_committee
if update_period == finalized_period:
sync_committee = store.current_sync_committee
assert update.next_sync_committee_branch == [Bytes32() for _ in range(floorlog2(NEXT_SYNC_COMMITTEE_INDEX))]
else:
sync_committee = snapshot.next_sync_committee
sync_committee = store.next_sync_committee
assert is_valid_merkle_branch(
leaf=hash_tree_root(update.next_sync_committee),
branch=update.next_sync_committee_branch,
depth=floorlog2(NEXT_SYNC_COMMITTEE_INDEX),
index=get_subtree_index(NEXT_SYNC_COMMITTEE_INDEX),
root=update.header.state_root,
root=active_header.state_root,
)
# Verify sync committee has sufficient participants
@ -186,20 +185,21 @@ def validate_light_client_update(snapshot: LightClientSnapshot,
# Verify sync committee aggregate signature
participant_pubkeys = [pubkey for (bit, pubkey) in zip(update.sync_committee_bits, sync_committee.pubkeys) if bit]
domain = compute_domain(DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
signing_root = compute_signing_root(get_signed_header(update), domain)
signing_root = compute_signing_root(update.attested_header, domain)
assert bls.FastAggregateVerify(participant_pubkeys, signing_root, update.sync_committee_signature)
```
#### `apply_light_client_update`
```python
def apply_light_client_update(snapshot: LightClientSnapshot, update: LightClientUpdate) -> None:
snapshot_period = compute_epoch_at_slot(snapshot.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
update_period = compute_epoch_at_slot(update.header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
if update_period == snapshot_period + 1:
snapshot.current_sync_committee = snapshot.next_sync_committee
snapshot.next_sync_committee = update.next_sync_committee
snapshot.header = update.header
def apply_light_client_update(store: LightClientStore, update: LightClientUpdate) -> None:
active_header = get_active_header(update)
finalized_period = compute_epoch_at_slot(store.finalized_header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
update_period = compute_epoch_at_slot(active_header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
if update_period == finalized_period + 1:
store.current_sync_committee = store.next_sync_committee
store.next_sync_committee = update.next_sync_committee
store.finalized_header = active_header
```
#### `process_light_client_update`
@ -210,13 +210,10 @@ def process_light_client_update(store: LightClientStore,
current_slot: Slot,
genesis_validators_root: Root) -> None:
validate_light_client_update(store.snapshot, update, genesis_validators_root)
validate_light_client_update(store, update, genesis_validators_root)
# Update the best update in case we have to force-update to it if the timeout elapses
if (
sum(update.sync_committee_bits) > sum(store.best_finalization_update.sync_committee_bits) and
get_signed_header(update).slot > store.snapshot.header.slot
):
if sum(update.sync_committee_bits) > sum(store.best_finalization_update.sync_committee_bits):
store.best_finalization_update = update
# Track the maximum attendance in the committee signatures
@ -228,20 +225,20 @@ def process_light_client_update(store: LightClientStore,
# Update the optimistic header
if (
sum(update.sync_committee_bits) > get_safety_threshold(store) and
update.header.slot > store.optimistic_header.slot
update.attested_header.slot > store.optimistic_header.slot
):
store.optimistic_header = update.header
store.optimistic_header = update.attested_header
# Update finalized header
if (
sum(update.sync_committee_bits) * 3 >= len(update.sync_committee_bits) * 2
and update.finality_header != BeaconBlockHeader()
and update.finalized_header != BeaconBlockHeader()
):
# Normal update through 2/3 threshold
apply_light_client_update(store.snapshot, update)
apply_light_client_update(store, update)
store.best_valid_update = None
elif current_slot > store.snapshot.header.slot + update_timeout:
elif current_slot > store.finalized_header.slot + update_timeout:
# Forced best update when the update timeout has elapsed
apply_light_client_update(store.snapshot, store.best_valid_update)
apply_light_client_update(store, store.best_valid_update)
store.best_valid_update = None
```