Remove fork_version from LightClientUpdate

The `fork_version` field in `LightClientUpdate` can be derived from the
`update.signature_slot` value by consulting the locally configured fork
schedule. The light client already needs access to the fork schedule to
determine the `GeneralizedIndex` values used for merkle proofs, and the
memory layouts of the structures (including `LightClientUpdate`). The
`fork_version` itself is network dependent and doesn't reveal that info.

specs/altair/beacon-chain.md

@@ -28,6 +28,7 @@
 - [Helper functions](#helper-functions)
   - [Crypto](#crypto)
   - [Misc](#misc-1)
+    - [`compute_fork_version`](#compute_fork_version)
     - [`add_flag`](#add_flag)
     - [`has_flag`](#has_flag)
   - [Beacon state accessors](#beacon-state-accessors)
@@ -226,6 +227,18 @@ the functionality described in those documents.
 
 ### Misc
 
+#### `compute_fork_version`
+
+```python
+def compute_fork_version(epoch: Epoch) -> Version:
+    """
+    Return the fork version at the given ``epoch``.
+    """
+    if epoch >= ALTAIR_FORK_EPOCH:
+        return ALTAIR_FORK_VERSION
+    return GENESIS_FORK_VERSION
+```
+
 #### `add_flag`
 
 ```python

specs/altair/sync-protocol.md

@@ -71,8 +71,6 @@ class LightClientUpdate(Container):
     finality_branch: Vector[Bytes32, floorlog2(FINALIZED_ROOT_INDEX)]
     # Sync committee aggregate signature
     sync_aggregate: SyncAggregate
-    # Fork version for the aggregate signature
-    fork_version: Version
     # Slot at which the aggregate signature was created (untrusted)
     signature_slot: Slot
 ```
@@ -211,7 +209,8 @@ def validate_light_client_update(store: LightClientStore,
         pubkey for (bit, pubkey) in zip(sync_aggregate.sync_committee_bits, sync_committee.pubkeys)
         if bit
     ]
-    domain = compute_domain(DOMAIN_SYNC_COMMITTEE, update.fork_version, genesis_validators_root)
+    fork_version = compute_fork_version(compute_epoch_at_slot(update.signature_slot))
+    domain = compute_domain(DOMAIN_SYNC_COMMITTEE, fork_version, genesis_validators_root)
     signing_root = compute_signing_root(update.attested_header, domain)
     assert bls.FastAggregateVerify(participant_pubkeys, signing_root, sync_aggregate.sync_committee_signature)
 ```

specs/bellatrix/beacon-chain.md

@@ -29,6 +29,7 @@
     - [`is_execution_enabled`](#is_execution_enabled)
   - [Misc](#misc)
     - [`compute_timestamp_at_slot`](#compute_timestamp_at_slot)
+    - [Modified `compute_fork_version`](#modified-compute_fork_version)
   - [Beacon state accessors](#beacon-state-accessors)
     - [Modified `get_inactivity_penalty_deltas`](#modified-get_inactivity_penalty_deltas)
   - [Beacon state mutators](#beacon-state-mutators)
@@ -242,6 +243,20 @@ def compute_timestamp_at_slot(state: BeaconState, slot: Slot) -> uint64:
     return uint64(state.genesis_time + slots_since_genesis * SECONDS_PER_SLOT)
 ```
 
+#### Modified `compute_fork_version`
+
+```python
+def compute_fork_version(epoch: Epoch) -> Version:
+    """
+    Return the fork version at the given ``epoch``.
+    """
+    if epoch >= BELLATRIX_FORK_EPOCH:
+        return BELLATRIX_FORK_VERSION
+    if epoch >= ALTAIR_FORK_EPOCH:
+        return ALTAIR_FORK_VERSION
+    return GENESIS_FORK_VERSION
+```
+
 ### Beacon state accessors
 
 #### Modified `get_inactivity_penalty_deltas`

specs/capella/beacon-chain.md

@@ -34,6 +34,8 @@
     - [`has_eth1_withdrawal_credential`](#has_eth1_withdrawal_credential)
     - [`is_fully_withdrawable_validator`](#is_fully_withdrawable_validator)
     - [`is_partially_withdrawable_validator`](#is_partially_withdrawable_validator)
+  - [Misc](#misc-1)
+    - [Modified `compute_fork_version`](#modified-compute_fork_version)
 - [Beacon chain state transition function](#beacon-chain-state-transition-function)
   - [Epoch processing](#epoch-processing)
     - [Full withdrawals](#full-withdrawals)
@@ -318,6 +320,24 @@ def is_partially_withdrawable_validator(validator: Validator, balance: Gwei) ->
     return has_eth1_withdrawal_credential(validator) and has_max_effective_balance and has_excess_balance
 ```
 
+### Misc
+
+#### Modified `compute_fork_version`
+
+```python
+def compute_fork_version(epoch: Epoch) -> Version:
+    """
+    Return the fork version at the given ``epoch``.
+    """
+    if epoch >= CAPELLA_FORK_EPOCH:
+        return CAPELLA_FORK_VERSION
+    if epoch >= BELLATRIX_FORK_EPOCH:
+        return BELLATRIX_FORK_VERSION
+    if epoch >= ALTAIR_FORK_EPOCH:
+        return ALTAIR_FORK_VERSION
+    return GENESIS_FORK_VERSION
+```
+
 ## Beacon chain state transition function
 
 ### Epoch processing

tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py

@@ -16,6 +16,7 @@ from eth2spec.test.helpers.constants import MINIMAL
 from eth2spec.test.helpers.light_client import (
     get_sync_aggregate,
     initialize_light_client_store,
+    override_config_fork_epochs,
 )
 from eth2spec.test.helpers.state import (
     next_slots,
@@ -27,6 +28,9 @@ from eth2spec.test.helpers.merkle import build_proof
 @with_altair_and_later
 @spec_state_test
 def test_process_light_client_update_not_timeout(spec, state):
+    old_config = spec.config
+    override_config_fork_epochs(spec, state)
+
     store = initialize_light_client_store(spec, state)
 
     # Block at slot 1 doesn't increase sync committee period, so it won't force update store.finalized_header
@@ -41,7 +45,7 @@ def test_process_light_client_update_not_timeout(spec, state):
     )
 
     # Sync committee signing the block_header
-    sync_aggregate, fork_version, signature_slot = get_sync_aggregate(spec, state, block_header)
+    sync_aggregate, signature_slot = get_sync_aggregate(spec, state, block_header)
     next_sync_committee_branch = [spec.Bytes32() for _ in range(spec.floorlog2(spec.NEXT_SYNC_COMMITTEE_INDEX))]
 
     # Ensure that finality checkpoint is genesis
@@ -57,7 +61,6 @@ def test_process_light_client_update_not_timeout(spec, state):
         finalized_header=finality_header,
         finality_branch=finality_branch,
         sync_aggregate=sync_aggregate,
-        fork_version=fork_version,
         signature_slot=signature_slot,
     )
 
@@ -70,11 +73,16 @@ def test_process_light_client_update_not_timeout(spec, state):
     assert store.finalized_header == pre_store.finalized_header
     assert store.best_valid_update == update
 
+    spec.config = old_config
+
 
 @with_altair_and_later
 @spec_state_test
 @with_presets([MINIMAL], reason="too slow")
 def test_process_light_client_update_at_period_boundary(spec, state):
+    old_config = spec.config
+    override_config_fork_epochs(spec, state)
+
     store = initialize_light_client_store(spec, state)
 
     # Forward to slot before next sync committee period so that next block is final one in period
@@ -94,7 +102,7 @@ def test_process_light_client_update_at_period_boundary(spec, state):
     )
 
     # Sync committee signing the block_header
-    sync_aggregate, fork_version, signature_slot = get_sync_aggregate(spec, state, block_header)
+    sync_aggregate, signature_slot = get_sync_aggregate(spec, state, block_header)
     next_sync_committee_branch = [spec.Bytes32() for _ in range(spec.floorlog2(spec.NEXT_SYNC_COMMITTEE_INDEX))]
 
     # Finality is unchanged
@@ -108,7 +116,6 @@ def test_process_light_client_update_at_period_boundary(spec, state):
         finalized_header=finality_header,
         finality_branch=finality_branch,
         sync_aggregate=sync_aggregate,
-        fork_version=fork_version,
         signature_slot=signature_slot,
     )
 
@@ -121,11 +128,16 @@ def test_process_light_client_update_at_period_boundary(spec, state):
     assert store.best_valid_update == update
     assert store.finalized_header == pre_store.finalized_header
 
+    spec.config = old_config
+
 
 @with_altair_and_later
 @spec_state_test
 @with_presets([MINIMAL], reason="too slow")
 def test_process_light_client_update_timeout(spec, state):
+    old_config = spec.config
+    override_config_fork_epochs(spec, state)
+
     store = initialize_light_client_store(spec, state)
 
     # Forward to next sync committee period
@@ -145,7 +157,7 @@ def test_process_light_client_update_timeout(spec, state):
     )
 
     # Sync committee signing the block_header
-    sync_aggregate, fork_version, signature_slot = get_sync_aggregate(spec, state, block_header)
+    sync_aggregate, signature_slot = get_sync_aggregate(spec, state, block_header)
 
     # Sync committee is updated
     next_sync_committee_branch = build_proof(state.get_backing(), spec.NEXT_SYNC_COMMITTEE_INDEX)
@@ -160,7 +172,6 @@ def test_process_light_client_update_timeout(spec, state):
         finalized_header=finality_header,
         finality_branch=finality_branch,
         sync_aggregate=sync_aggregate,
-        fork_version=fork_version,
         signature_slot=signature_slot,
     )
 
@@ -173,11 +184,16 @@ def test_process_light_client_update_timeout(spec, state):
     assert store.best_valid_update == update
     assert store.finalized_header == pre_store.finalized_header
 
+    spec.config = old_config
+
 
 @with_altair_and_later
 @spec_state_test
 @with_presets([MINIMAL], reason="too slow")
 def test_process_light_client_update_finality_updated(spec, state):
+    old_config = spec.config
+    override_config_fork_epochs(spec, state)
+
     store = initialize_light_client_store(spec, state)
 
     # Change finality
@@ -211,7 +227,7 @@ def test_process_light_client_update_finality_updated(spec, state):
     )
 
     # Sync committee signing the block_header
-    sync_aggregate, fork_version, signature_slot = get_sync_aggregate(spec, state, block_header)
+    sync_aggregate, signature_slot = get_sync_aggregate(spec, state, block_header)
 
     update = spec.LightClientUpdate(
         attested_header=block_header,
@@ -220,7 +236,6 @@ def test_process_light_client_update_finality_updated(spec, state):
         finalized_header=finalized_block_header,
         finality_branch=finality_branch,
         sync_aggregate=sync_aggregate,
-        fork_version=fork_version,
         signature_slot=signature_slot,
     )
 
@@ -230,3 +245,5 @@ def test_process_light_client_update_finality_updated(spec, state):
     assert store.optimistic_header == update.attested_header
     assert store.finalized_header == update.finalized_header
     assert store.best_valid_update is None
+
+    spec.config = old_config

tests/core/pyspec/eth2spec/test/helpers/light_client.py

@@ -1,3 +1,5 @@
+from copy import deepcopy
+
 from eth2spec.test.helpers.state import (
     transition_to,
 )
@@ -7,6 +9,35 @@ from eth2spec.test.helpers.sync_committee import (
 )
 
 
+def override_config_fork_epochs(spec, state):
+    # Test framework adjusts state fork but leaves spec config constants inconsistent
+    config_overrides = {}
+    if state.fork.current_version == spec.config.GENESIS_FORK_VERSION:
+        pass
+    elif state.fork.current_version == spec.config.ALTAIR_FORK_VERSION:
+        config_overrides['ALTAIR_FORK_EPOCH'] = spec.GENESIS_EPOCH
+    elif state.fork.current_version == spec.config.BELLATRIX_FORK_VERSION:
+        config_overrides['ALTAIR_FORK_EPOCH'] = spec.GENESIS_EPOCH
+        config_overrides['BELLATRIX_FORK_EPOCH'] = spec.GENESIS_EPOCH
+    elif state.fork.current_version == spec.config.CAPELLA_FORK_VERSION:
+        config_overrides['ALTAIR_FORK_EPOCH'] = spec.GENESIS_EPOCH
+        config_overrides['BELLATRIX_FORK_EPOCH'] = spec.GENESIS_EPOCH
+        config_overrides['CAPELLA_FORK_EPOCH'] = spec.GENESIS_EPOCH
+    elif state.fork.current_version == spec.config.SHARDING_FORK_VERSION:
+        config_overrides['ALTAIR_FORK_EPOCH'] = spec.GENESIS_EPOCH
+        config_overrides['BELLATRIX_FORK_EPOCH'] = spec.GENESIS_EPOCH
+        config_overrides['CAPELLA_FORK_EPOCH'] = spec.GENESIS_EPOCH
+        config_overrides['SHARDING_FORK_EPOCH'] = spec.GENESIS_EPOCH
+    else:
+        assert False
+
+    tmp_config = deepcopy(spec.config._asdict())
+    tmp_config.update(config_overrides)
+    config_types = spec.Configuration.__annotations__
+    test_config = {k: config_types[k](v) for k, v in tmp_config.items()}
+    spec.config = spec.Configuration(**test_config)
+
+
 def initialize_light_client_store(spec, state):
     return spec.LightClientStore(
         finalized_header=spec.BeaconBlockHeader(),
@@ -45,5 +76,4 @@ def get_sync_aggregate(spec, state, block_header, signature_slot=None):
         sync_committee_bits=sync_committee_bits,
         sync_committee_signature=sync_committee_signature,
     )
-    fork_version = signature_state.fork.current_version
+    return sync_aggregate, signature_slot
-    return sync_aggregate, fork_version, signature_slot