add `historical_state_roots`

* enables proving the state for empty slots
* allows verifying the backfilled fields against the previous
`historical_roots` field
* allows trivially computing `historical_roots`, for any existing use
cases

presets/mainnet/altair.yaml

@@ -1,10 +1,5 @@
 # Mainnet preset - Altair
 
-# State vector lengths
-# ---------------------------------------------------------------
-# 2**24 (= 16,777,216) historical roots, ~26,131 years
-HISTORICAL_BLOCK_ROOTS_LIMIT: 16777216
-
 # Updated penalty values
 # ---------------------------------------------------------------
 # 3 * 2**24 (= 50,331,648)

presets/minimal/altair.yaml

@@ -1,10 +1,5 @@
 # Minimal preset - Altair
 
-# State vector lengths
-# ---------------------------------------------------------------
-# 2**24 (= 16,777,216) historical roots, ~26,131 years
-HISTORICAL_BLOCK_ROOTS_LIMIT: 16777216
-
 # Updated penalty values
 # ---------------------------------------------------------------
 # 3 * 2**24 (= 50,331,648)

specs/altair/beacon-chain.md

@@ -16,7 +16,6 @@
 - [Preset](#preset)
   - [Updated penalty values](#updated-penalty-values)
   - [Sync committee](#sync-committee)
-  - [State list lengths](#state-list-lengths)
 - [Configuration](#configuration)
   - [Inactivity penalties](#inactivity-penalties)
 - [Containers](#containers)
@@ -133,12 +132,6 @@ This patch updates a few configuration values to move penalty parameters closer
 | `SYNC_COMMITTEE_SIZE` | `uint64(2**9)` (= 512) | Validators | |
 | `EPOCHS_PER_SYNC_COMMITTEE_PERIOD` | `uint64(2**9)` (= 512) | epochs | ~54 hours |
 
-### State list lengths
-
-| Name | Value | Unit | Duration |
-| - | - | :-: | :-: |
-| `HISTORICAL_BLOCK_ROOTS_LIMIT` | `uint64(2**24)` (= 16,777,216) | historical roots | ~52,262 years |
-
 ## Configuration
 
 ### Inactivity penalties
@@ -182,8 +175,8 @@ class BeaconState(Container):
     latest_block_header: BeaconBlockHeader
     block_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
     state_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
-    historical_root: Root
-    historical_block_roots: List[Root, HISTORICAL_BLOCK_ROOTS_LIMIT]
+    historical_block_roots: List[Root, HISTORICAL_ROOTS_LIMIT]
+    historical_state_roots: List[Root, HISTORICAL_ROOTS_LIMIT]
     # Eth1
     eth1_data: Eth1Data
     eth1_data_votes: List[Eth1Data, EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH]
@@ -607,7 +600,7 @@ def process_epoch(state: BeaconState) -> None:
     process_effective_balance_updates(state)
     process_slashings_reset(state)
     process_randao_mixes_reset(state)
-    process_historical_block_roots_update(state)
+    process_historical_roots_update(state)
     process_participation_flag_updates(state)  # [New in Altair]
     process_sync_committee_updates(state)  # [New in Altair]
 ```
@@ -698,14 +691,15 @@ def process_participation_flag_updates(state: BeaconState) -> None:
 
 #### Historical roots updates
 
-*Note*: The function `process_historical_block_roots_update` is new and replaces the `process_historical_roots_update` function from phase0.
+*Note*: The function `process_historical_roots_update` changes definition compared to phase0.
 
 ```python
-def process_historical_block_roots_update(state: BeaconState) -> None:
+def process_historical_roots_update(state: BeaconState) -> None:
     # Set historical block root accumulator
     next_epoch = Epoch(get_current_epoch(state) + 1)
     if next_epoch % (SLOTS_PER_HISTORICAL_ROOT // SLOTS_PER_EPOCH) == 0:
         state.historical_block_roots.append(hash_tree_root(state.block_roots))
+        state.historical_state_roots.append(hash_tree_root(state.state_roots))
 ```
 
 #### Sync committee updates

specs/altair/fork.md

@@ -58,13 +58,17 @@ def translate_participation(state: BeaconState, pending_attestations: Sequence[p
             for flag_index in participation_flag_indices:
                 epoch_participation[index] = add_flag(epoch_participation[index], flag_index)
 
-def backfill_historical_block_roots(pre: BeaconState) -> List[Root, HISTORICAL_BLOCK_ROOTS_LIMIT]:
-    # Split historical blocks into chunks of SLOTS_PER_HISTORICAL_ROOT
-    # For each chunk, compute hash_tree_root(chunk) and add to returned list
-    # For new altair-based chains, use an empty list
-    # Backfilling requires access to external block storage.
+def backfill_historical_roots(state: BeaconState, pre: BeaconState) -> None:
+    # Clients need to recreate `historical_block_roots` and
+    # `historical_state_roots` from their history. A large part can be
+    # precalculated and verified against historical_roots in state.
+    # Before the fork, clients should start collecting the part that has not
+    # been pre-calculated.
+    #
+    # The calclulated split historical roots can be verified against the existing
+    # historical_roots field up to the fork.
+
 
-    return []
 
 def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
     epoch = phase0.get_current_epoch(pre)
@@ -82,8 +86,8 @@ def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
         latest_block_header=pre.latest_block_header,
         block_roots=pre.block_roots,
         state_roots=pre.state_roots,
-        historical_root=hash_tree_root(pre.historical_roots),
-        historical_block_roots=backfill_historical_block_roots(pre),
+        historical_block_roots=[],
+        historical_state_roots=[],
         # Eth1
         eth1_data=pre.eth1_data,
         eth1_data_votes=pre.eth1_data_votes,
@@ -109,6 +113,8 @@ def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
     # Fill in previous epoch participation from the pre state's pending attestations
     translate_participation(post, pre.previous_epoch_attestations)
 
+    backfill_historical_roots(post, pre)
+
     # Fill in sync committees
     # Note: A duplicate committee is assigned for the current and next committee at the fork boundary
     post.current_sync_committee = get_next_sync_committee(post)