Altair

Author: MegaRedHand

spec/fork.md

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+# Fork Logic
+
+## Table of contents
+
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
+- [Introduction](#introduction)
+- [Configuration](#configuration)
+- [Helper functions](#helper-functions)
+  - [Misc](#misc)
+    - [`compute_fork_version`](#compute_fork_version)
+- [Fork to Altair](#fork-to-altair)
+  - [Fork trigger](#fork-trigger)
+  - [Upgrading the state](#upgrading-the-state)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+## Introduction
+
+This document describes the process of the upgrade of the beacon chain.
+
+## Configuration
+
+Warning: this configuration is not definitive.
+
+| Name | Value |
+| - | - |
+| `ALTAIR_FORK_VERSION` | `Version('0x01000000')` |
+| `ALTAIR_FORK_EPOCH` | `Epoch(74240)` (Oct 27, 2021, 10:56:23am UTC) |
+
+## Helper functions
+
+### 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
+```
+
+## Fork to Altair
+
+### Fork trigger
+
+The fork is triggered at epoch `ALTAIR_FORK_EPOCH`.
+
+Note that for the pure Altair networks, we don't apply `upgrade_to_altair` since it starts with Altair version logic.
+
+### Upgrading the state
+
+If `state.slot % SLOTS_PER_EPOCH == 0` and `compute_epoch_at_slot(state.slot) == ALTAIR_FORK_EPOCH`, an irregular state change is made to upgrade to Altair.
+
+The upgrade occurs after the completion of the inner loop of `process_slots` that sets `state.slot` equal to `ALTAIR_FORK_EPOCH * SLOTS_PER_EPOCH`.
+Care must be taken when transitioning through the fork boundary as implementations will need a modified [state transition function](../phase0/beacon-chain.md#beacon-chain-state-transition-function) that deviates from the Phase 0 document.
+In particular, the outer `state_transition` function defined in the Phase 0 document will not expose the precise fork slot to execute the upgrade in the presence of skipped slots at the fork boundary. Instead the logic must be within `process_slots`.
+
+```python
+def translate_participation(state: BeaconState, pending_attestations: Sequence[phase0.PendingAttestation]) -> None:
+    for attestation in pending_attestations:
+        data = attestation.data
+        inclusion_delay = attestation.inclusion_delay
+        # Translate attestation inclusion info to flag indices
+        participation_flag_indices = get_attestation_participation_flag_indices(state, data, inclusion_delay)
+
+        # Apply flags to all attesting validators
+        epoch_participation = state.previous_epoch_participation
+        for index in get_attesting_indices(state, data, attestation.aggregation_bits):
+            for flag_index in participation_flag_indices:
+                epoch_participation[index] = add_flag(epoch_participation[index], flag_index)
+
+
+def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
+    epoch = phase0.get_current_epoch(pre)
+    post = BeaconState(
+        # Versioning
+        genesis_time=pre.genesis_time,
+        genesis_validators_root=pre.genesis_validators_root,
+        slot=pre.slot,
+        fork=Fork(
+            previous_version=pre.fork.current_version,
+            current_version=ALTAIR_FORK_VERSION,
+            epoch=epoch,
+        ),
+        # History
+        latest_block_header=pre.latest_block_header,
+        block_roots=pre.block_roots,
+        state_roots=pre.state_roots,
+        historical_roots=pre.historical_roots,
+        # Eth1
+        eth1_data=pre.eth1_data,
+        eth1_data_votes=pre.eth1_data_votes,
+        eth1_deposit_index=pre.eth1_deposit_index,
+        # Registry
+        validators=pre.validators,
+        balances=pre.balances,
+        # Randomness
+        randao_mixes=pre.randao_mixes,
+        # Slashings
+        slashings=pre.slashings,
+        # Participation
+        previous_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
+        current_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
+        # Finality
+        justification_bits=pre.justification_bits,
+        previous_justified_checkpoint=pre.previous_justified_checkpoint,
+        current_justified_checkpoint=pre.current_justified_checkpoint,
+        finalized_checkpoint=pre.finalized_checkpoint,
+        # Inactivity
+        inactivity_scores=[uint64(0) for _ in range(len(pre.validators))],
+    )
+    # Fill in previous epoch participation from the pre state's pending attestations
+    translate_participation(post, pre.previous_epoch_attestations)
+
+    # 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)
+    post.next_sync_committee = get_next_sync_committee(post)
+    return post
+```