docs: add fork logic spec

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) |
+| `BELLATRIX_FORK_VERSION` | `Version('0x02000000')` |
+| `BELLATRIX_FORK_EPOCH` | `Epoch(144896)` (Sept 6, 2022, 11:34:47am UTC) |
+| `CAPELLA_FORK_VERSION` | `Version('0x03000000')` |
+| `CAPELLA_FORK_EPOCH` | `Epoch(194048)` (April 12, 2023, 10:27:35pm 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 >= 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
+```
+
+## 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
+```
+
+## Fork to Bellatrix
+
+### Fork trigger
+
+TBD. Social consensus, along with state conditions such as epoch boundary, finality, deposits, active validator count, etc. may be part of the decision process to trigger the fork. For now we assume the condition will be triggered at epoch `BELLATRIX_FORK_EPOCH`.
+
+Note that for the pure Bellatrix networks, we don't apply `upgrade_to_bellatrix` since it starts with Bellatrix version logic.
+
+### Upgrading the state
+
+As with the Phase0-to-Altair upgrade, the `state_transition` is modified to upgrade the `BeaconState`.
+The `BeaconState` upgrade runs as part of `process_slots`, slots with missing block proposals do not affect the upgrade time.
+
+If `state.slot % SLOTS_PER_EPOCH == 0` and `compute_epoch_at_slot(state.slot) == BELLATRIX_FORK_EPOCH`, an irregular state change is made to upgrade to Bellatrix.
+The upgrade occurs after the completion of the inner loop of `process_slots` that sets `state.slot` equal to `BELLATRIX_FORK_EPOCH * SLOTS_PER_EPOCH`.
+
+When multiple upgrades are scheduled for the same epoch (common for test-networks),
+all the upgrades run in sequence before resuming the regular state transition.
+
+```python
+def upgrade_to_bellatrix(pre: altair.BeaconState) -> BeaconState:
+    epoch = altair.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=BELLATRIX_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=pre.previous_epoch_participation,
+        current_epoch_participation=pre.current_epoch_participation,
+        # 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=pre.inactivity_scores,
+        # Sync
+        current_sync_committee=pre.current_sync_committee,
+        next_sync_committee=pre.next_sync_committee,
+        # Execution-layer
+        latest_execution_payload_header=ExecutionPayloadHeader(),
+    )
+
+    return post
+```
+
+## Fork to Capella
+
+### Fork trigger
+
+The fork is triggered at epoch `CAPELLA_FORK_EPOCH`.
+
+Note that for the pure Capella networks, we don't apply `upgrade_to_capella` since it starts with Capella version logic.
+
+### Upgrading the state
+
+If `state.slot % SLOTS_PER_EPOCH == 0` and `compute_epoch_at_slot(state.slot) == CAPELLA_FORK_EPOCH`,
+an irregular state change is made to upgrade to Capella.
+
+The upgrade occurs after the completion of the inner loop of `process_slots` that sets `state.slot` equal to `CAPELLA_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 upgrade_to_capella(pre: bellatrix.BeaconState) -> BeaconState:
+    epoch = bellatrix.get_current_epoch(pre)
+    latest_execution_payload_header = ExecutionPayloadHeader(
+        parent_hash=pre.latest_execution_payload_header.parent_hash,
+        fee_recipient=pre.latest_execution_payload_header.fee_recipient,
+        state_root=pre.latest_execution_payload_header.state_root,
+        receipts_root=pre.latest_execution_payload_header.receipts_root,
+        logs_bloom=pre.latest_execution_payload_header.logs_bloom,
+        prev_randao=pre.latest_execution_payload_header.prev_randao,
+        block_number=pre.latest_execution_payload_header.block_number,
+        gas_limit=pre.latest_execution_payload_header.gas_limit,
+        gas_used=pre.latest_execution_payload_header.gas_used,
+        timestamp=pre.latest_execution_payload_header.timestamp,
+        extra_data=pre.latest_execution_payload_header.extra_data,
+        base_fee_per_gas=pre.latest_execution_payload_header.base_fee_per_gas,
+        block_hash=pre.latest_execution_payload_header.block_hash,
+        transactions_root=pre.latest_execution_payload_header.transactions_root,
+        withdrawals_root=Root(),  # [New in Capella]
+    )
+    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=CAPELLA_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=pre.previous_epoch_participation,
+        current_epoch_participation=pre.current_epoch_participation,
+        # 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=pre.inactivity_scores,
+        # Sync
+        current_sync_committee=pre.current_sync_committee,
+        next_sync_committee=pre.next_sync_committee,
+        # Execution-layer
+        latest_execution_payload_header=latest_execution_payload_header,
+        # Withdrawals
+        next_withdrawal_index=WithdrawalIndex(0),  # [New in Capella]
+        next_withdrawal_validator_index=ValidatorIndex(0),  # [New in Capella]
+        # Deep history valid from Capella onwards
+        historical_summaries=List[HistoricalSummary, HISTORICAL_ROOTS_LIMIT]([]),  # [New in Capella]
+    )
+
+    return post
+```