Add limit churn feature

specs/_features/limit_churn/beacon_chain.md

@@ -0,0 +1,52 @@
+Limit churn -- The Beacon Chain
+
+## Table of contents
+
+<!-- TOC -->
+<!-- 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)
+ - [Validator cycle](#validator-cycle)
+- [Helper functions](#helper-functions)
+ - [Beacon state accessors](#beacon-state-accessors)
+ - [modified `get_validator_churn_limit`](#modified-get_validator_churn_limit)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## Introduction
+
+This is the beacon chain specification to limit the max churn value, motivated to limit the validator active set growth rate.
+
+*Note:* This specification is built upon [Capella](../../capella/beacon_chain.md) and is under active development.
+
+## Configuration
+
+### Validator cycle
+
+| Name | Value |
+| - | - |
+| `MAX_PER_EPOCH_CHURN_LIMIT` | `uint64(12)` (= 12) |
+
+## Helper functions
+
+### Beacon state accessors
+
+#### modified `get_validator_churn_limit`
+
+```python
+def get_validator_churn_limit(state: BeaconState) -> uint64:
+ """
+ Return the validator churn limit for the current epoch.
+ """
+ active_validator_indices = get_active_validator_indices(state, get_current_epoch(state))
+ return min(
+ MAX_PER_EPOCH_CHURN_LIMIT,
+ max(
+ MIN_PER_EPOCH_CHURN_LIMIT,
+ uint64(len(active_validator_indices)) // CHURN_LIMIT_QUOTIENT
+ )
+ )
+```