Add separate get_safe_beacon_block function

fork_choice/safe-block.md

@@ -0,0 +1,31 @@
+# Fork Choice -- Safe Block
+
+## 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)
+- [`get_safe_beacon_block`](#get_safe_beacon_block)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## Introduction
+
+Under honest majority and certain network synchronicity assumptions
+there exist a block that is safe from re-orgs. Normally this block is
+pretty close to the head of canonical chain which makes it valuable
+to expose a safe block to users.
+
+This section describes an algorithm to find a safe block.
+
+## `get_safe_beacon_block`
+
+```python
+def get_safe_beacon_block(store: Store) -> Root:
+    # Use most recent justified block as a stopgap
+    return store.justified_checkpoint.root
+```
+*Note*: Currently safe block algorithm simply returns `store.justified_checkpoint.root`
+and is meant to be improved in the future.

specs/bellatrix/fork-choice.md

@@ -80,8 +80,7 @@ MUST be set to the return value of the following function:
 
 ```python
 def get_safe_block_hash(store: Store) -> Hash32:
-    # Use most recent justified block as a stopgap
-    safe_block_root = store.justified_checkpoint.root
+    safe_block_root = get_safe_beacon_block(store)
     safe_block = store.blocks[safe_block_root]
 
     # Return Hash32() if no payload is yet justified