Bellatrix: pass justified as a safe block

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

@@ -12,6 +12,7 @@
 - [Protocols](#protocols)
   - [`ExecutionEngine`](#executionengine)
     - [`notify_forkchoice_updated`](#notify_forkchoice_updated)
+      - [`safe_block_hash`](#safe_block_hash)
 - [Helpers](#helpers)
   - [`PayloadAttributes`](#payloadattributes)
   - [`PowBlock`](#powblock)
@@ -72,7 +73,22 @@ As per EIP-3675, before a post-transition block is finalized, `notify_forkchoice
 
 *Note*: Client software MUST call this function to initiate the payload build process to produce the merge transition block; the `head_block_hash` parameter MUST be set to the hash of a terminal PoW block in this case.
 
-*Note*: Until safe head function is implemented, `safe_block_hash` parameter MUST be stubbed with the `head_block_hash` value.
+##### `safe_block_hash`
+
+The `safe_block_hash` parameter in a call to `notify_forkchoice_updated` function
+MUST be set to the return value of the following function:
+
+```python
+def get_safe_block_hash(store: Store) -> Hash32:
+    safe_block_root = get_safe_beacon_block(store)
+    safe_block = store.blocks[safe_block_root]
+
+    # Return Hash32() if no payload is yet justified
+    if compute_epoch_at_slot(safe_block.slot) >= BELLATRIX_FORK_EPOCH:
+        return safe_block.body.execution_payload.block_hash
+    else:
+        return Hash32()
+```
 
 ## Helpers
 

specs/bellatrix/validator.md

@@ -110,16 +110,18 @@ All validator responsibilities remain unchanged other than those noted below. Na
 
 To obtain an execution payload, a block proposer building a block on top of a `state` must take the following actions:
 
-1. Set `payload_id = prepare_execution_payload(state, pow_chain, finalized_block_hash, suggested_fee_recipient, execution_engine)`, where:
+1. Set `payload_id = prepare_execution_payload(state, pow_chain, finalized_block_hash, safe_block_hash, suggested_fee_recipient, execution_engine)`, where:
     * `state` is the state object after applying `process_slots(state, slot)` transition to the resulting state of the parent block processing
     * `pow_chain` is a `Dict[Hash32, PowBlock]` dictionary that abstractly represents all blocks in the PoW chain with block hash as the dictionary key
+    * `safe_block_hash` is the return value of the `get_safe_block_hash(store: Store)` function call
     * `finalized_block_hash` is the hash of the latest finalized execution payload (`Hash32()` if none yet finalized)
     * `suggested_fee_recipient` is the value suggested to be used for the `fee_recipient` field of the execution payload
 
 
 ```python
 def prepare_execution_payload(state: BeaconState,
                               pow_chain: Dict[Hash32, PowBlock],
+                              safe_block_hash: Hash32,
                               finalized_block_hash: Hash32,
                               suggested_fee_recipient: ExecutionAddress,
                               execution_engine: ExecutionEngine) -> Optional[PayloadId]:
@@ -146,11 +148,9 @@ def prepare_execution_payload(state: BeaconState,
         prev_randao=get_randao_mix(state, get_current_epoch(state)),
         suggested_fee_recipient=suggested_fee_recipient,
     )
-    # Set safe and head block hashes to the same value
     return execution_engine.notify_forkchoice_updated(
         head_block_hash=parent_hash,
-        # TODO: Use `parent_hash` as a stub for now.
-        safe_block_hash=parent_hash,
+        safe_block_hash=safe_block_hash,
         finalized_block_hash=finalized_block_hash,
         payload_attributes=payload_attributes,
     )

tests/core/pyspec/eth2spec/test/bellatrix/unittests/validator/test_validator.py

@@ -143,6 +143,7 @@ def test_prepare_execution_payload(spec, state):
 
         # Dummy arguments
         finalized_block_hash = b'\x56' * 32
+        safe_block_hash = b'\x58' * 32
         suggested_fee_recipient = b'\x78' * 20
 
         # Mock execution_engine
@@ -158,6 +159,7 @@ def notify_forkchoice_updated(self,
             state=state,
             pow_chain=pow_chain.to_dict(),
             finalized_block_hash=finalized_block_hash,
+            safe_block_hash=safe_block_hash,
             suggested_fee_recipient=suggested_fee_recipient,
             execution_engine=TestEngine(),
         )