refactor: introduce safe-block to fork-choice (#8618)

**Motivation**

There is a concept documented in the specs called
[safe-block](https://github.com/ethereum/consensus-specs/blob/master/fork_choice/safe-block.md).
Wanted to introduce that concept in our codebase so upcoming feature of
`fcr` have less invasive changes.

**Description**

- Expose functions `getSafeBeaconBlockRoot` and
`getSafeExecutionBlockHash` from `fork-choice` package.
- Update the usage of `safeBlock` to use those functions

---------

Co-authored-by: Nico Flaig <nflaig@protonmail.com>

Author: nazarhussain

packages/beacon-node/src/chain/blocks/importBlock.ts

@@ -6,6 +6,7 @@ import {
   ForkChoiceError,
   ForkChoiceErrorCode,
   NotReorgedReason,
+  getSafeExecutionBlockHash,
 } from "@lodestar/fork-choice";
 import {ForkPostAltair, ForkPostElectra, ForkSeq, MAX_SEED_LOOKAHEAD, SLOTS_PER_EPOCH} from "@lodestar/params";
 import {
@@ -394,7 +395,7 @@ export async function importBlock(
      * the current finalized block does not contain any execution payload at all (pre MERGE_EPOCH) or if it contains a
      * zero block hash (pre TTD)
      */
-    const safeBlockHash = this.forkChoice.getJustifiedBlock().executionPayloadBlockHash ?? ZERO_HASH_HEX;
+    const safeBlockHash = getSafeExecutionBlockHash(this.forkChoice);
     const finalizedBlockHash = this.forkChoice.getFinalizedBlock().executionPayloadBlockHash ?? ZERO_HASH_HEX;
     if (headBlockHash !== ZERO_HASH_HEX) {
       this.executionEngine

packages/beacon-node/src/chain/prepareNextSlot.ts

@@ -1,5 +1,6 @@
 import {routes} from "@lodestar/api";
 import {ChainForkConfig} from "@lodestar/config";
+import {getSafeExecutionBlockHash} from "@lodestar/fork-choice";
 import {ForkPostBellatrix, ForkSeq, SLOTS_PER_EPOCH, isForkPostElectra} from "@lodestar/params";
 import {
   BeaconStateElectra,
@@ -166,7 +167,7 @@ export class PrepareNextSlotScheduler {
             computeTimeAtSlot(this.config, prepareSlot, this.chain.genesisTime) - Date.now() / 1000;
           this.metrics?.blockPayload.payloadAdvancePrepTime.observe(preparationTime);
 
-          const safeBlockHash = this.chain.forkChoice.getJustifiedBlock().executionPayloadBlockHash ?? ZERO_HASH_HEX;
+          const safeBlockHash = getSafeExecutionBlockHash(this.chain.forkChoice);
           const finalizedBlockHash =
             this.chain.forkChoice.getFinalizedBlock().executionPayloadBlockHash ?? ZERO_HASH_HEX;
           // awaiting here instead of throwing an async call because there is no other task

packages/beacon-node/src/chain/produceBlock/produceBlockBody.ts

@@ -1,4 +1,5 @@
 import {ChainForkConfig} from "@lodestar/config";
+import {getSafeExecutionBlockHash} from "@lodestar/fork-choice";
 import {
   ForkName,
   ForkPostBellatrix,
@@ -191,7 +192,7 @@ export async function produceBlockBody<T extends BlockType>(
 
     // We don't deal with blinded blocks, execution engine, blobs and execution requests post-gloas
   } else if (isForkPostBellatrix(fork)) {
-    const safeBlockHash = this.forkChoice.getJustifiedBlock().executionPayloadBlockHash ?? ZERO_HASH_HEX;
+    const safeBlockHash = getSafeExecutionBlockHash(this.forkChoice);
     const finalizedBlockHash = this.forkChoice.getFinalizedBlock().executionPayloadBlockHash ?? ZERO_HASH_HEX;
     const feeRecipient = requestedFeeRecipient ?? this.beaconProposerCache.getOrDefault(proposerIndex);
     const feeRecipientType = requestedFeeRecipient

packages/fork-choice/src/forkChoice/safeBlocks.ts

@@ -0,0 +1,27 @@
+import {ZERO_HASH_HEX} from "@lodestar/params";
+import {Root, RootHex} from "@lodestar/types";
+import {IForkChoice} from "./interface.js";
+
+/**
+ * Under honest majority and certain network synchronicity assumptions there exists 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.
+ *
+ * https://github.com/ethereum/consensus-specs/blob/v1.6.0/fork_choice/safe-block.md#get_safe_beacon_block_root
+ */
+export function getSafeBeaconBlockRoot(fc: IForkChoice): Root {
+  return fc.getJustifiedCheckpoint().root;
+}
+
+/**
+ * Get execution payload hash for the safe block
+ * This function assumes that safe block is post Bellatrix and function should not be called otherwise.
+ *
+ * As our existing usage is aligned with above condition so not adding fork-check inside this function
+ *
+ *
+ * https://github.com/ethereum/consensus-specs/blob/v1.6.0/fork_choice/safe-block.md#get_safe_execution_block_hash
+ */
+export function getSafeExecutionBlockHash(forkChoice: IForkChoice): RootHex {
+  return forkChoice.getJustifiedBlock().executionPayloadBlockHash ?? ZERO_HASH_HEX;
+}

packages/fork-choice/src/index.ts

@@ -15,6 +15,7 @@ export {
   NotReorgedReason,
   type PowBlockHex,
 } from "./forkChoice/interface.js";
+export * from "./forkChoice/safeBlocks.js";
 export {
   type CheckpointWithHex,
   ForkChoiceStore,