set fee recipient in empty payload fallback (#4291)

When the EL/Builder fails to produce an execution payload, we fall back
to an empty `ExecutionPayload`. Even though it contains no transactions
it should refer to the configured fee recipient. This is useful for
privacy reasons (do not reveal the reason for the empty payload) and for
compliance with additional fee recipient rules by staking pools.

AllTests-mainnet.md

@@ -440,10 +440,11 @@ OK: 12/12 Fail: 0/12 Skip: 0/12
 OK: 1/1 Fail: 0/1 Skip: 0/1
 ## Spec helpers
 ```diff
++ build_empty_execution_payload                                                              OK
 + build_proof - BeaconState                                                                  OK
 + integer_squareroot                                                                         OK
 ```
-OK: 2/2 Fail: 0/2 Skip: 0/2
+OK: 3/3 Fail: 0/3 Skip: 0/3
 ## Specific field types
 ```diff
 + root update                                                                                OK
@@ -604,4 +605,4 @@ OK: 1/1 Fail: 0/1 Skip: 0/1
 OK: 9/9 Fail: 0/9 Skip: 0/9
 
 ---TOTAL---
-OK: 333/338 Fail: 0/338 Skip: 5/338
+OK: 334/339 Fail: 0/339 Skip: 5/339

beacon_chain/spec/helpers.nim

@@ -385,7 +385,8 @@ proc emptyPayloadToBlockHeader*(
   )
 
 func build_empty_execution_payload*(
-    state: bellatrix.BeaconState): bellatrix.ExecutionPayload =
+    state: bellatrix.BeaconState,
+    feeRecipient: Eth1Address): bellatrix.ExecutionPayload =
   ## Assuming a pre-state of the same slot, build a valid ExecutionPayload
   ## without any transactions.
   let
@@ -398,6 +399,7 @@ func build_empty_execution_payload*(
 
   var payload = bellatrix.ExecutionPayload(
     parent_hash: latest.block_hash,
+    fee_recipient: bellatrix.ExecutionAddress(data: distinctBase(feeRecipient)),
     state_root: latest.state_root, # no changes to the state
     receipts_root: EMPTY_ROOT_HASH,
     block_number: latest.block_number + 1,

beacon_chain/validators/validator_duties.nim

@@ -348,12 +348,20 @@ proc getExecutionPayload[T](
     epoch: Epoch, validator_index: ValidatorIndex): Future[Opt[T]] {.async.} =
   # https://github.com/ethereum/consensus-specs/blob/v1.1.10/specs/bellatrix/validator.md#executionpayload
 
+  let feeRecipient = block:
+    let pubkey = node.dag.validatorKey(validator_index)
+    if pubkey.isNone():
+      error "Cannot get proposer pubkey, bug?", validator_index
+      default(Eth1Address)
+    else:
+      node.getFeeRecipient(pubkey.get().toPubKey(), validator_index, epoch)
+
   template empty_execution_payload(): auto =
     withState(proposalState[]):
       when stateFork >= BeaconStateFork.Capella:
         raiseAssert $capellaImplementationMissing
       elif stateFork >= BeaconStateFork.Bellatrix:
-        build_empty_execution_payload(forkyState.data)
+        build_empty_execution_payload(forkyState.data, feeRecipient)
       else:
         default(T)
 
@@ -381,13 +389,6 @@ proc getExecutionPayload[T](
           default(Eth2Digest)
       latestSafe = beaconHead.safeExecutionPayloadHash
       latestFinalized = beaconHead.finalizedExecutionPayloadHash
-      feeRecipient = block:
-        let pubkey = node.dag.validatorKey(validator_index)
-        if pubkey.isNone():
-          error "Cannot get proposer pubkey, bug?", validator_index
-          default(Eth1Address)
-        else:
-          node.getFeeRecipient(pubkey.get().toPubKey(), validator_index, epoch)
       lastFcU = node.consensusManager.forkchoiceUpdatedInfo
       timestamp = withState(proposalState[]):
         compute_timestamp_at_slot(forkyState.data, forkyState.data.slot)

tests/test_helpers.nim

@@ -10,8 +10,10 @@
 import
   # Status libraries
   stew/bitops2,
+  web3/ethtypes,
   # Beacon chain internals
   ../beacon_chain/spec/[forks, helpers, state_transition],
+  ../beacon_chain/spec/datatypes/bellatrix,
   # Test utilities
   ./unittest2, mocking/mock_genesis
 
@@ -58,3 +60,19 @@ suite "Spec helpers":
           process(fieldVar, i shl childDepth)
         i += 1
     process(state, state.numLeaves)
+
+  test "build_empty_execution_payload":
+    var cfg = defaultRuntimeConfig
+    cfg.ALTAIR_FORK_EPOCH = GENESIS_EPOCH
+    cfg.BELLATRIX_FORK_EPOCH = GENESIS_EPOCH
+
+    let state = newClone(initGenesisState(cfg = cfg).bellatrixData)
+
+    template testCase(recipient: Eth1Address): untyped =
+      block:
+        let payload = build_empty_execution_payload(state[].data, recipient)
+        check payload.fee_recipient ==
+          bellatrix.ExecutionAddress(data: distinctBase(recipient))
+
+    testCase default(Eth1Address)
+    testCase Eth1Address.fromHex("0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b")

tests/testblockutil.nim

@@ -145,7 +145,8 @@ proc addTestBlock*(
         if  forkyState.data.slot >
             cfg.BELLATRIX_FORK_EPOCH * SLOTS_PER_EPOCH + 10:
           if is_merge_transition_complete(forkyState.data):
-            build_empty_execution_payload(forkyState.data)
+            const feeRecipient = default(Eth1Address)
+            build_empty_execution_payload(forkyState.data, feeRecipient)
           else:
             build_empty_merge_execution_payload(forkyState.data)
         else: