Don't create a execution payload with same timestamp as terminal block (#3331)

## Issue Addressed

Resolves #3316 

## Proposed Changes

This PR fixes an issue where lighthouse created a transition block with `block.execution_payload().timestamp == terminal_block.timestamp` if the terminal block was created at the slot boundary.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -3908,14 +3908,13 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                 ForkName::Base | ForkName::Altair => return Ok(()),
                 _ => {
                     // We are post-bellatrix
-                    if execution_layer
+                    if let Some(payload_attributes) = execution_layer
                         .payload_attributes(next_slot, params.head_root)
                         .await
-                        .is_some()
                     {
                         // We are a proposer, check for terminal_pow_block_hash
                         if let Some(terminal_pow_block_hash) = execution_layer
-                            .get_terminal_pow_block_hash(&self.spec)
+                            .get_terminal_pow_block_hash(&self.spec, payload_attributes.timestamp)
                             .await
                             .map_err(Error::ForkchoiceUpdate)?
                         {

beacon_node/beacon_chain/src/execution_payload.rs

@@ -393,7 +393,7 @@ where
         }
 
         let terminal_pow_block_hash = execution_layer
-            .get_terminal_pow_block_hash(spec)
+            .get_terminal_pow_block_hash(spec, timestamp)
             .await
             .map_err(BlockProductionError::TerminalPoWBlockLookupFailed)?;
 

beacon_node/beacon_chain/src/test_utils.rs

@@ -30,6 +30,7 @@ use rayon::prelude::*;
 use sensitive_url::SensitiveUrl;
 use slog::Logger;
 use slot_clock::TestingSlotClock;
+use state_processing::per_block_processing::compute_timestamp_at_slot;
 use state_processing::{
     state_advance::{complete_state_advance, partial_state_advance},
     StateRootStrategy,
@@ -521,6 +522,11 @@ where
         self.chain.head_beacon_state_cloned()
     }
 
+    pub fn get_timestamp_at_slot(&self) -> u64 {
+        let state = self.get_current_state();
+        compute_timestamp_at_slot(&state, &self.spec).unwrap()
+    }
+
     pub fn get_current_state_and_root(&self) -> (BeaconState<E>, Hash256) {
         let head = self.chain.head_snapshot();
         let state_root = head.beacon_state_root();

beacon_node/beacon_chain/tests/merge.rs

@@ -1,7 +1,7 @@
 #![cfg(not(debug_assertions))] // Tests run too slow in debug.
 
 use beacon_chain::test_utils::BeaconChainHarness;
-use execution_layer::test_utils::{generate_pow_block, DEFAULT_TERMINAL_BLOCK};
+use execution_layer::test_utils::{generate_pow_block, Block, DEFAULT_TERMINAL_BLOCK};
 use types::*;
 
 const VALIDATOR_COUNT: usize = 32;
@@ -22,6 +22,7 @@ fn verify_execution_payload_chain<T: EthSpec>(chain: &[FullPayload<T>]) {
                 prev_ep.execution_payload.block_number + 1,
                 ep.execution_payload.block_number
             );
+            assert!(ep.execution_payload.timestamp > prev_ep.execution_payload.timestamp);
         }
         prev_ep = Some(ep.clone());
     }
@@ -169,6 +170,30 @@ async fn base_altair_merge_with_terminal_block_after_fork() {
         .move_to_terminal_block()
         .unwrap();
 
+    // Add a slot duration to get to the next slot
+    let timestamp = harness.get_timestamp_at_slot() + harness.spec.seconds_per_slot;
+
+    harness
+        .execution_block_generator()
+        .modify_last_block(|block| {
+            if let Block::PoW(terminal_block) = block {
+                terminal_block.timestamp = timestamp;
+            }
+        });
+
+    harness.extend_slots(1).await;
+
+    let one_after_merge_head = &harness.chain.head_snapshot().beacon_block;
+    assert_eq!(
+        *one_after_merge_head
+            .message()
+            .body()
+            .execution_payload()
+            .unwrap(),
+        FullPayload::default()
+    );
+    assert_eq!(one_after_merge_head.slot(), merge_fork_slot + 2);
+
     /*
      * Next merge block should include an exec payload.
      */