Whisk: don't mutate candidates during cooldown

specs/_features/whisk/beacon-chain.md

@@ -318,30 +318,25 @@ def get_shuffle_indices(randao_reveal: BLSSignature) -> Sequence[uint64]:
 
 ```python
 def process_shuffled_trackers(state: BeaconState, body: BeaconBlockBody) -> None:
-    # Check the shuffle proof
-    shuffle_indices = get_shuffle_indices(body.randao_reveal)
-    pre_shuffle_trackers = [state.whisk_candidate_trackers[i] for i in shuffle_indices]
-
     shuffle_epoch = get_current_epoch(state) % WHISK_EPOCHS_PER_SHUFFLING_PHASE
     if shuffle_epoch + WHISK_PROPOSER_SELECTION_GAP + 1 >= WHISK_EPOCHS_PER_SHUFFLING_PHASE:
         # Require trackers set to zero during cooldown
         assert body.whisk_post_shuffle_trackers == Vector[WhiskTracker, WHISK_VALIDATORS_PER_SHUFFLE]()
         assert body.whisk_shuffle_proof_M_commitment == BLSG1Point()
         assert body.whisk_shuffle_proof == WhiskShuffleProof()
-        post_shuffle_trackers = pre_shuffle_trackers
     else:
         # Require shuffled trackers during shuffle
+        shuffle_indices = get_shuffle_indices(body.randao_reveal)
+        pre_shuffle_trackers = [state.whisk_candidate_trackers[i] for i in shuffle_indices]
         assert IsValidWhiskShuffleProof(
             pre_shuffle_trackers,
             body.whisk_post_shuffle_trackers,
             body.whisk_shuffle_proof_M_commitment,
             body.whisk_shuffle_proof,
         )
-        post_shuffle_trackers = body.whisk_post_shuffle_trackers
-
-    # Shuffle candidate trackers
-    for i, shuffle_index in enumerate(shuffle_indices):
-        state.whisk_candidate_trackers[shuffle_index] = post_shuffle_trackers[i]
+        # Shuffle candidate trackers
+        for i, shuffle_index in enumerate(shuffle_indices):
+            state.whisk_candidate_trackers[shuffle_index] = body.whisk_post_shuffle_trackers[i]
 ```
 
 ```python