Specify proposing_on_time condition

Author: michaelsproul

specs/bellatrix/fork-choice.md

@@ -129,7 +129,7 @@ def should_override_forkchoice_update(
     time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT
     current_time_ok = (head_block.slot == current_slot or
                        (proposal_slot == current_slot and
-                        time_into_slot < SECONDS_PER_SLOT // INTERVALS_PER_SLOT))
+                        time_into_slot <= SECONDS_PER_SLOT // INTERVALS_PER_SLOT // 2))
     single_slot_reorg = parent_slot_ok and current_time_ok
 
     # Shuffling stable.

specs/phase0/fork-choice.md

@@ -308,6 +308,10 @@ def get_proposer_head(store: Store, head_root: Root, slot: Slot) -> Root:
     # Only re-org a single slot at most.
     single_slot_reorg = parent_block.slot + 1 == head_block.slot and head_block.slot + 1 == slot
 
+    # Only re-org if we are proposing on-time.
+    time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT
+    proposing_on_time = time_into_slot <= SECONDS_PER_SLOT // INTERVALS_PER_SLOT // 2
+
     # Do not re-org on an epoch boundary where the proposer shuffling could change.
     shuffling_stable = slot % SLOTS_PER_EPOCH != 0
 
@@ -326,8 +330,8 @@ def get_proposer_head(store: Store, head_root: Root, slot: Slot) -> Root:
     reorg_threshold = calculate_committee_fraction(justified_state, REORG_WEIGHT_THRESHOLD)
     head_weak = head_weight < reorg_threshold
 
-    if all([head_late, finalization_ok, single_slot_reorg, shuffling_stable, ffg_competitive,
-            head_weak]):
+    if all([head_late, finalization_ok, single_slot_reorg, proposing_on_time, shuffling_stable,
+            ffg_competitive, head_weak]):
         # We can re-org the current head by building upon its parent block.
         return parent_root
     else: