Merge pull request #2844 from sigp/opt-import

Optimistically import any post-merge block

sync/optimistic.md

@@ -81,10 +81,17 @@ def is_execution_block(block: BeaconBlock) -> bool:
 
 ```python
 def is_optimistic_candidate_block(opt_store: OptimisticStore, current_slot: Slot, block: BeaconBlock) -> bool:
+    if is_execution_block(opt_store.blocks[block.parent_root]):
+        return True
+
     justified_root = opt_store.block_states[opt_store.head_block_root].current_justified_checkpoint.root
-    justified_is_execution_block = is_execution_block(opt_store.blocks[justified_root])
-    block_is_deep = block.slot + SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY <= current_slot
-    return justified_is_execution_block or block_is_deep
+    if is_execution_block(opt_store.blocks[justified_root]):
+        return True
+
+    if block.slot + SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY <= current_slot:
+        return True
+
+    return False
 ```
 
 Let only a node which returns `is_optimistic(opt_store, head) is True` be an *optimistic
@@ -99,14 +106,21 @@ behaviours without regard for optimistic sync.
 ### When to optimistically import blocks
 
 A block MAY be optimistically imported when
-`is_optimistic_candidate_block(opt_store, current_slot, block)` returns
-`True`. This ensures that blocks are only optimistically imported if either:
+`is_optimistic_candidate_block(opt_store, current_slot, block)` returns `True`.
+This ensures that blocks are only optimistically imported if one or more of the
+following are true:
 
+1. The parent of the block has execution enabled.
 1. The justified checkpoint has execution enabled.
 1. The current slot (as per the system clock) is at least
    `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY` ahead of the slot of the block being
    imported.
 
+In effect, there are restrictions on when a *merge block* can be optimistically
+imported. The merge block is the first block in any chain where
+`is_execution_block(block) == True`. Any descendant of a merge block may be
+imported optimistically at any time.
+
 *See [Fork Choice Poisoning](#fork-choice-poisoning) for the motivations behind
 these conditions.*