Add first version of safe check

consensus/fork_choice/src/fork_choice.rs

@@ -910,6 +910,45 @@ where
             .is_descendant(self.fc_store.finalized_checkpoint().root, block_root)
     }
 
+    pub fn safe_to_import_optimistically(
+        &self,
+        current_slot: Slot,
+        block_slot: Slot,
+        block_parent_root: &Hash256,
+        spec: &ChainSpec,
+    ) -> Result<bool, Error<T::Error>> {
+        // If the block is sufficiently old, import it.
+        if block_slot + spec.safe_slots_to_import_optimistically > current_slot {
+            return Ok(true);
+        }
+
+        // If the block has an ancestor with a verified parent, import this block.
+        //
+        // TODO(paul): this is not in the spec, add it.
+        //
+        // ## Note
+        //
+        // If `block_parent_root` is unknown this iter will always return `None`.
+        if self
+            .proto_array
+            .iter_nodes(block_parent_root)
+            .any(|node| node.execution_status.is_valid())
+        {
+            return Ok(true);
+        }
+
+        // If the justified block has execution enabled, then optimistically import any block.
+        if !self
+            .get_justified_block()?
+            .execution_status
+            .is_execution_enabled()
+        {
+            return Ok(true);
+        }
+
+        Ok(false)
+    }
+
     /// Return the current finalized checkpoint.
     pub fn finalized_checkpoint(&self) -> Checkpoint {
         *self.fc_store.finalized_checkpoint()

consensus/proto_array/src/proto_array_fork_choice.rs

@@ -1,5 +1,5 @@
 use crate::error::Error;
-use crate::proto_array::{ProposerBoost, ProtoArray};
+use crate::proto_array::{Iter, ProposerBoost, ProtoArray};
 use crate::ssz_container::SszContainer;
 use serde_derive::{Deserialize, Serialize};
 use ssz::{Decode, Encode};
@@ -37,6 +37,14 @@ pub enum ExecutionStatus {
 }
 
 impl ExecutionStatus {
+    pub fn is_valid(&self) -> bool {
+        matches!(self, ExecutionStatus::Valid(_))
+    }
+
+    pub fn is_execution_enabled(&self) -> bool {
+        !matches!(self, ExecutionStatus::Irrelevant(_))
+    }
+
     pub fn irrelevant() -> Self {
         ExecutionStatus::Irrelevant(false)
     }
@@ -302,6 +310,11 @@ impl ProtoArrayForkChoice {
         }
     }
 
+    /// See `ProtoArray::iter_nodes`
+    pub fn iter_nodes<'a>(&'a self, block_root: &Hash256) -> Iter<'a> {
+        self.proto_array.iter_nodes(block_root)
+    }
+
     pub fn as_bytes(&self) -> Vec<u8> {
         SszContainer::from(self).as_ssz_bytes()
     }