Add debug fork choice api (#4003)

## Issue Addressed

Which issue # does this PR address?
https://github.com/sigp/lighthouse/issues/3669

## Proposed Changes

Please list or describe the changes introduced by this PR.
- A new API to fetch fork choice data, as specified [here](https://github.com/ethereum/beacon-APIs/pull/232)
- A new integration test to test the new API

## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.

- `extra_data` field specified in the beacon-API spec is not implemented, please let me know if I should instead.

Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/http_api/src/lib.rs

@@ -30,7 +30,8 @@ use beacon_chain::{
 pub use block_id::BlockId;
 use directory::DEFAULT_ROOT_DIR;
 use eth2::types::{
-    self as api_types, EndpointVersion, SkipRandaoVerification, ValidatorId, ValidatorStatus,
+    self as api_types, EndpointVersion, ForkChoice, ForkChoiceNode, SkipRandaoVerification,
+    ValidatorId, ValidatorStatus,
 };
 use lighthouse_network::{types::SyncState, EnrExt, NetworkGlobals, PeerId, PubsubMessage};
 use lighthouse_version::version_with_platform;
@@ -2148,6 +2149,58 @@ pub fn serve<T: BeaconChainTypes>(
             },
         );
 
+    // GET debug/fork_choice
+    let get_debug_fork_choice = eth_v1
+        .and(warp::path("debug"))
+        .and(warp::path("fork_choice"))
+        .and(warp::path::end())
+        .and(chain_filter.clone())
+        .and_then(|chain: Arc<BeaconChain<T>>| {
+            blocking_json_task(move || {
+                let beacon_fork_choice = chain.canonical_head.fork_choice_read_lock();
+
+                let proto_array = beacon_fork_choice.proto_array().core_proto_array();
+
+                let fork_choice_nodes = proto_array
+                    .nodes
+                    .iter()
+                    .map(|node| {
+                        let execution_status = if node.execution_status.is_execution_enabled() {
+                            Some(node.execution_status.to_string())
+                        } else {
+                            None
+                        };
+
+                        ForkChoiceNode {
+                            slot: node.slot,
+                            block_root: node.root,
+                            parent_root: node
+                                .parent
+                                .and_then(|index| proto_array.nodes.get(index))
+                                .map(|parent| parent.root),
+                            justified_epoch: node
+                                .justified_checkpoint
+                                .map(|checkpoint| checkpoint.epoch),
+                            finalized_epoch: node
+                                .finalized_checkpoint
+                                .map(|checkpoint| checkpoint.epoch),
+                            weight: node.weight,
+                            validity: execution_status,
+                            execution_block_hash: node
+                                .execution_status
+                                .block_hash()
+                                .map(|block_hash| block_hash.into_root()),
+                        }
+                    })
+                    .collect::<Vec<_>>();
+                Ok(ForkChoice {
+                    justified_checkpoint: proto_array.justified_checkpoint,
+                    finalized_checkpoint: proto_array.finalized_checkpoint,
+                    fork_choice_nodes,
+                })
+            })
+        });
+
     /*
      * node
      */
@@ -3676,6 +3729,7 @@ pub fn serve<T: BeaconChainTypes>(
                 .uor(get_config_deposit_contract)
                 .uor(get_debug_beacon_states)
                 .uor(get_debug_beacon_heads)
+                .uor(get_debug_fork_choice)
                 .uor(get_node_identity)
                 .uor(get_node_version)
                 .uor(get_node_syncing)