Allow proposer duties request for the next epoch (#2963)

## Issue Addressed

Closes #2880 

## Proposed Changes

Support requests to the next epoch in proposer_duties api.

## Additional Info

Implemented with skipping proposer cache for this case because the cache for the future epoch will be missed every new slot as dependent_root is changed and we don't want to "wash it out" by saving additional values.

Cargo.lock

@@ -2442,6 +2442,7 @@ dependencies = [
  "lighthouse_network",
  "lighthouse_version",
  "network",
+ "safe_arith",
  "sensitive_url",
  "serde",
  "slog",

beacon_node/http_api/Cargo.toml

@@ -27,6 +27,7 @@ slot_clock = { path = "../../common/slot_clock" }
 eth2_ssz = "0.4.1"
 bs58 = "0.4.0"
 futures = "0.3.8"
+safe_arith = {path = "../../consensus/safe_arith"}
 
 [dev-dependencies]
 store = { path = "../store" }

beacon_node/http_api/src/proposer_duties.rs

@@ -5,11 +5,12 @@ use beacon_chain::{
     BeaconChain, BeaconChainError, BeaconChainTypes, MAXIMUM_GOSSIP_CLOCK_DISPARITY,
 };
 use eth2::types::{self as api_types};
+use safe_arith::SafeArith;
 use slog::{debug, Logger};
 use slot_clock::SlotClock;
 use state_processing::state_advance::partial_state_advance;
 use std::cmp::Ordering;
-use types::{BeaconState, ChainSpec, CloneConfig, Epoch, EthSpec, Hash256, Slot};
+use types::{BeaconState, ChainSpec, CloneConfig, Epoch, EthSpec, Fork, Hash256, Slot};
 
 /// The struct that is returned to the requesting HTTP client.
 type ApiDuties = api_types::DutiesResponse<Vec<api_types::ProposerData>>;
@@ -49,11 +50,21 @@ pub fn proposer_duties<T: BeaconChainTypes>(
             );
             compute_and_cache_proposer_duties(request_epoch, chain)
         }
-    } else if request_epoch > current_epoch {
-        // Reject queries about the future as they're very expensive there's no look-ahead for
-        // proposer duties.
+    } else if request_epoch
+        == current_epoch
+            .safe_add(1)
+            .map_err(warp_utils::reject::arith_error)?
+    {
+        let (proposers, dependent_root, _) = compute_proposer_duties(request_epoch, chain)?;
+        convert_to_api_response(chain, request_epoch, dependent_root, proposers)
+    } else if request_epoch
+        > current_epoch
+            .safe_add(1)
+            .map_err(warp_utils::reject::arith_error)?
+    {
+        // Reject queries about the future epochs for which lookahead is not possible
         Err(warp_utils::reject::custom_bad_request(format!(
-            "request epoch {} is ahead of the current epoch {}",
+            "request epoch {} is ahead of the next epoch {}",
             request_epoch, current_epoch
         )))
     } else {
@@ -119,6 +130,24 @@ fn compute_and_cache_proposer_duties<T: BeaconChainTypes>(
     current_epoch: Epoch,
     chain: &BeaconChain<T>,
 ) -> Result<ApiDuties, warp::reject::Rejection> {
+    let (indices, dependent_root, fork) = compute_proposer_duties(current_epoch, chain)?;
+
+    // Prime the proposer shuffling cache with the newly-learned value.
+    chain
+        .beacon_proposer_cache
+        .lock()
+        .insert(current_epoch, dependent_root, indices.clone(), fork)
+        .map_err(BeaconChainError::from)
+        .map_err(warp_utils::reject::beacon_chain_error)?;
+
+    convert_to_api_response(chain, current_epoch, dependent_root, indices)
+}
+
+/// Compute the proposer duties using the head state without cache.
+fn compute_proposer_duties<T: BeaconChainTypes>(
+    current_epoch: Epoch,
+    chain: &BeaconChain<T>,
+) -> Result<(Vec<usize>, Hash256, Fork), warp::reject::Rejection> {
     // Take a copy of the head of the chain.
     let head = chain
         .head()
@@ -140,20 +169,7 @@ fn compute_and_cache_proposer_duties<T: BeaconChainTypes>(
         .map_err(BeaconChainError::from)
         .map_err(warp_utils::reject::beacon_chain_error)?;
 
-    // Prime the proposer shuffling cache with the newly-learned value.
-    chain
-        .beacon_proposer_cache
-        .lock()
-        .insert(
-            state.current_epoch(),
-            dependent_root,
-            indices.clone(),
-            state.fork(),
-        )
-        .map_err(BeaconChainError::from)
-        .map_err(warp_utils::reject::beacon_chain_error)?;
-
-    convert_to_api_response(chain, current_epoch, dependent_root, indices)
+    Ok((indices, dependent_root, state.fork()))
 }
 
 /// Compute some proposer duties by reading a `BeaconState` from disk, completely ignoring the

beacon_node/http_api/tests/tests.rs

@@ -1671,7 +1671,7 @@ impl ApiTester {
     pub async fn test_get_validator_duties_proposer(self) -> Self {
         let current_epoch = self.chain.epoch().unwrap();
 
-        for epoch in 0..=self.chain.epoch().unwrap().as_u64() {
+        for epoch in 0..=self.chain.epoch().unwrap().as_u64() + 1 {
             let epoch = Epoch::from(epoch);
 
             let dependent_root = self
@@ -1780,9 +1780,9 @@ impl ApiTester {
             }
         }
 
-        // Requests to future epochs should fail.
+        // Requests to the epochs after the next epoch should fail.
         self.client
-            .get_validator_duties_proposer(current_epoch + 1)
+            .get_validator_duties_proposer(current_epoch + 2)
             .await
             .unwrap_err();
 
@@ -1802,15 +1802,27 @@ impl ApiTester {
             current_epoch_start - MAXIMUM_GOSSIP_CLOCK_DISPARITY - Duration::from_millis(1),
         );
 
-        assert_eq!(
-            self.client
-                .get_validator_duties_proposer(current_epoch)
-                .await
-                .unwrap_err()
-                .status()
-                .map(Into::into),
-            Some(400),
-            "should not get proposer duties outside of tolerance"
+        let dependent_root = self
+            .chain
+            .block_root_at_slot(
+                current_epoch.start_slot(E::slots_per_epoch()) - 1,
+                WhenSlotSkipped::Prev,
+            )
+            .unwrap()
+            .unwrap_or(self.chain.head_beacon_block_root().unwrap());
+
+        self.client
+            .get_validator_duties_proposer(current_epoch)
+            .await
+            .expect("should get proposer duties for the next epoch outside of tolerance");
+
+        assert!(
+            self.chain
+                .beacon_proposer_cache
+                .lock()
+                .get_epoch::<E>(dependent_root, current_epoch)
+                .is_none(),
+            "should not prime the proposer cache outside of tolerance"
         );
 
         assert_eq!(
@@ -1832,6 +1844,16 @@ impl ApiTester {
             .get_validator_duties_proposer(current_epoch)
             .await
             .expect("should get proposer duties within tolerance");
+
+        assert!(
+            self.chain
+                .beacon_proposer_cache
+                .lock()
+                .get_epoch::<E>(dependent_root, current_epoch)
+                .is_some(),
+            "should prime the proposer cache inside the tolerance"
+        );
+
         self.client
             .post_validator_duties_attester(next_epoch, &[0])
             .await