Updated computeSubnetForAttestation (#9872)

During non finality it's possible to have long periods without slots, and in these cases we would need to advance the state slot to be able to compute the attestation subnet.

Ordinarily this code will not be in range to be used, but we will need to start computing epochs to ensure we're in range.

This was found by diamond tests scenario nfin-001

Author: rolfyone

ethereum/spec/src/main/java/tech/pegasys/teku/spec/Spec.java

@@ -37,6 +37,8 @@
 import java.util.function.Function;
 import java.util.function.Supplier;
 import javax.annotation.CheckReturnValue;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.apache.tuweni.bytes.Bytes;
 import org.apache.tuweni.bytes.Bytes32;
 import tech.pegasys.teku.bls.BLSPublicKey;
@@ -112,6 +114,7 @@
 import tech.pegasys.teku.spec.schemas.registry.SchemaRegistryBuilder;
 
 public class Spec {
+  private static final Logger LOG = LogManager.getLogger();
   private final Map<SpecMilestone, SpecVersion> specVersions;
   private final ForkSchedule forkSchedule;
   private final StateTransition stateTransition;
@@ -626,6 +629,28 @@ public UInt64 computeNextEpochBoundary(final UInt64 slot) {
   }
 
   public int computeSubnetForAttestation(final BeaconState state, final Attestation attestation) {
+    final UInt64 stateEpoch = getCurrentEpoch(state);
+    final UInt64 attestationEpoch =
+        atEpoch(stateEpoch).miscHelpers().computeEpochAtSlot(attestation.getData().getSlot());
+    final UInt64 earliestEpochInRange =
+        attestationEpoch.minusMinZero(atEpoch(attestationEpoch).getConfig().getMaxSeedLookahead());
+    if (stateEpoch.isLessThan(earliestEpochInRange)) {
+      final UInt64 earliestSlot =
+          atEpoch(stateEpoch).miscHelpers().computeStartSlotAtEpoch(earliestEpochInRange);
+      LOG.debug(
+          "Processing empty slots for state at slot {}, target slot {}",
+          state.getSlot(),
+          earliestSlot);
+      try {
+        final BeaconState advancedState = processSlots(state, earliestSlot);
+        return atState(advancedState)
+            .getBeaconStateUtil()
+            .computeSubnetForAttestation(advancedState, attestation);
+      } catch (SlotProcessingException | EpochProcessingException e) {
+        LOG.debug("Couldn't wind state at slot {} forward", state.getSlot(), e);
+      }
+    }
+
     return atState(state).getBeaconStateUtil().computeSubnetForAttestation(state, attestation);
   }
 

ethereum/spec/src/test/java/tech/pegasys/teku/spec/SpecTest.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright Consensys Software Inc., 2025
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+
+package tech.pegasys.teku.spec;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.List;
+import java.util.Optional;
+import org.junit.jupiter.api.Test;
+import tech.pegasys.teku.spec.datastructures.blocks.SignedBlockAndState;
+import tech.pegasys.teku.spec.generator.ChainBuilder;
+import tech.pegasys.teku.spec.util.DataStructureUtil;
+import tech.pegasys.teku.storage.server.StateStorageMode;
+import tech.pegasys.teku.storage.storageSystem.InMemoryStorageSystemBuilder;
+import tech.pegasys.teku.storage.storageSystem.StorageSystem;
+
+class SpecTest {
+  final Spec spec = TestSpecFactory.createMinimalAltair();
+  final DataStructureUtil dataStructureUtil = new DataStructureUtil(spec);
+  private final StorageSystem storageSystem =
+      InMemoryStorageSystemBuilder.buildDefault(StateStorageMode.PRUNE, spec);
+  private final ChainBuilder chainBuilder = storageSystem.chainBuilder();
+
+  @Test
+  void shouldWindStateForwardIfOutsidePeriod() {
+    chainBuilder.generateGenesis();
+    chainBuilder.generateBlocksUpToSlot(8);
+    final List<SignedBlockAndState> chain = chainBuilder.finalizeCurrentChain(Optional.empty());
+
+    // the important thing here is that we get a response, we don't fail, even though slot 100 is
+    // well beyond the finalized slot (in epoch 4)
+    assertThat(
+            spec.computeSubnetForAttestation(
+                chain.getLast().getState(), dataStructureUtil.randomAttestation(100)))
+        .isEqualTo(48);
+  }
+}