Make checkpoint sync work when finalized state is transitioned with empty slots

Author: StefanBratanov

ethereum/spec/src/main/java/tech/pegasys/teku/spec/datastructures/state/AnchorPoint.java

@@ -94,7 +94,19 @@ public static AnchorPoint fromInitialState(final Spec spec, final BeaconState st
     if (isGenesisState(state)) {
       return fromGenesisState(spec, state);
     } else {
-      final BeaconBlockHeader header = BeaconBlockHeader.fromState(state);
+      BeaconBlockHeader header = BeaconBlockHeader.fromState(state);
+
+      if (state.getSlot().isGreaterThan(header.getSlot())) {
+        // there have been empty slot(s) since the latest block header so replace the state root
+        // with the current state root
+        header =
+            new BeaconBlockHeader(
+                header.getSlot(),
+                header.getProposerIndex(),
+                header.getParentRoot(),
+                state.hashTreeRoot(),
+                header.getBodyRoot());
+      }
 
       // Calculate closest epoch boundary to use for the checkpoint
       final UInt64 epoch = spec.computeNextEpochBoundary(state.getSlot());

infrastructure/logging/src/main/java/tech/pegasys/teku/infrastructure/logging/StatusLogger.java

@@ -322,21 +322,12 @@ public void loadedInitialStateResource(
     }
   }
 
-  public void errorIncompatibleInitialState(final UInt64 epoch) {
-    log.error(
-        "Cannot start with provided initial state for the epoch {}, "
-            + "checkpoint occurred on the empty slot, which is not yet supported.\n"
-            + "If you are using remote checkpoint source, "
-            + "please wait for the next epoch to finalize and retry.",
-        epoch);
-  }
-
   public void warnInitialStateIgnored() {
     log.warn("Not loading specified initial state as chain data already exists.");
   }
 
-  public void warnFailedToLoadInitialState(final String message) {
-    log.warn(message);
+  public void warnFailedToLoadInitialState(final Throwable throwable) {
+    log.warn("Failed to load initial state", throwable);
   }
 
   public void warnOnInitialStateWithSkippedSlots(

services/beaconchain/src/main/java/tech/pegasys/teku/services/beaconchain/BeaconChainController.java

@@ -1392,7 +1392,7 @@ private Optional<AnchorPoint> tryLoadingAnchorPointFromInitialState(
           && !stateBoostrapConfig.isUsingCheckpointSync()) {
         throw e;
       }
-      STATUS_LOG.warnFailedToLoadInitialState(e.getMessage());
+      STATUS_LOG.warnFailedToLoadInitialState(e);
     }
 
     return initialAnchor;

services/beaconchain/src/main/java/tech/pegasys/teku/services/beaconchain/WeakSubjectivityInitializer.java

@@ -13,7 +13,6 @@
 
 package tech.pegasys.teku.services.beaconchain;
 
-import static tech.pegasys.teku.infrastructure.exceptions.ExitConstants.ERROR_EXIT_CODE;
 import static tech.pegasys.teku.infrastructure.logging.StatusLogger.STATUS_LOG;
 import static tech.pegasys.teku.networks.Eth2NetworkConfiguration.FINALIZED_STATE_URL_PATH;
 
@@ -86,10 +85,6 @@ private AnchorPoint getAnchorPoint(Spec spec, String stateResource, String sanit
       throws IOException {
     STATUS_LOG.loadingInitialStateResource(sanitizedResource);
     final BeaconState state = ChainDataLoader.loadState(spec, stateResource);
-    if (state.getSlot().isGreaterThan(state.getLatestBlockHeader().getSlot())) {
-      STATUS_LOG.errorIncompatibleInitialState(spec.computeEpochAtSlot(state.getSlot()));
-      System.exit(ERROR_EXIT_CODE);
-    }
     final AnchorPoint anchor = AnchorPoint.fromInitialState(spec, state);
     STATUS_LOG.loadedInitialStateResource(
         state.hashTreeRoot(),