Make findEpochRoot work with missing blocks at the end of an epoch (#…

…12061)

This is for example used in attestations rewards calculation, which was
failing with the error message "no block found for this epoch".

In reality only the last block was not found in the epoch, and that
already triggered the error.

With this change, we find the last non-missed block in this case.

cl/beacon/handler/lighthouse.go

@@ -37,18 +37,24 @@ type LighthouseValidatorInclusionGlobal struct {
 	PreviousEpochHeadAttestingGwei   uint64 `json:"previous_epoch_head_attesting_gwei"`
 }
 
+// the block root hash of the highest numbered slot that actually exists
 func (a *ApiHandler) findEpochRoot(tx kv.Tx, epoch uint64) (common.Hash, error) {
 	var currentBlockRoot common.Hash
+	var validBlockRoot common.Hash
 	var err error
 	for i := epoch * a.beaconChainCfg.SlotsPerEpoch; i < (epoch+1)*a.beaconChainCfg.SlotsPerEpoch; i++ {
 		// read the block root
 		currentBlockRoot, err = beacon_indicies.ReadCanonicalBlockRoot(tx, i)
 		if err != nil {
 			return common.Hash{}, err
 		}
+		if currentBlockRoot != (common.Hash{}) {
+			// only accept a block root, if it was not missed
+			validBlockRoot = currentBlockRoot
+		}
 	}
-	return currentBlockRoot, nil
-
+	// return the last valid block root that was not missed and we found
+	return validBlockRoot, nil
 }
 
 func (a *ApiHandler) GetLighthouseValidatorInclusionGlobal(w http.ResponseWriter, r *http.Request) (*beaconhttp.BeaconResponse, error) {