eth/downloader: ignore zero size header batch for importing (#24569)

* eth/downloader: ignore zero size header batch for importing

* core, light: reject empty header batch for importing

core/blockchain.go

@@ -2309,6 +2309,9 @@ Error: %v
 // of the header retrieval mechanisms already need to verify nonces, as well as
 // because nonces can be verified sparsely, not needing to check each.
 func (bc *BlockChain) InsertHeaderChain(chain []*types.Header, checkFreq int) (int, error) {
+	if len(chain) == 0 {
+		return 0, nil
+	}
 	start := time.Now()
 	if i, err := bc.hc.ValidateHeaderChain(chain, checkFreq); err != nil {
 		return i, err

eth/downloader/downloader.go

@@ -1377,6 +1377,7 @@ func (d *Downloader) processHeaders(origin uint64, td, ttd *big.Int, beaconMode
 							}
 						}
 					}
+					if len(chunkHeaders) > 0 {
 						if n, err := d.lightchain.InsertHeaderChain(chunkHeaders, frequency); err != nil {
 							rollbackErr = err
 
@@ -1387,7 +1388,7 @@ func (d *Downloader) processHeaders(origin uint64, td, ttd *big.Int, beaconMode
 							log.Warn("Invalid header encountered", "number", chunkHeaders[n].Number, "hash", chunkHashes[n], "parent", chunkHeaders[n].ParentHash, "err", err)
 							return fmt.Errorf("%w: %v", errInvalidChain, err)
 						}
-					// All verifications passed, track all headers within the alloted limits
+						// All verifications passed, track all headers within the allowed limits
 						if mode == SnapSync {
 							head := chunkHeaders[len(chunkHeaders)-1].Number.Uint64()
 							if head-rollback > uint64(fsHeaderSafetyNet) {
@@ -1396,6 +1397,7 @@ func (d *Downloader) processHeaders(origin uint64, td, ttd *big.Int, beaconMode
 								rollback = 1
 							}
 						}
+					}
 					if len(rejected) != 0 {
 						// Merge threshold reached, stop importing, but don't roll back
 						rollback = 0

light/lightchain.go

@@ -419,6 +419,9 @@ func (lc *LightChain) SetChainHead(header *types.Header) error {
 // In the case of a light chain, InsertHeaderChain also creates and posts light
 // chain events when necessary.
 func (lc *LightChain) InsertHeaderChain(chain []*types.Header, checkFreq int) (int, error) {
+	if len(chain) == 0 {
+		return 0, nil
+	}
 	if atomic.LoadInt32(&lc.disableCheckFreq) == 1 {
 		checkFreq = 0
 	}