Maintain P2P connections active during sync (#1417)

* maintain P2P connections active during sync (instead of dropping them when getting the Stopped Engine error)

* change log statement for ignoring stopped engine errors from warn to debug (such errors occur during node sync since mining is automatically disabled)

eth/handler.go

@@ -29,6 +29,7 @@ import (
 	"github.com/ethereum/go-ethereum/consensus"
 	"github.com/ethereum/go-ethereum/consensus/clique"
 	"github.com/ethereum/go-ethereum/consensus/ethash"
+	"github.com/ethereum/go-ethereum/consensus/istanbul"
 	"github.com/ethereum/go-ethereum/core"
 	"github.com/ethereum/go-ethereum/core/forkid"
 	"github.com/ethereum/go-ethereum/core/types"
@@ -755,6 +756,12 @@ func (h *handler) handleConsensusLoop(p *eth.Peer, protoRW p2p.MsgReadWriter, fa
 	// Handle incoming messages until the connection is torn down
 	for {
 		if err := h.handleConsensus(p, protoRW, fallThroughBackend); err != nil {
+			// allow the P2P connection to remain active during sync (when the engine is stopped)
+			if errors.Is(err, istanbul.ErrStoppedEngine) && h.downloader.Synchronising() {
+				// should this be warn or debug
+				p.Log().Debug("Ignoring `stopped engine` consensus error due to active sync.")
+				continue
+			}
 			p.Log().Debug("Ethereum quorum message handling failed", "err", err)
 			return err
 		}