fix: When getting the key in the slot where migration failed, codis-proxy exited abnormally (#3238)

Co-authored-by: chejinge <chejinge@360.cn>

Author: chejinge

codis/pkg/proxy/backend.go

@@ -270,8 +270,9 @@ func (bc *BackendConn) run() {
 }
 
 var (
-	errRespMasterDown = []byte("MASTERDOWN")
-	errRespLoading    = []byte("LOADING")
+	errRespMasterDown        = []byte("MASTERDOWN")
+	errRespLoading           = []byte("LOADING")
+	errRespSlotMigrateClosed = []byte("ERR please open slotmigrate and reload slot")
 )
 
 func (bc *BackendConn) loopReader(tasks <-chan *Request, c *redis.Conn, round int) (err error) {
@@ -301,6 +302,11 @@ func (bc *BackendConn) loopReader(tasks <-chan *Request, c *redis.Conn, round in
 					log.Warnf("backend conn [%p] to %s, db-%d state = DataStale, caused by 'LOADING'",
 						bc, bc.addr, bc.database)
 				}
+			case bytes.HasPrefix(resp.Value, errRespSlotMigrateClosed):
+				if bc.state.CompareAndSwap(stateConnected, stateDataStale) {
+					log.Warnf("backend conn [%p] to %s, db-%d state = DataStale, caused by 'slotmigrate disabled'",
+						bc, bc.addr, bc.database)
+				}
 			}
 		}
 		bc.setResponse(r, resp, nil)