*: fix panic in get cause error (#1344)

Signed-off-by: crazycs520 <crazycs520@gmail.com>

internal/locate/region_request.go

@@ -180,6 +180,17 @@ func (r *RequestErrorStats) RecordRPCErrorStats(errLabel string) {
 	}
 }
 
+// getErrMsg returns error message. if the error has cause error, then return cause error message.
+func getErrMsg(err error) string {
+	if err == nil {
+		return ""
+	}
+	if causeErr := errors.Cause(err); causeErr != nil {
+		return causeErr.Error()
+	}
+	return err.Error()
+}
+
 // String implements fmt.Stringer interface.
 func (r *RegionRequestRuntimeStats) String() string {
 	if r == nil {
@@ -2115,7 +2126,7 @@ func (s *RegionRequestSender) sendReqToRegion(
 	if err != nil {
 		s.rpcError = err
 		if s.Stats != nil {
-			errStr := errors.Cause(err).Error()
+			errStr := getErrMsg(err)
 			s.Stats.RecordRPCErrorStats(errStr)
 			s.recordRPCAccessInfo(req, rpcCtx, errStr)
 		}
@@ -2211,7 +2222,11 @@ func (s *RegionRequestSender) onSendFail(bo *retry.Backoffer, ctx *RPCContext, r
 			}
 		}
 	}
-	metrics.TiKVRPCErrorCounter.WithLabelValues(errors.Cause(err).Error(), storeLabel).Inc()
+	if errStr := getErrMsg(err); len(errStr) > 0 {
+		metrics.TiKVRPCErrorCounter.WithLabelValues(getErrMsg(err), storeLabel).Inc()
+	} else {
+		metrics.TiKVRPCErrorCounter.WithLabelValues("unknown", storeLabel).Inc()
+	}
 
 	if ctx.Store != nil && ctx.Store.storeType == tikvrpc.TiFlashCompute {
 		s.regionCache.InvalidateTiFlashComputeStoresIfGRPCError(err)

internal/locate/region_request_test.go

@@ -56,6 +56,7 @@ import (
 	"github.com/pingcap/kvproto/pkg/mpp"
 	"github.com/pingcap/kvproto/pkg/tikvpb"
 	"github.com/pkg/errors"
+	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
 	"github.com/tikv/client-go/v2/config"
 	"github.com/tikv/client-go/v2/config/retry"
@@ -872,3 +873,20 @@ func (s *testRegionRequestToSingleStoreSuite) TestRegionRequestStats() {
 	}
 	s.Contains(expecteds, access.String())
 }
+
+type noCauseError struct {
+	error
+}
+
+func (_ noCauseError) Cause() error {
+	return nil
+}
+
+func TestGetErrMsg(t *testing.T) {
+	err := noCauseError{error: errors.New("no cause err")}
+	require.Equal(t, nil, errors.Cause(err))
+	require.Panicsf(t, func() {
+		_ = errors.Cause(err).Error()
+	}, "should panic")
+	require.Equal(t, "no cause err", getErrMsg(err))
+}