Properly handle grpc dial errors in the throttler metric aggregation

go/vt/vttablet/tabletserver/throttle/base/metric_result.go

@@ -19,6 +19,9 @@ package base
 import (
 	"errors"
 	"net"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 )
 
 // MetricResult is what we expect our probes to return. This can be a numeric result, or
@@ -58,6 +61,11 @@ func IsDialTCPError(err error) bool {
 	if err == nil {
 		return false
 	}
+
+	if s, ok := status.FromError(err); ok {
+		return s.Code() == codes.Unavailable || s.Code() == codes.DeadlineExceeded
+	}
+
 	switch err := err.(type) {
 	case *net.OpError:
 		return err.Op == "dial" && err.Net == "tcp"

go/vt/vttablet/tabletserver/throttle/throttler.go

@@ -907,7 +907,7 @@ func (throttler *Throttler) generateTabletProbeFunction(scope base.Scope, probe
 		req := &tabletmanagerdatapb.CheckThrottlerRequest{} // We leave AppName empty; it will default to VitessName anyway, and we can save some proto space
 		resp, gRPCErr := tmClient.CheckThrottler(ctx, probe.Tablet, req)
 		if gRPCErr != nil {
-			return metricsWithError(fmt.Errorf("gRPC error accessing tablet %v. Err=%v", probe.Alias, gRPCErr))
+			return metricsWithError(fmt.Errorf("gRPC error accessing tablet %v. Err=%w", probe.Alias, gRPCErr))
 		}
 		throttleMetric.Value = resp.Value
 		if resp.ResponseCode == tabletmanagerdatapb.CheckThrottlerResponseCode_INTERNAL_ERROR {

go/vt/vttablet/tabletserver/throttle/throttler_test.go

@@ -30,9 +30,15 @@ import (
 	"github.com/stretchr/testify/require"
 	"golang.org/x/exp/maps"
 
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+
 	"vitess.io/vitess/go/protoutil"
+
+	"vitess.io/vitess/go/vt/grpcclient"
 	"vitess.io/vitess/go/vt/topo"
 	"vitess.io/vitess/go/vt/vtenv"
+	"vitess.io/vitess/go/vt/vttablet/grpctmclient"
 	"vitess.io/vitess/go/vt/vttablet/tabletserver/connpool"
 	"vitess.io/vitess/go/vt/vttablet/tabletserver/tabletenv"
 	"vitess.io/vitess/go/vt/vttablet/tabletserver/throttle/base"
@@ -837,6 +843,110 @@ func TestApplyThrottlerConfigAppCheckedMetrics(t *testing.T) {
 	})
 }
 
+func TestIsDialTCPError(t *testing.T) {
+	// Verify that IsDialTCPError actually recognizes grpc dial errors
+	cc, err := grpcclient.DialContext(t.Context(), ":0", true, grpc.WithTransportCredentials(insecure.NewCredentials()))
+	require.NoError(t, err)
+	defer cc.Close()
+
+	err = cc.Invoke(context.Background(), "/Fail", nil, nil)
+
+	require.True(t, base.IsDialTCPError(err))
+	require.True(t, base.IsDialTCPError(fmt.Errorf("wrapped: %w", err)))
+}
+
+func TestProbeWithUnavailableHost(t *testing.T) {
+	throttler := Throttler{
+		throttledApps:   cache.New(cache.NoExpiration, 0),
+		heartbeatWriter: &FakeHeartbeatWriter{},
+	}
+
+	alias := &topodatapb.TabletAlias{
+		Cell: "cell1",
+		Uid:  100,
+	}
+
+	// The hostname used here is not routable, so the connection will fail.
+	tablet := &topo.TabletInfo{
+		Tablet: &topodatapb.Tablet{
+			Alias:         alias,
+			Hostname:      "192.0.2.0",
+			MysqlHostname: "192.0.2.0",
+			MysqlPort:     3306,
+			PortMap:       map[string]int32{"grpc": 5000},
+			Type:          topodatapb.TabletType_PRIMARY,
+		},
+	}
+
+	probe := &base.Probe{
+		Alias:       "cell1-100",
+		Tablet:      tablet.Tablet,
+		CacheMillis: 100,
+	}
+
+	tmClient := grpctmclient.NewClient()
+
+	probeFunc := throttler.generateTabletProbeFunction(base.ShardScope, probe)
+
+	metrics := probeFunc(t.Context(), tmClient)
+	require.True(t, base.IsDialTCPError(metrics["custom"].Err))
+
+	tabletResultsMap := base.TabletResultMap{
+		"cell1-100": base.MetricResultMap{
+			"custom": metrics["custom"],
+		},
+	}
+
+	worstMetric := base.AggregateTabletMetricResults("custom", tabletResultsMap, 0, true, 0.0)
+	require.Equal(t, base.NoHostsMetricResult, worstMetric)
+}
+
+func TestProbeWithEmptyHostAndPort(t *testing.T) {
+	throttler := Throttler{
+		throttledApps:   cache.New(cache.NoExpiration, 0),
+		heartbeatWriter: &FakeHeartbeatWriter{},
+	}
+
+	alias := &topodatapb.TabletAlias{
+		Cell: "cell1",
+		Uid:  100,
+	}
+
+	// The hostname used here is not routable, so the connection will fail.
+	tablet := &topo.TabletInfo{
+		Tablet: &topodatapb.Tablet{
+			Alias:         alias,
+			Hostname:      "",
+			MysqlHostname: "192.0.2.0",
+			MysqlPort:     3306,
+			PortMap:       map[string]int32{"grpc": 0},
+			Type:          topodatapb.TabletType_PRIMARY,
+		},
+	}
+
+	probe := &base.Probe{
+		Alias:       "cell1-100",
+		Tablet:      tablet.Tablet,
+		CacheMillis: 100,
+	}
+
+	tmClient := grpctmclient.NewClient()
+
+	probeFunc := throttler.generateTabletProbeFunction(base.ShardScope, probe)
+
+	metrics := probeFunc(t.Context(), tmClient)
+	require.True(t, base.IsDialTCPError(metrics["custom"].Err))
+
+	tabletResultsMap := base.TabletResultMap{
+		"cell1-100": base.MetricResultMap{
+			"custom": metrics["custom"],
+		},
+	}
+
+	worstMetric := base.AggregateTabletMetricResults("custom", tabletResultsMap, 0, true, 0.0)
+	require.Equal(t, base.NoHostsMetricResult, worstMetric)
+}
+
 func TestIsAppThrottled(t *testing.T) {
 	plusOneHour := time.Now().Add(time.Hour)
 	throttler := Throttler{