Add startup argument to stop tablet from reparent

go/vt/vttablet/tabletmanager/rpc_external_reparent.go

@@ -21,6 +21,9 @@ import (
 	"sync"
 	"time"
 
+	"vitess.io/vitess/go/vt/proto/vtrpc"
+	"vitess.io/vitess/go/vt/vterrors"
+
 	"github.com/golang/protobuf/proto"
 	"golang.org/x/net/context"
 	"vitess.io/vitess/go/event"
@@ -40,8 +43,18 @@ var (
 	finalizeReparentTimeout = flag.Duration("finalize_external_reparent_timeout", 30*time.Second, "Timeout for the finalize stage of a fast external reparent reconciliation.")
 
 	externalReparentStats = stats.NewTimings("ExternalReparents", "External reparenting time", "stage", "NewMasterVisible", "FullRebuild")
+
+	neverReparent = flag.Bool("never_reparent", false, "if set to true, this tablet will not become MASTER")
 )
 
+// IsItSafeToReparent returns nil if we can reparent, and an error if we shouldn't
+func IsItSafeToReparent() error {
+	if *neverReparent {
+		return vterrors.New(vtrpc.Code_INVALID_ARGUMENT, "never_reparent set to true, so will not reparent")
+	}
+	return nil
+}
+
 // SetReparentFlags changes flag values. It should only be used in tests.
 func SetReparentFlags(timeout time.Duration) {
 	*finalizeReparentTimeout = timeout
@@ -50,6 +63,10 @@ func SetReparentFlags(timeout time.Duration) {
 // TabletExternallyReparented updates all topo records so the current
 // tablet is the new master for this shard.
 func (agent *ActionAgent) TabletExternallyReparented(ctx context.Context, externalID string) error {
+	err := IsItSafeToReparent()
+	if err != nil {
+		return err
+	}
 	if err := agent.lock(ctx); err != nil {
 		return err
 	}

go/vt/vttablet/tabletmanager/rpc_external_reparent_test.go

@@ -19,6 +19,8 @@ package tabletmanager
 import (
 	"context"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestTabletExternallyReparentedAlwaysUpdatesTimestamp(t *testing.T) {
@@ -43,3 +45,13 @@ func TestTabletExternallyReparentedAlwaysUpdatesTimestamp(t *testing.T) {
 		t.Fatalf("subsequent TER call did not update the timestamp: %v = %v", ter1, ter2)
 	}
 }
+
+func TestShouldNotReparent(t *testing.T) {
+	ctx := context.Background()
+	agent := createTestAgent(ctx, t, nil)
+	soTrue := true
+	neverReparent = &soTrue
+
+	err := agent.TabletExternallyReparented(ctx, "unused_id")
+	assert.Error(t, err)
+}

go/vt/wrangler/reparent.go

@@ -26,6 +26,8 @@ import (
 	"sync"
 	"time"
 
+	"vitess.io/vitess/go/vt/vttablet/tabletmanager"
+
 	"vitess.io/vitess/go/event"
 	"vitess.io/vitess/go/mysql"
 	"vitess.io/vitess/go/sqlescape"
@@ -329,6 +331,10 @@ func (wr *Wrangler) initShardMasterLocked(ctx context.Context, ev *events.Repare
 // PlannedReparentShard will make the provided tablet the master for the shard,
 // when both the current and new master are reachable and in good shape.
 func (wr *Wrangler) PlannedReparentShard(ctx context.Context, keyspace, shard string, masterElectTabletAlias, avoidMasterAlias *topodatapb.TabletAlias, waitSlaveTimeout time.Duration) (err error) {
+	err = tabletmanager.IsItSafeToReparent()
+	if err != nil {
+		return err
+	}
 	// lock the shard
 	lockAction := fmt.Sprintf(
 		"PlannedReparentShard(%v, avoid_master=%v)",
@@ -604,6 +610,11 @@ func (wr *Wrangler) chooseNewMaster(
 // EmergencyReparentShard will make the provided tablet the master for
 // the shard, when the old master is completely unreachable.
 func (wr *Wrangler) EmergencyReparentShard(ctx context.Context, keyspace, shard string, masterElectTabletAlias *topodatapb.TabletAlias, waitSlaveTimeout time.Duration) (err error) {
+	err = tabletmanager.IsItSafeToReparent()
+	if err != nil {
+		return err
+	}
+
 	// lock the shard
 	ctx, unlock, lockErr := wr.ts.LockShard(ctx, keyspace, shard, fmt.Sprintf("EmergencyReparentShard(%v)", topoproto.TabletAliasString(masterElectTabletAlias)))
 	if lockErr != nil {