add a hack to enable routing to RDONLY tablets for REPLICA queries

Signed-off-by: Michael Demmer <mdemmer@slack-corp.com>

go/vt/vtgate/discoverygateway.go

@@ -17,6 +17,7 @@ limitations under the License.
 package vtgate
 
 import (
+	"flag"
 	"fmt"
 	"math/rand"
 	"sort"
@@ -40,6 +41,10 @@ import (
 	vtrpcpb "vitess.io/vitess/go/vt/proto/vtrpc"
 )
 
+var (
+	routeReplicaToRdonly = flag.Bool("gateway_route_replica_to_rdonly", false, "route REPLICA queries to RDONLY tablets as well as REPLICA tablets")
+)
+
 const (
 	// GatewayImplementationDiscovery defines the string value used as the implementation key for DiscoveryGateway
 	GatewayImplementationDiscovery = "discoverygateway"
@@ -286,6 +291,13 @@ func (dg *DiscoveryGateway) withRetry(ctx context.Context, target *querypb.Targe
 		}
 
 		tablets := dg.tsc.GetHealthyTabletStats(target.Keyspace, target.Shard, target.TabletType)
+
+		// temporary hack to enable REPLICA type queries to address both REPLICA tablets and RDONLY tablets
+		// original commit - https://github.com/tinyspeck/vitess/pull/166/commits/2552b4ce25a9fdb41ff07fa69f2ccf485fea83ac
+		if *routeReplicaToRdonly && target.TabletType == topodatapb.TabletType_REPLICA {
+			tablets = append(tablets, dg.tsc.GetHealthyTabletStats(target.Keyspace, target.Shard, topodatapb.TabletType_RDONLY)...)
+		}
+
 		if len(tablets) == 0 {
 			// fail fast if there is no tablet
 			err = vterrors.New(vtrpcpb.Code_UNAVAILABLE, "no valid tablet")