Improve target_shards handling

Signed-off-by: Matt Lord <mattalord@gmail.com>

go/cmd/vtctldclient/command/vreplication/vdiff/vdiff.go

@@ -198,7 +198,7 @@ vtctldclient --server localhost:15999 vdiff --workflow commerce2customer --targe
 
 			for _, shard := range resumeOptions.TargetShards {
 				if !key.IsValidKeyRange(shard) {
-					return fmt.Errorf("invalid target shard provided: %s", shard)
+					return fmt.Errorf("invalid target shard provided: %q", shard)
 				}
 			}
 
@@ -250,7 +250,7 @@ vtctldclient --server localhost:15999 vdiff --workflow commerce2customer --targe
 
 			for _, shard := range stopOptions.TargetShards {
 				if !key.IsValidKeyRange(shard) {
-					return fmt.Errorf("invalid target shard provided: %s", shard)
+					return fmt.Errorf("invalid target shard provided: %q", shard)
 				}
 			}
 

go/vt/vtctl/workflow/server.go

@@ -1927,10 +1927,8 @@ func (s *Server) VDiffResume(ctx context.Context, req *vtctldatapb.VDiffResumeRe
 	}
 
 	if len(targetShards) > 0 {
-		for key, target := range ts.targets {
-			if !slices.Contains(targetShards, target.GetShard().ShardName()) {
-				delete(ts.targets, key)
-			}
+		if err := applyTargetShards(ts, targetShards); err != nil {
+			return nil, err
 		}
 	}
 
@@ -2012,10 +2010,8 @@ func (s *Server) VDiffStop(ctx context.Context, req *vtctldatapb.VDiffStopReques
 	}
 
 	if len(targetShards) > 0 {
-		for key, target := range ts.targets {
-			if !slices.Contains(targetShards, target.GetShard().ShardName()) {
-				delete(ts.targets, key)
-			}
+		if err := applyTargetShards(ts, targetShards); err != nil {
+			return nil, err
 		}
 	}
 

go/vt/vtctl/workflow/utils.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"hash/fnv"
 	"math"
+	"slices"
 	"sort"
 	"strconv"
 	"strings"
@@ -967,3 +968,28 @@ func defaultErrorHandler(logger logutil.Logger, message string, err error) (*[]s
 	logger.Error(werr)
 	return nil, werr
 }
+
+// applyTargetShards applies the targetShards, coming from a command, to the trafficSwitcher.
+// It will return an error if the targetShards list contains a shard that does not exist in
+// the target keyspace.
+// It will remove any target shards in the trafficSwitcher that are not in the targetShards list.
+func applyTargetShards(ts *trafficSwitcher, targetShards []string) error {
+	if ts == nil {
+		return nil
+	}
+	if ts.targets == nil {
+		return vterrors.Errorf(vtrpcpb.Code_FAILED_PRECONDITION, "no targets found for workflow %s", ts.workflow)
+	}
+	for _, targetShard := range targetShards {
+		if _, ok := ts.targets[targetShard]; !ok {
+			return vterrors.Errorf(vtrpcpb.Code_INVALID_ARGUMENT, "specified target shard %s not a valid target for workflow %s",
+				targetShard, ts.workflow)
+		}
+	}
+	for key, target := range ts.targets {
+		if !slices.Contains(targetShards, target.GetShard().ShardName()) {
+			delete(ts.targets, key)
+		}
+	}
+	return nil
+}