Make the replica transformer kind aware.

The previous implementation had a bug and poorly handled
types that should not have a `spec: replica:` field.

Documentation is updated to reflect the change in behavior,
and better highlights the cases where a patch should be
used instead of this shorthand.

plugin/builtin/ReplicaCountTransformer.go

@@ -7,19 +7,17 @@ import (
 	"sigs.k8s.io/kustomize/pkg/ifc"
 	"sigs.k8s.io/kustomize/pkg/resid"
 	"sigs.k8s.io/kustomize/pkg/resmap"
+	"sigs.k8s.io/kustomize/pkg/transformers"
+	"sigs.k8s.io/kustomize/pkg/transformers/config"
 	"sigs.k8s.io/kustomize/pkg/types"
 	"sigs.k8s.io/yaml"
 )
 
-const (
-	fldReplica = "replicas"
-	fldSpec    = "spec"
-)
-
 // Find matching replicas declarations and replace the count.
 // Eases the kustomization configuration of replica changes.
 type ReplicaCountTransformerPlugin struct {
-	Replica types.Replica `json:"replica,omitempty" yaml:"replica,omitempty"`
+	Replica    types.Replica      `json:"replica,omitempty" yaml:"replica,omitempty"`
+	FieldSpecs []config.FieldSpec `json:"fieldSpecs,omitempty" yaml:"fieldSpecs,omitempty"`
 }
 
 //noinspection GoUnusedGlobalVariable
@@ -31,30 +29,46 @@ func (p *ReplicaCountTransformerPlugin) Config(
 	ldr ifc.Loader, rf *resmap.Factory, c []byte) (err error) {
 
 	p.Replica = types.Replica{}
+	p.FieldSpecs = nil
 	return yaml.Unmarshal(c, p)
 }
 
 func (p *ReplicaCountTransformerPlugin) Transform(m resmap.ResMap) error {
-	matcher := func(r resid.ResId) bool {
-		return r.Name == p.Replica.Name
-	}
-
-	for _, r := range m.GetMatchingResourcesByOriginalId(matcher) {
-		kMap := r.Map()
-
-		specInterface, ok := kMap[fldSpec]
-		if !ok {
-			return fmt.Errorf("object %s missing field %s, cannot update %s",
-				p.Replica.Name, fldSpec, fldReplica)
-		}
-
-		if spec, ok := specInterface.(map[string]interface{}); ok {
-			spec[fldReplica] = p.Replica.Count
-			kMap[fldSpec] = spec
-		} else {
-			return fmt.Errorf("object %s has a malformed %s", p.Replica.Name, fldSpec)
+	for i, replicaSpec := range p.FieldSpecs {
+		for _, res := range m.GetMatchingResourcesByOriginalId(p.createMatcher(i)) {
+			err := transformers.MutateField(
+				res.Map(), replicaSpec.PathSlice(),
+				replicaSpec.CreateIfNotPresent, p.addReplicas)
+			if err != nil {
+				return err
+			}
 		}
 	}
 
 	return nil
 }
+
+// Match Replica.Name and FieldSpec
+func (p *ReplicaCountTransformerPlugin) createMatcher(i int) resmap.IdMatcher {
+	return func(r resid.ResId) bool {
+		return r.Name == p.Replica.Name &&
+			r.Gvk.IsSelected(&p.FieldSpecs[i].Gvk)
+	}
+}
+
+func (p *ReplicaCountTransformerPlugin) addReplicas(in interface{}) (interface{}, error) {
+	switch m := in.(type) {
+	case int64:
+		// Was already in the field.
+	case map[string]interface{}:
+		if len(m) != 0 {
+			// A map was already in the replicas field, don't want to
+			// discard this data silently.
+			return nil, fmt.Errorf("%#v is expected to be %T", in, m)
+		}
+		// Just got added, default type is map, but we can return anything.
+	default:
+		return nil, fmt.Errorf("%#v is expected to be %T", in, m)
+	}
+	return p.Replica.Count, nil
+}