Update patch factory and add multi transformer with checking conflicts

pkg/transformers/multitransformer.go

@@ -16,11 +16,16 @@ limitations under the License.
 
 package transformers
 
-import "github.com/kubernetes-sigs/kustomize/pkg/resmap"
+import (
+	"fmt"
+
+	"github.com/kubernetes-sigs/kustomize/pkg/resmap"
+)
 
 // multiTransformer contains a list of transformers.
 type multiTransformer struct {
-	transformers []Transformer
+	transformers         []Transformer
+	checkConflictEnabled bool
 }
 
 var _ Transformer = &multiTransformer{}
@@ -28,13 +33,29 @@ var _ Transformer = &multiTransformer{}
 // NewMultiTransformer constructs a multiTransformer.
 func NewMultiTransformer(t []Transformer) Transformer {
 	r := &multiTransformer{
-		transformers: make([]Transformer, len(t))}
+		transformers:         make([]Transformer, len(t)),
+		checkConflictEnabled: false}
+	copy(r.transformers, t)
+	return r
+}
+
+// NewMultiTransformerWithConflictCheck constructs a multiTransformer with checking of conflicts.
+func NewMultiTransformerWithConflictCheck(t []Transformer) Transformer {
+	r := &multiTransformer{
+		transformers:         make([]Transformer, len(t)),
+		checkConflictEnabled: true}
 	copy(r.transformers, t)
 	return r
 }
 
 // Transform prepends the name prefix.
 func (o *multiTransformer) Transform(m resmap.ResMap) error {
+	if o.checkConflictEnabled {
+		return o.transformWithCheckConflict(m)
+	}
+	return o.transform(m)
+}
+func (o *multiTransformer) transform(m resmap.ResMap) error {
 	for _, t := range o.transformers {
 		err := t.Transform(m)
 		if err != nil {
@@ -43,3 +64,30 @@ func (o *multiTransformer) Transform(m resmap.ResMap) error {
 	}
 	return nil
 }
+
+// Of the len(o.transformers)! possible transformer orderings, compare to a reversed order.
+// A spot check to perform when the transformations are supposed to be commutative.
+// Fail if there's a difference in the result.
+func (o *multiTransformer) transformWithCheckConflict(m resmap.ResMap) error {
+	mcopy := m.DeepCopy()
+	err := o.transform(m)
+	if err != nil {
+		return err
+	}
+	o.reverseTransformers()
+	err = o.transform(mcopy)
+	if err != nil {
+		return err
+	}
+	err = m.ErrorIfNotEqual(mcopy)
+	if err != nil {
+		return fmt.Errorf("found conflict between different patches\n%v", err)
+	}
+	return nil
+}
+
+func (o *multiTransformer) reverseTransformers() {
+	for i, j := 0, len(o.transformers)-1; i < j; i, j = i+1, j-1 {
+		o.transformers[i], o.transformers[j] = o.transformers[j], o.transformers[i]
+	}
+}