Fix error during expansion of !!merge <<: anchor tags (#4383)

* WIP

* Fix merge corner cases

* Add test for explicit !!merge tag

* Fix tests

* Cleanup

* Cleanup

* Fix deanchoring lists

* Add test case for keeping comments

* Add MapEntrySetter and fix json marshalling after deanchoring

* Keep duplicated keys

* Move MergeTag definition to yaml alias

* Remove go-spew from api

* Add support for sequence nodes on merge tags

* Add docstring to MapEntrySetter.Key

* Add docstring to MapEntrySetter struct

* Add tests to MapEntrySetter

* Fix duplicate merge key

* Revert whitespace changes on forked go-yaml

* Remove AssocMapEntry function

* Refactoring merge order

* Return errors on VisitFields and PipeE

* Add tests for each non-conforming map merges

kyaml/yaml/rnode.go

@@ -932,7 +932,17 @@ func deAnchor(yn *yaml.Node) (res *yaml.Node, err error) {
 		return yn, nil
 	case yaml.AliasNode:
 		return deAnchor(yn.Alias)
-	case yaml.DocumentNode, yaml.MappingNode, yaml.SequenceNode:
+	case yaml.MappingNode:
+		toMerge, err := removeMergeTags(yn)
+		if err != nil {
+			return nil, err
+		}
+		err = mergeAll(yn, toMerge)
+		if err != nil {
+			return nil, err
+		}
+		fallthrough
+	case yaml.DocumentNode, yaml.SequenceNode:
 		for i := range yn.Content {
 			yn.Content[i], err = deAnchor(yn.Content[i])
 			if err != nil {
@@ -945,6 +955,103 @@ func deAnchor(yn *yaml.Node) (res *yaml.Node, err error) {
 	}
 }
 
+// isMerge returns if the node is tagged with !!merge
+func isMerge(yn *yaml.Node) bool {
+	return yn.Tag == MergeTag
+}
+
+// findMergeValues receives either a MappingNode, a AliasNode or a potentially
+// mixed list of MappingNodes and AliasNodes. It returns a list of MappingNodes.
+func findMergeValues(yn *yaml.Node) ([]*yaml.Node, error) {
+	if yn == nil {
+		return []*yaml.Node{}, nil
+	}
+	switch yn.Kind {
+	case MappingNode:
+		return []*yaml.Node{yn}, nil
+	case AliasNode:
+		if yn.Alias != nil && yn.Alias.Kind != MappingNode {
+			return nil, errors.Errorf("invalid map merge: received alias for a non-map value")
+		}
+		return []*yaml.Node{yn.Alias}, nil
+	case SequenceNode:
+		mergeValues := []*yaml.Node{}
+		for i := 0; i < len(yn.Content); i++ {
+			if yn.Content[i].Kind == SequenceNode {
+				return nil, errors.Errorf("invalid map merge: received a nested sequence")
+			}
+			newMergeValues, err := findMergeValues(yn.Content[i])
+			if err != nil {
+				return nil, err
+			}
+			mergeValues = append(newMergeValues, mergeValues...)
+		}
+		return mergeValues, nil
+	default:
+		return nil, errors.Errorf("map merge requires map or sequence of maps as the value")
+	}
+}
+
+// getMergeTagValue receives a MappingNode yaml node, and it searches for
+// merge tagged keys and return its value yaml node. If the key is duplicated,
+// it fails.
+func getMergeTagValue(yn *yaml.Node) (*yaml.Node, error) {
+	var result *yaml.Node
+	for i := 0; i < len(yn.Content); i += 2 {
+		key := yn.Content[i]
+		value := yn.Content[i+1]
+		if isMerge(key) {
+			if result != nil {
+				return nil, fmt.Errorf("duplicate merge key")
+			}
+			result = value
+		}
+	}
+	return result, nil
+}
+
+// removeMergeTags removes all merge tags and returns a ordered list of yaml
+// nodes to merge and a error
+func removeMergeTags(yn *yaml.Node) ([]*yaml.Node, error) {
+	if yn == nil || yn.Content == nil {
+		return nil, nil
+	}
+	if yn.Kind != yaml.MappingNode {
+		return nil, nil
+	}
+	value, err := getMergeTagValue(yn)
+	if err != nil {
+		return nil, err
+	}
+	toMerge, err := findMergeValues(value)
+	if err != nil {
+		return nil, err
+	}
+	err = NewRNode(yn).PipeE(Clear("<<"))
+	if err != nil {
+		return nil, err
+	}
+	return toMerge, nil
+}
+
+func mergeAll(yn *yaml.Node, toMerge []*yaml.Node) error {
+	// We only need to start with a copy of the existing node because we need to
+	// maintain duplicated keys and style
+	rn := NewRNode(yn).Copy()
+	toMerge = append(toMerge, yn)
+	for i := range toMerge {
+		rnToMerge := NewRNode(toMerge[i]).Copy()
+		err := rnToMerge.VisitFields(func(node *MapNode) error {
+			return rn.PipeE(MapEntrySetter{Key: node.Key, Value: node.Value})
+		})
+		if err != nil {
+			return err
+		}
+	}
+	*yn = *rn.value
+	return nil
+}
+
 // GetValidatedMetadata returns metadata after subjecting it to some tests.
 func (rn *RNode) GetValidatedMetadata() (ResourceMeta, error) {
 	m, err := rn.GetMeta()