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/fns.go

@@ -612,6 +612,50 @@ func Set(value *RNode) FieldSetter {
 	return FieldSetter{Value: value}
 }
 
+// MapEntrySetter sets a map entry to a value. If it finds a key with the same
+// value, it will override both Key and Value RNodes, including style and any
+// other metadata. If it doesn't find the key, it will insert a new map entry.
+// It will set the field, even if it's empty or nil, unlike the FieldSetter.
+// This is useful for rebuilding some pre-existing RNode structure.
+type MapEntrySetter struct {
+	// Name is the name of the field or key to lookup in a MappingNode.
+	// If Name is unspecified, it will use the Key's Value
+	Name string `yaml:"name,omitempty"`
+
+	// Value is the value to set.
+	Value *RNode `yaml:"value,omitempty"`
+
+	// Key is the map key to set.
+	Key *RNode `yaml:"key,omitempty"`
+}
+
+func (s MapEntrySetter) Filter(rn *RNode) (*RNode, error) {
+	if rn == nil {
+		return nil, errors.Errorf("Can't set map entry on a nil RNode")
+	}
+	if err := ErrorIfInvalid(rn, yaml.MappingNode); err != nil {
+		return nil, err
+	}
+	if s.Name == "" {
+		s.Name = GetValue(s.Key)
+	}
+	for i := 0; i < len(rn.Content()); i = IncrementFieldIndex(i) {
+		isMatchingField := rn.Content()[i].Value == s.Name
+		if isMatchingField {
+			rn.Content()[i] = s.Key.YNode()
+			rn.Content()[i+1] = s.Value.YNode()
+			return rn, nil
+		}
+	}
+
+	// create the field
+	rn.YNode().Content = append(
+		rn.YNode().Content,
+		s.Key.YNode(),
+		s.Value.YNode())
+	return rn, nil
+}
+
 // FieldSetter sets a field or map entry to a value.
 type FieldSetter struct {
 	Kind string `yaml:"kind,omitempty"`