Use OpenAPI when merging (3way) resources

- When merging (3way) resources use the patch strategy from the openAPI if the definition exists for the field
- Allow disabling of guessing patch strategy merge keys when no definition exists
- Support defining strategy and key directly on configuration fields through line and header coments
- Support attaching schema to parent fields of lists, and propagating -- e.g. that a field is a PodTemplate

kyaml/yaml/schema/schema.go

@@ -0,0 +1,34 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package schema contains libraries for working with the yaml and openapi packages.
+package schema
+
+import (
+	"sigs.k8s.io/kustomize/kyaml/openapi"
+	"sigs.k8s.io/kustomize/kyaml/yaml"
+)
+
+// IsAssociative returns true if all elements in the list contain an AssociativeSequenceKey
+// as a field.
+func IsAssociative(schema *openapi.ResourceSchema, nodes []*yaml.RNode, infer bool) bool {
+	if schema != nil {
+		// use the schema to identify if the list is associative
+		s, _ := schema.PatchStrategyAndKey()
+		return s == "merge"
+	}
+	if !infer {
+		return false
+	}
+
+	for i := range nodes {
+		node := nodes[i]
+		if yaml.IsEmpty(node) {
+			continue
+		}
+		if node.IsAssociative() {
+			return true
+		}
+	}
+	return false
+}