Add structured-merge-diff vendor

This is where most of the logic actually lives.

vendor/sigs.k8s.io/structured-merge-diff/merge/conflict.go

@@ -0,0 +1,91 @@
+/*
+Copyright 2018 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package merge
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+
+	"sigs.k8s.io/structured-merge-diff/fieldpath"
+)
+
+// Conflict is a conflict on a specific field with the current manager of
+// that field. It does implement the error interface so that it can be
+// used as an error.
+type Conflict struct {
+	Manager string
+	Path    fieldpath.Path
+}
+
+// Conflict is an error.
+var _ error = Conflict{}
+
+// Error formats the conflict as an error.
+func (c Conflict) Error() string {
+	return fmt.Sprintf("conflict with %q: %v", c.Manager, c.Path)
+}
+
+// Conflicts accumulates multiple conflicts and aggregates them by managers.
+type Conflicts []Conflict
+
+var _ error = Conflicts{}
+
+// Error prints the list of conflicts, grouped by sorted managers.
+func (conflicts Conflicts) Error() string {
+	if len(conflicts) == 1 {
+		return conflicts[0].Error()
+	}
+
+	m := map[string][]fieldpath.Path{}
+	for _, conflict := range conflicts {
+		m[conflict.Manager] = append(m[conflict.Manager], conflict.Path)
+	}
+
+	managers := []string{}
+	for manager := range m {
+		managers = append(managers, manager)
+	}
+
+	// Print conflicts by sorted managers.
+	sort.Strings(managers)
+
+	messages := []string{}
+	for _, manager := range managers {
+		messages = append(messages, fmt.Sprintf("conflicts with %q:", manager))
+		for _, path := range m[manager] {
+			messages = append(messages, fmt.Sprintf("- %v", path))
+		}
+	}
+	return strings.Join(messages, "\n")
+}
+
+// ConflictsFromManagers creates a list of conflicts given Managers sets.
+func ConflictsFromManagers(sets fieldpath.ManagedFields) Conflicts {
+	conflicts := []Conflict{}
+
+	for manager, set := range sets {
+		set.Iterate(func(p fieldpath.Path) {
+			conflicts = append(conflicts, Conflict{
+				Manager: manager,
+				Path:    p,
+			})
+		})
+	}
+
+	return conflicts
+}