navigation: Cache and copy Menu for sorting

.Site.Menus is mutated when it is sorted for now and this causes concurrency problem (#7594)
In this patch, each related sort function copies Menu before sorting to prevent
race condition.

Pages already have such a sort and cache logic and this patch is identical to it.

Closes #7594

navigation/menu_cache.go

@@ -0,0 +1,113 @@
+// Copyright 2021 The Hugo Authors. All rights reserved.
+//
+// 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 navigation
+
+import (
+	"sync"
+)
+
+type menuCacheEntry struct {
+	in  []Menu
+	out Menu
+}
+
+func (entry menuCacheEntry) matches(menuList []Menu) bool {
+	if len(entry.in) != len(menuList) {
+		return false
+	}
+	for i, m := range menuList {
+		if !menuEqual(m, entry.in[i]) {
+			return false
+		}
+	}
+
+	return true
+}
+
+func newMenuCache() *menuCache {
+	return &menuCache{m: make(map[string][]menuCacheEntry)}
+}
+
+func (c *menuCache) clear() {
+	c.Lock()
+	defer c.Unlock()
+	c.m = make(map[string][]menuCacheEntry)
+}
+
+type menuCache struct {
+	sync.RWMutex
+	m map[string][]menuCacheEntry
+}
+
+func menuEqual(m1, m2 Menu) bool {
+	if m1 == nil && m2 == nil {
+		return true
+	}
+
+	if m1 == nil || m2 == nil {
+		return false
+	}
+
+	if len(m1) != len(m2) {
+		return false
+	}
+
+	if len(m1) == 0 {
+		return true
+	}
+
+	for i := 0; i < len(m1); i++ {
+		if m1[i] != m2[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func (c *menuCache) get(key string, apply func(m Menu), menuLists ...Menu) (Menu, bool) {
+	return c.getP(key, func(m *Menu) {
+		if apply != nil {
+			apply(*m)
+		}
+	}, menuLists...)
+}
+
+func (c *menuCache) getP(key string, apply func(m *Menu), menuLists ...Menu) (Menu, bool) {
+	c.Lock()
+	defer c.Unlock()
+
+	if cached, ok := c.m[key]; ok {
+		for _, entry := range cached {
+			if entry.matches(menuLists) {
+				return entry.out, true
+			}
+		}
+	}
+
+	m := menuLists[0]
+	menuCopy := append(Menu(nil), m...)
+
+	if apply != nil {
+		apply(&menuCopy)
+	}
+
+	entry := menuCacheEntry{in: menuLists, out: menuCopy}
+	if v, ok := c.m[key]; ok {
+		c.m[key] = append(v, entry)
+	} else {
+		c.m[key] = []menuCacheEntry{entry}
+	}
+
+	return menuCopy, false
+}