golang学习之ngrok源码分析

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从去年开始就对go语言产生一点兴趣，总感觉java有时太过臃肿，是时候尝试一种新语言了。我看了几本关于go的书，不过看完就忘了，最近开发一个微信公众号项目，使用ngrok做内网穿透，顺道研究一下ngrok源码，巩固一下go语言。

网上有两篇文章已对ngrok原理讲得很清晰了：

https://blog.messyidea.com/archives/41/

https://tonybai.com/2015/05/14/ngrok-source-intro/

我试图记录一些细节，然后实现在个简单的ngrok。

源码/ngrok/cache/lru.go,这是随手点开的一个文件，从名字可以看出它的用处：实现一个lru（Least Recently Used）缓存,在以往的项目中，我使用的本地缓存是guava提供的工具，很久前我还用过WeakReferenceMap作为一个简单缓存。以下是ngrok中的源码


type LRUCache struct {
   mu sync.Mutex

   // list & table of *entry objects
   list  *list.List
   table map[string]*list.Element

   // Our current size, in bytes. Obviously a gross simplification and low-grade
   // approximation.
   size uint64

   // How many bytes we are limiting the cache to.
   capacity uint64
}

type Item struct {
   Key   string
   Value Value
}

虽然算法很简单，但有几个地方还是值得把玩的。list是一个链表，存储的是item，它可以看成是一个具有优先级的队列，每次访问一个元素时，都会对将相应元素移至队首。这样当list长度超过capacity时，就可以移除队尾的元素。而table是用作索引，通过一个键查询缓存时，通过table得到list中一个element的引用。而list保存的item有一个key属性对应用table中的key,这样很方便对table做移除操作。

以前我想在java中用PriorityBolckingQueue,和ConcurrentHashMap实现一个类似的缓存（不仅是LRU缓存，它们组合实现多种缓存算法），但只想到用map保存键值对，而queue用来做优先队列（这种双向索引的方式我没想到）。说不定用JDK中的HashLinkedMap更容易实现LRU,因为他已同时具备链表和索引的功能。

完整代码如下：

// Copyright 2012, Google Inc. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// The implementation borrows heavily from SmallLRUCache (originally by Nathan
// Schrenk). The object maintains a doubly-linked list of elements in the
// When an element is accessed it is promoted to the head of the list, and when
// space is needed the element at the tail of the list (the least recently used
// element) is evicted.
package cache

import (
 "container/list"
 "encoding/gob"
 "fmt"
 "io"
 "os"
 "sync"
 "time"
)

type LRUCache struct {
 mu sync.Mutex

 // list & table of *entry objects
 list  *list.List
 table map[string]*list.Element

 // Our current size, in bytes. Obviously a gross simplification and low-grade
 // approximation.
 size uint64

 // How many bytes we are limiting the cache to.
 capacity uint64
}

// Values that go into LRUCache need to satisfy this interface.
type Value interface {
 Size() int
}

type Item struct {
 Key   string
 Value Value
}

type entry struct {
 key           string
 value         Value
 size          int
 time_accessed time.Time
}

func NewLRUCache(capacity uint64) *LRUCache {
 return &LRUCache{
     list:     list.New(),
     table:    make(map[string]*list.Element),
     capacity: capacity,
 }
}

func (lru *LRUCache) Get(key string) (v Value, ok bool) {
 lru.mu.Lock()
 defer lru.mu.Unlock()

 element := lru.table[key]
 if element == nil {
     return nil, false
 }
 lru.moveToFront(element)
 return element.Value.(*entry).value, true
}

func (lru *LRUCache) Set(key string, value Value) {
 lru.mu.Lock()
 defer lru.mu.Unlock()

 if element := lru.table[key]; element != nil {
     lru.updateInplace(element, value)
 } else {
     lru.addNew(key, value)
 }
}

func (lru *LRUCache) SetIfAbsent(key string, value Value) {
 lru.mu.Lock()
 defer lru.mu.Unlock()

 if element := lru.table[key]; element != nil {
     lru.moveToFront(element)
 } else {
     lru.addNew(key, value)
 }
}

func (lru *LRUCache) Delete(key string) bool {
 lru.mu.Lock()
 defer lru.mu.Unlock()

 element := lru.table[key]
 if element == nil {
     return false
 }

 lru.list.Remove(element)
 delete(lru.table, key)
 lru.size -= uint64(element.Value.(*entry).size)
 return true
}

func (lru *LRUCache) Clear() {
 lru.mu.Lock()
 defer lru.mu.Unlock()

 lru.list.Init()
 lru.table = make(map[string]*list.Element)
 lru.size = 0
}

func (lru *LRUCache) SetCapacity(capacity uint64) {
 lru.mu.Lock()
 defer lru.mu.Unlock()

 lru.capacity = capacity
 lru.checkCapacity()
}

func (lru *LRUCache) Stats() (length, size, capacity uint64, oldest time.Time) {
 lru.mu.Lock()
 defer lru.mu.Unlock()
 if lastElem := lru.list.Back(); lastElem != nil {
     oldest = lastElem.Value.(*entry).time_accessed
 }
 return uint64(lru.list.Len()), lru.size, lru.capacity, oldest
}

func (lru *LRUCache) StatsJSON() string {
 if lru == nil {
     return "{}"
 }
 l, s, c, o := lru.Stats()
 return fmt.Sprintf("{\"Length\": %v, \"Size\": %v, \"Capacity\": %v, \"OldestAccess\": \"%v\"}", l, s, c, o)
}

func (lru *LRUCache) Keys() []string {
 lru.mu.Lock()
 defer lru.mu.Unlock()

 keys := make([]string, 0, lru.list.Len())
 for e := lru.list.Front(); e != nil; e = e.Next() {
     keys = append(keys, e.Value.(*entry).key)
 }
 return keys
}

func (lru *LRUCache) Items() []Item {
 lru.mu.Lock()
 defer lru.mu.Unlock()

 items := make([]Item, 0, lru.list.Len())
 for e := lru.list.Front(); e != nil; e = e.Next() {
     v := e.Value.(*entry)
     items = append(items, Item{Key: v.key, Value: v.value})
 }
 return items
}

func (lru *LRUCache) SaveItems(w io.Writer) error {
 items := lru.Items()
 encoder := gob.NewEncoder(w)
 return encoder.Encode(items)
}

func (lru *LRUCache) SaveItemsToFile(path string) error {
 if wr, err := os.OpenFile(path, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0644); err != nil {
     return err
 } else {
     defer wr.Close()
     return lru.SaveItems(wr)
 }
}

func (lru *LRUCache) LoadItems(r io.Reader) error {
 items := make([]Item, 0)
 decoder := gob.NewDecoder(r)
 if err := decoder.Decode(&items); err != nil {
     return err
 }

 lru.mu.Lock()
 defer lru.mu.Unlock()
 for _, item := range items {
     // XXX: copied from Set()
     if element := lru.table[item.Key]; element != nil {
         lru.updateInplace(element, item.Value)
     } else {
         lru.addNew(item.Key, item.Value)
     }
 }

 return nil
}

func (lru *LRUCache) LoadItemsFromFile(path string) error {
 if rd, err := os.Open(path); err != nil {
     return err
 } else {
     defer rd.Close()
     return lru.LoadItems(rd)
 }
}

func (lru *LRUCache) updateInplace(element *list.Element, value Value) {
 valueSize := value.Size()
 sizeDiff := valueSize - element.Value.(*entry).size
 element.Value.(*entry).value = value
 element.Value.(*entry).size = valueSize
 lru.size += uint64(sizeDiff)
 lru.moveToFront(element)
 lru.checkCapacity()
}

func (lru *LRUCache) moveToFront(element *list.Element) {
 lru.list.MoveToFront(element)
 element.Value.(*entry).time_accessed = time.Now()
}

func (lru *LRUCache) addNew(key string, value Value) {
 newEntry := &entry{key, value, value.Size(), time.Now()}
 element := lru.list.PushFront(newEntry)
 lru.table[key] = element
 lru.size += uint64(newEntry.size)
 lru.checkCapacity()
}

func (lru *LRUCache) checkCapacity() {
 // Partially duplicated from Delete
 for lru.size > lru.capacity {
     delElem := lru.list.Back()
     delValue := delElem.Value.(*entry)
     lru.list.Remove(delElem)
     delete(lru.table, delValue.key)
     lru.size -= uint64(delValue.size)
 }
}

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