Go sync.Map 源码实现

type syncMap struct{
	lock sync.RWMutex
	m map[interface{}]interface{}
}

type Map struct {
    // 带锁写dirty结构使用
	mu Mutex

    // read域，原子性+可见性
	read atomic.Value // readOnly

    // 全部map数据，改数据都体现到这里
	dirty map[interface{}]*entry

	// 读操作超过miss次数，则把dirty map赋值给read域，dirty置为nil
	misses int
}

// immutable结构，注意map的value是指针，指针可以改
type readOnly struct {
	m       map[interface{}]*entry
	amended bool // 如果dirty有，而read.m没有 则为true
}

// 删除状态 map的value值
var expunged = unsafe.Pointer(new(interface{}))

// map的value指针具体值，map的value是*entry
type entry struct {
	p unsafe.Pointer // *interface{}
}

func newEntry(i interface{}) *entry {
	return &entry{p: unsafe.Pointer(&i)}
}

func (m *Map) Store(key, value interface{}) {
	read, _ := m.read.Load().(readOnly)
	if e, ok := read.m[key]; ok && e.tryStore(&value) {
		return
	}

	m.mu.Lock()
	read, _ = m.read.Load().(readOnly)
	if e, ok := read.m[key]; ok {
		if e.unexpungeLocked() {
			// The entry was previously expunged, which implies that there is a
			// non-nil dirty map and this entry is not in it.
			m.dirty[key] = e
		}
		e.storeLocked(&value)
	} else if e, ok := m.dirty[key]; ok {
		e.storeLocked(&value)
	} else {
		if !read.amended {
			// We're adding the first new key to the dirty map.
			// Make sure it is allocated and mark the read-only map as incomplete.
			m.dirtyLocked()
			m.read.Store(readOnly{m: read.m, amended: true})
		}
		m.dirty[key] = newEntry(value)
	}
	m.mu.Unlock()
}

func (e *entry) tryStore(i *interface{}) bool {
	for {
		p := atomic.LoadPointer(&e.p)
		if p == expunged {
			return false
		}
		// CAS操作替换value
		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
			return true
		}
	}
}

func (m *Map) dirtyLocked() {
	if m.dirty != nil {
		return
	}

	read, _ := m.read.Load().(readOnly)
	// dirty重置，从read拷贝过来
	m.dirty = make(map[interface{}]*entry, len(read.m))
	for k, e := range read.m {
	    // 去掉删除态的value 这里有nil和expunged的区别
		if !e.tryExpungeLocked() {
			m.dirty[k] = e
		}
	}
}

func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
	read, _ := m.read.Load().(readOnly)
	e, ok := read.m[key]
	if !ok && read.amended {
		m.mu.Lock()
		// Avoid reporting a spurious miss if m.dirty got promoted while we were
		// blocked on m.mu. (If further loads of the same key will not miss, it's
		// not worth copying the dirty map for this key.)
		read, _ = m.read.Load().(readOnly)
		e, ok = read.m[key]
		if !ok && read.amended {
			e, ok = m.dirty[key]
			// Regardless of whether the entry was present, record a miss: this key
			// will take the slow path until the dirty map is promoted to the read
			// map.
			m.missLocked()
		}
		m.mu.Unlock()
	}
	if !ok {
		return nil, false
	}
	return e.load()
}


func (m *Map) missLocked() {
	m.misses++
	if m.misses < len(m.dirty) {
		return
	}
	m.read.Store(readOnly{m: m.dirty})
	m.dirty = nil
	m.misses = 0
}

func (e *entry) load() (value interface{}, ok bool) {
	p := atomic.LoadPointer(&e.p)
	if p == nil || p == expunged {
		return nil, false
	}
	return *(*interface{})(p), true
}

func (m *Map) LoadAndDelete(key interface{}) (value interface{}, loaded bool) {
	read, _ := m.read.Load().(readOnly)
	e, ok := read.m[key]
	if !ok && read.amended {
		m.mu.Lock()
		read, _ = m.read.Load().(readOnly)
		e, ok = read.m[key]
		if !ok && read.amended {
			e, ok = m.dirty[key]
			delete(m.dirty, key)
			// Regardless of whether the entry was present, record a miss: this key
			// will take the slow path until the dirty map is promoted to the read
			// map.
			m.missLocked()
		}
		m.mu.Unlock()
	}
	if ok {
		return e.delete()
	}
	return nil, false
}

// Delete deletes the value for a key.
func (m *Map) Delete(key interface{}) {
	m.LoadAndDelete(key)
}

func (e *entry) delete() (value interface{}, ok bool) {
	for {
		p := atomic.LoadPointer(&e.p)
		if p == nil || p == expunged {
			return nil, false
		}
		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
			return *(*interface{})(p), true
		}
	}
}