go设计之sync.Map

type Map struct {
	mu Mutex

	// read contains the portion of the map's contents that are safe for
	// concurrent access (with or without mu held).
	//
	// The read field itself is always safe to load, but must only be stored with
	// mu held.
	//
	// Entries stored in read may be updated concurrently without mu, but updating
	// a previously-expunged entry requires that the entry be copied to the dirty
	// map and unexpunged with mu held.
	read atomic.Value // readOnly

	// dirty contains the portion of the map's contents that require mu to be
	// held. To ensure that the dirty map can be promoted to the read map quickly,
	// it also includes all of the non-expunged entries in the read map.
	//
	// Expunged entries are not stored in the dirty map. An expunged entry in the
	// clean map must be unexpunged and added to the dirty map before a new value
	// can be stored to it.
	//
	// If the dirty map is nil, the next write to the map will initialize it by
	// making a shallow copy of the clean map, omitting stale entries.
	dirty map[any]*entry

	// misses counts the number of loads since the read map was last updated that
	// needed to lock mu to determine whether the key was present.
	//
	// Once enough misses have occurred to cover the cost of copying the dirty
	// map, the dirty map will be promoted to the read map (in the unamended
	// state) and the next store to the map will make a new dirty copy.
	misses int
}

// Load returns the value stored in the map for a key, or nil if no
// value is present.
// The ok result indicates whether value was found in the map.
func (m *Map) Load(key any) (value any, 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
}

// Store sets the value for a key.
func (m *Map) Store(key, value any) {
	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()
}

// tryStore stores a value if the entry has not been expunged.
//
// If the entry is expunged, tryStore returns false and leaves the entry
// unchanged.
func (e *entry) tryStore(i *any) bool {
	for {
		p := atomic.LoadPointer(&e.p)
		if p == expunged {
			return false
		}
		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
			return true
		}
	}
}

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

func (m *Map) LoadAndDelete(key any) (value any, 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
}

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

func (m *Map) Range(f func(key, value any) bool) {
	// We need to be able to iterate over all of the keys that were already
	// present at the start of the call to Range.
	// If read.amended is false, then read.m satisfies that property without
	// requiring us to hold m.mu for a long time.
	read, _ := m.read.Load().(readOnly)
	if read.amended {
		// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
		// (assuming the caller does not break out early), so a call to Range
		// amortizes an entire copy of the map: we can promote the dirty copy
		// immediately!
		m.mu.Lock()
		read, _ = m.read.Load().(readOnly)
		if read.amended {
			read = readOnly{m: m.dirty}
			m.read.Store(read)
			m.dirty = nil
			m.misses = 0
		}
		m.mu.Unlock()
	}

	for k, e := range read.m {
		v, ok := e.load()
		if !ok {
			continue
		}
		if !f(k, v) {
			break
		}
	}
}