【Golang语言社区】源码篇--sync包map

package sync

import (

"sync/atomic"

"unsafe"

)

// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.

// It is safe for multiple goroutines to call a Map's methods concurrently.

//

// The zero Map is valid and empty.

//

// A Map must not be copied after first use.

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[interface{}]*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

}

// readOnly is an immutable struct stored atomically in the Map.read field.

type readOnly struct {

m map[interface{}]*entry

amended bool // true if the dirty map contains some key not in m.

}

// expunged is an arbitrary pointer that marks entries which have been deleted

// from the dirty map.

var expunged = unsafe.Pointer(new(interface{}))

// An entry is a slot in the map corresponding to a particular key.

type entry struct {

// p points to the interface{} value stored for the entry.

//

// If p == nil, the entry has been deleted and m.dirty == nil.

//

// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry

// is missing from m.dirty.

//

// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty

// != nil, in m.dirty[key].

//

// An entry can be deleted by atomic replacement with nil: when m.dirty is

// next created, it will atomically replace nil with expunged and leave

// m.dirty[key] unset.

//

// An entry's associated value can be updated by atomic replacement, provided

// p != expunged. If p == expunged, an entry's associated value can be updated

// only after first setting m.dirty[key] = e so that lookups using the dirty

// map find the entry.

p unsafe.Pointer // *interface{}

}

func newEntry(i interface{}) *entry {

return &entry{p: unsafe.Pointer(&i)}

}

// 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 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 (e *entry) load() (value interface{}, ok bool) {

p := atomic.LoadPointer(&e.p)

if p == nil || p == expunged {

return nil, false

}

return *(*interface{})(p), true

}

// Store sets the value for a key.

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()

}

// 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 *interface{}) bool {

p := atomic.LoadPointer(&e.p)

if p == expunged {

return false

}

for {

if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {

return true

}

p = atomic.LoadPointer(&e.p)

if p == expunged {

return false

}

}

}

// unexpungeLocked ensures that the entry is not marked as expunged.

//

// If the entry was previously expunged, it must be added to the dirty map

// before m.mu is unlocked.

func (e *entry) unexpungeLocked() (wasExpunged bool) {

return atomic.CompareAndSwapPointer(&e.p, expunged, nil)

}

// storeLocked unconditionally stores a value to the entry.

//

// The entry must be known not to be expunged.

func (e *entry) storeLocked(i *interface{}) {

atomic.StorePointer(&e.p, unsafe.Pointer(i))

}

// LoadOrStore returns the existing value for the key if present.

// Otherwise, it stores and returns the given value.

// The loaded result is true if the value was loaded, false if stored.

func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {

// Avoid locking if it's a clean hit.

read, _ := m.read.Load().(readOnly)

if e, ok := read.m[key]; ok {

actual, loaded, ok := e.tryLoadOrStore(value)

if ok {

return actual, loaded

}

}

m.mu.Lock()

read, _ = m.read.Load().(readOnly)

if e, ok := read.m[key]; ok {

if e.unexpungeLocked() {

m.dirty[key] = e

}

actual, loaded, _ = e.tryLoadOrStore(value)

} else if e, ok := m.dirty[key]; ok {

actual, loaded, _ = e.tryLoadOrStore(value)

m.missLocked()

} 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)

actual, loaded = value, false

}

m.mu.Unlock()

return actual, loaded

}

// tryLoadOrStore atomically loads or stores a value if the entry is not

// expunged.

//

// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and

// returns with ok==false.

func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {

p := atomic.LoadPointer(&e.p)

if p == expunged {

return nil, false, false

}

if p != nil {

return *(*interface{})(p), true, true

}

// Copy the interface after the first load to make this method more amenable

// to escape analysis: if we hit the "load" path or the entry is expunged, we

// shouldn't bother heap-allocating.

ic := i

for {

if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {

return i, false, true

}

p = atomic.LoadPointer(&e.p)

if p == expunged {

return nil, false, false

}

if p != nil {

return *(*interface{})(p), true, true

}

}

}

// Delete deletes the value for a key.

func (m *Map) Delete(key interface{}) {

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 {

delete(m.dirty, key)

}

m.mu.Unlock()

}

if ok {

e.delete()

}

}

func (e *entry) delete() (hadValue bool) {

for {

p := atomic.LoadPointer(&e.p)

if p == nil || p == expunged {

return false

}

if atomic.CompareAndSwapPointer(&e.p, p, nil) {

return true

}

}

}

// Range calls f sequentially for each key and value present in the map.

// If f returns false, range stops the iteration.

//

// Range does not necessarily correspond to any consistent snapshot of the Map's

// contents: no key will be visited more than once, but if the value for any key

// is stored or deleted concurrently, Range may reflect any mapping for that key

// from any point during the Range call.

//

// Range may be O(N) with the number of elements in the map even if f returns

// false after a constant number of calls.

func (m *Map) Range(f func(key, value interface{}) 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

}

}

}

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 (m *Map) dirtyLocked() {

if m.dirty != nil {

return

}

read, _ := m.read.Load().(readOnly)

m.dirty = make(map[interface{}]*entry, len(read.m))

for k, e := range read.m {

if !e.tryExpungeLocked() {

m.dirty[k] = e

}

}

}

func (e *entry) tryExpungeLocked() (isExpunged bool) {

p := atomic.LoadPointer(&e.p)

for p == nil {

if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {

return true

}

p = atomic.LoadPointer(&e.p)

}

return p == expunged

}