golang map 代码
文件路径：/src/sync/map.go
/ Copyright 2016 The Go Authors. All rights reserved.
/ Use of this source code is governed by a BSD-style
/ license that can be found in the LICENSE file.
package sync
import (
"sync/atomic"
"unsafe"
)
/ Map is like a Go map[interface{}]interface{} but is safe for concurrent use
/ by multiple goroutines without additional locking or coordination.
/ Loads, stores, and deletes run in amortized constant time.
/
/ The Map type is specialized. Most code should use a plain Go map instead,
/ with separate locking or coordination, for better type safety and to make it
/ easier to maintain other invariants along with the map content.
/
/ The Map type is optimized for two common use cases: (1) when the entry for a given
/ key is only ever written once but read many times, as in caches that only grow,
/ or (2) when multiple goroutines read, write, and overwrite entries for disjoint
/ sets of keys. In these two cases, use of a Map may significantly reduce lock
/ contention compared to a Go map paired with a separate Mutex or RWMutex.
/
/ The zero Map is empty and ready for use. A Map must not be copied after first use.
/
/ In the terminology of the Go memory model, Map arranges that a write operation
/ “synchronizes before” any read operation that observes the effect of the write, where
/ read and write operations are defined as follows.
/ Load, LoadAndDelete, LoadOrStore are read operations;
/ Delete, LoadAndDelete, and Store are write operations;
/ and LoadOrStore is a write operation when it returns loaded set to false.
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
}
/ readOnly is an immutable struct stored atomically in the Map.read field.
type readOnly struct {
m map[any]*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(any))
/ 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 either m.dirty == nil or
/ m.dirty[key] is e.
/
/ 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 any) *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 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 (e *entry) load() (value any, ok bool) {
p := atomic.LoadPointer(&e.p)
if p == nil || p == expunged {
return nil, false
}
return *(*any)(p), true
}
/ 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
}
}
}
/ 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 *any) {
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 any) (actual any, 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 any) (actual any, loaded, ok bool) {
p := atomic.LoadPointer(&e.p)
if p == expunged {
return nil, false, false
}
if p != nil {
return *(*any)(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 *(*any)(p), true, true
}
}
}
/ LoadAndDelete deletes the value for a key, returning the previous value if any.
/ The loaded result reports whether the key was present.
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
}
/ Delete deletes the value for a key.
func (m *Map) Delete(key any) {
m.LoadAndDelete(key)
}
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
}
}
}
/ 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 (including by f), Range may reflect any
/ mapping for that key from any point during the Range call. Range does not
/ block other methods on the receiver; even f itself may call any method on m.
/
/ 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 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
}
}
}
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[any]*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
}