syncmap: use type alias for Map

sync.Map was introduced in Go 1.9, this change updates the syncmap package to use a type alias to sync.Map for 1.9 and later. It also updates the comments to reflect this change. Fixes golang/go#33867 Change-Id: Ia58ad2bc409f8fcb8a7539ef165148315da5cfb1 Reviewed-on: https://go-review.googlesource.com/c/sync/+/192737 Reviewed-by: Bryan C. Mills <bcmills@google.com> Run-TryBot: Bryan C. Mills <bcmills@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: erutherford

syncmap/go19.go

@@ -0,0 +1,17 @@
+// Copyright 2019 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.
+
+// +build go1.9
+
+package syncmap
+
+import "sync" // home to the standard library's sync.map implementation as of Go 1.9
+
+// 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 = sync.Map

syncmap/map.go

@@ -1,372 +1,8 @@
-// Copyright 2016 The Go Authors. All rights reserved.
+// Copyright 2019 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 syncmap provides a concurrent map implementation.
-// It is a prototype for a proposed addition to the sync package
-// in the standard library.
-// (https://golang.org/issue/18177)
+// This was the prototype for sync.Map which was added to the standard library's
+// sync package in Go 1.9. https://golang.org/pkg/sync/#Map.
 package syncmap
-
-import (
-"sync"
-"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 sync.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
-}