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372 lines
11 KiB
372 lines
11 KiB
// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package syncmap provides a concurrent map implementation.
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// It is a prototype for a proposed addition to the sync package
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// in the standard library.
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// (https://golang.org/issue/18177)
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package syncmap
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import (
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"sync"
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"sync/atomic"
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"unsafe"
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)
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// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
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// It is safe for multiple goroutines to call a Map's methods concurrently.
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//
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// The zero Map is valid and empty.
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//
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// A Map must not be copied after first use.
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type Map struct {
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mu sync.Mutex
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// read contains the portion of the map's contents that are safe for
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// concurrent access (with or without mu held).
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//
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// The read field itself is always safe to load, but must only be stored with
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// mu held.
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//
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// Entries stored in read may be updated concurrently without mu, but updating
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// a previously-expunged entry requires that the entry be copied to the dirty
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// map and unexpunged with mu held.
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read atomic.Value // readOnly
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// dirty contains the portion of the map's contents that require mu to be
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// held. To ensure that the dirty map can be promoted to the read map quickly,
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// it also includes all of the non-expunged entries in the read map.
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//
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// Expunged entries are not stored in the dirty map. An expunged entry in the
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// clean map must be unexpunged and added to the dirty map before a new value
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// can be stored to it.
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//
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// If the dirty map is nil, the next write to the map will initialize it by
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// making a shallow copy of the clean map, omitting stale entries.
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dirty map[interface{}]*entry
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// misses counts the number of loads since the read map was last updated that
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// needed to lock mu to determine whether the key was present.
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//
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// Once enough misses have occurred to cover the cost of copying the dirty
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// map, the dirty map will be promoted to the read map (in the unamended
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// state) and the next store to the map will make a new dirty copy.
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misses int
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}
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// readOnly is an immutable struct stored atomically in the Map.read field.
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type readOnly struct {
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m map[interface{}]*entry
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amended bool // true if the dirty map contains some key not in m.
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}
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// expunged is an arbitrary pointer that marks entries which have been deleted
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// from the dirty map.
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var expunged = unsafe.Pointer(new(interface{}))
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// An entry is a slot in the map corresponding to a particular key.
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type entry struct {
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// p points to the interface{} value stored for the entry.
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//
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// If p == nil, the entry has been deleted and m.dirty == nil.
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//
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// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
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// is missing from m.dirty.
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//
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// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
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// != nil, in m.dirty[key].
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//
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// An entry can be deleted by atomic replacement with nil: when m.dirty is
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// next created, it will atomically replace nil with expunged and leave
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// m.dirty[key] unset.
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//
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// An entry's associated value can be updated by atomic replacement, provided
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// p != expunged. If p == expunged, an entry's associated value can be updated
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// only after first setting m.dirty[key] = e so that lookups using the dirty
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// map find the entry.
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p unsafe.Pointer // *interface{}
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}
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func newEntry(i interface{}) *entry {
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return &entry{p: unsafe.Pointer(&i)}
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}
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// Load returns the value stored in the map for a key, or nil if no
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// value is present.
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// The ok result indicates whether value was found in the map.
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func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
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read, _ := m.read.Load().(readOnly)
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e, ok := read.m[key]
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if !ok && read.amended {
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m.mu.Lock()
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// Avoid reporting a spurious miss if m.dirty got promoted while we were
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// blocked on m.mu. (If further loads of the same key will not miss, it's
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// not worth copying the dirty map for this key.)
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read, _ = m.read.Load().(readOnly)
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e, ok = read.m[key]
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if !ok && read.amended {
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e, ok = m.dirty[key]
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// Regardless of whether the entry was present, record a miss: this key
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// will take the slow path until the dirty map is promoted to the read
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// map.
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m.missLocked()
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}
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m.mu.Unlock()
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}
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if !ok {
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return nil, false
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}
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return e.load()
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}
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func (e *entry) load() (value interface{}, ok bool) {
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p := atomic.LoadPointer(&e.p)
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if p == nil || p == expunged {
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return nil, false
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}
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return *(*interface{})(p), true
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}
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// Store sets the value for a key.
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func (m *Map) Store(key, value interface{}) {
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read, _ := m.read.Load().(readOnly)
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if e, ok := read.m[key]; ok && e.tryStore(&value) {
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return
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}
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m.mu.Lock()
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read, _ = m.read.Load().(readOnly)
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if e, ok := read.m[key]; ok {
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if e.unexpungeLocked() {
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// The entry was previously expunged, which implies that there is a
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// non-nil dirty map and this entry is not in it.
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m.dirty[key] = e
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}
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e.storeLocked(&value)
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} else if e, ok := m.dirty[key]; ok {
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e.storeLocked(&value)
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} else {
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if !read.amended {
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// We're adding the first new key to the dirty map.
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// Make sure it is allocated and mark the read-only map as incomplete.
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m.dirtyLocked()
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m.read.Store(readOnly{m: read.m, amended: true})
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}
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m.dirty[key] = newEntry(value)
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}
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m.mu.Unlock()
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}
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// tryStore stores a value if the entry has not been expunged.
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//
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// If the entry is expunged, tryStore returns false and leaves the entry
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// unchanged.
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func (e *entry) tryStore(i *interface{}) bool {
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p := atomic.LoadPointer(&e.p)
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if p == expunged {
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return false
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}
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for {
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if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
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return true
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}
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p = atomic.LoadPointer(&e.p)
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if p == expunged {
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return false
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}
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}
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}
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// unexpungeLocked ensures that the entry is not marked as expunged.
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//
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// If the entry was previously expunged, it must be added to the dirty map
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// before m.mu is unlocked.
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func (e *entry) unexpungeLocked() (wasExpunged bool) {
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return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
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}
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// storeLocked unconditionally stores a value to the entry.
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//
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// The entry must be known not to be expunged.
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func (e *entry) storeLocked(i *interface{}) {
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atomic.StorePointer(&e.p, unsafe.Pointer(i))
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}
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// LoadOrStore returns the existing value for the key if present.
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// Otherwise, it stores and returns the given value.
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// The loaded result is true if the value was loaded, false if stored.
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func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
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// Avoid locking if it's a clean hit.
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read, _ := m.read.Load().(readOnly)
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if e, ok := read.m[key]; ok {
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actual, loaded, ok := e.tryLoadOrStore(value)
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if ok {
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return actual, loaded
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}
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}
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m.mu.Lock()
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read, _ = m.read.Load().(readOnly)
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if e, ok := read.m[key]; ok {
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if e.unexpungeLocked() {
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m.dirty[key] = e
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}
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actual, loaded, _ = e.tryLoadOrStore(value)
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} else if e, ok := m.dirty[key]; ok {
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actual, loaded, _ = e.tryLoadOrStore(value)
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m.missLocked()
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} else {
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if !read.amended {
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// We're adding the first new key to the dirty map.
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// Make sure it is allocated and mark the read-only map as incomplete.
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m.dirtyLocked()
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m.read.Store(readOnly{m: read.m, amended: true})
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}
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m.dirty[key] = newEntry(value)
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actual, loaded = value, false
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}
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m.mu.Unlock()
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return actual, loaded
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}
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// tryLoadOrStore atomically loads or stores a value if the entry is not
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// expunged.
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//
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// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
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// returns with ok==false.
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func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {
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p := atomic.LoadPointer(&e.p)
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if p == expunged {
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return nil, false, false
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}
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if p != nil {
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return *(*interface{})(p), true, true
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}
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// Copy the interface after the first load to make this method more amenable
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// to escape analysis: if we hit the "load" path or the entry is expunged, we
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// shouldn't bother heap-allocating.
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ic := i
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for {
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if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
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return i, false, true
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}
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p = atomic.LoadPointer(&e.p)
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if p == expunged {
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return nil, false, false
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}
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if p != nil {
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return *(*interface{})(p), true, true
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}
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}
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}
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// Delete deletes the value for a key.
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func (m *Map) Delete(key interface{}) {
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read, _ := m.read.Load().(readOnly)
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e, ok := read.m[key]
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if !ok && read.amended {
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m.mu.Lock()
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read, _ = m.read.Load().(readOnly)
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e, ok = read.m[key]
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if !ok && read.amended {
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delete(m.dirty, key)
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}
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m.mu.Unlock()
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}
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if ok {
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e.delete()
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}
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}
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func (e *entry) delete() (hadValue bool) {
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for {
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p := atomic.LoadPointer(&e.p)
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if p == nil || p == expunged {
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return false
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}
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if atomic.CompareAndSwapPointer(&e.p, p, nil) {
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return true
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}
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}
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}
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// Range calls f sequentially for each key and value present in the map.
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// If f returns false, range stops the iteration.
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//
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// Range does not necessarily correspond to any consistent snapshot of the Map's
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// contents: no key will be visited more than once, but if the value for any key
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// is stored or deleted concurrently, Range may reflect any mapping for that key
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// from any point during the Range call.
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//
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// Range may be O(N) with the number of elements in the map even if f returns
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// false after a constant number of calls.
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func (m *Map) Range(f func(key, value interface{}) bool) {
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// We need to be able to iterate over all of the keys that were already
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// present at the start of the call to Range.
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// If read.amended is false, then read.m satisfies that property without
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// requiring us to hold m.mu for a long time.
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read, _ := m.read.Load().(readOnly)
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if read.amended {
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// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
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// (assuming the caller does not break out early), so a call to Range
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// amortizes an entire copy of the map: we can promote the dirty copy
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// immediately!
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m.mu.Lock()
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read, _ = m.read.Load().(readOnly)
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if read.amended {
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read = readOnly{m: m.dirty}
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m.read.Store(read)
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m.dirty = nil
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m.misses = 0
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}
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m.mu.Unlock()
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}
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for k, e := range read.m {
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v, ok := e.load()
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if !ok {
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continue
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}
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if !f(k, v) {
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break
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}
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}
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}
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func (m *Map) missLocked() {
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m.misses++
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if m.misses < len(m.dirty) {
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return
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}
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m.read.Store(readOnly{m: m.dirty})
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m.dirty = nil
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m.misses = 0
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}
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func (m *Map) dirtyLocked() {
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if m.dirty != nil {
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return
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}
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read, _ := m.read.Load().(readOnly)
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m.dirty = make(map[interface{}]*entry, len(read.m))
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for k, e := range read.m {
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if !e.tryExpungeLocked() {
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m.dirty[k] = e
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}
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}
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}
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func (e *entry) tryExpungeLocked() (isExpunged bool) {
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p := atomic.LoadPointer(&e.p)
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for p == nil {
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if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
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return true
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}
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p = atomic.LoadPointer(&e.p)
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}
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return p == expunged
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}
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