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+// 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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+ return actual, loaded
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+}
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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Daniel Nephin