--- /dev/null
+// 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.
+
+// +build !go1.9
+
+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
+}
projects / dotfiles / .git / blobdiff