--- /dev/null
+// Copyright 2014 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 atomic
+
+import (
+ "unsafe"
+)
+
+// A Value provides an atomic load and store of a consistently typed value.
+// Values can be created as part of other data structures.
+// The zero value for a Value returns nil from Load.
+// Once Store has been called, a Value must not be copied.
+type Value struct {
+ v interface{}
+}
+
+// ifaceWords is interface{} internal representation.
+type ifaceWords struct {
+ typ unsafe.Pointer
+ data unsafe.Pointer
+}
+
+// Load returns the value set by the most recent Store.
+// It returns nil if there has been no call to Store for this Value.
+func (v *Value) Load() (x interface{}) {
+ vp := (*ifaceWords)(unsafe.Pointer(v))
+ typ := LoadPointer(&vp.typ)
+ if typ == nil || uintptr(typ) == ^uintptr(0) {
+ // First store not yet completed.
+ return nil
+ }
+ data := LoadPointer(&vp.data)
+ xp := (*ifaceWords)(unsafe.Pointer(&x))
+ xp.typ = typ
+ xp.data = data
+ if raceenabled {
+ raceAcquire(unsafe.Pointer(v))
+ }
+ return
+}
+
+// Store sets the value of the Value to v.
+// All calls to Store for a given Value must use values of the same concrete type.
+// Store of an inconsistent type panics, as does Store(nil).
+func (v *Value) Store(x interface{}) {
+ if x == nil {
+ panic("sync/atomic: store of nil value into Value")
+ }
+ if raceenabled {
+ raceReleaseMerge(unsafe.Pointer(v))
+ }
+ vp := (*ifaceWords)(unsafe.Pointer(v))
+ xp := (*ifaceWords)(unsafe.Pointer(&x))
+ for {
+ typ := LoadPointer(&vp.typ)
+ if typ == nil {
+ // Attempt to start first store.
+ // Disable preemption so that other goroutines can use
+ // active spin wait to wait for completion; and so that
+ // GC does not see the fake type accidentally.
+ runtime_procPin()
+ if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
+ runtime_procUnpin()
+ continue
+ }
+ // Complete first store.
+ StorePointer(&vp.data, xp.data)
+ StorePointer(&vp.typ, xp.typ)
+ runtime_procUnpin()
+ return
+ }
+ if uintptr(typ) == ^uintptr(0) {
+ // First store in progress. Wait.
+ // Since we disable preemption around the first store,
+ // we can wait with active spinning.
+ continue
+ }
+ // First store completed. Check type and overwrite data.
+ if typ != xp.typ {
+ panic("sync/atomic: store of inconsistently typed value into Value")
+ }
+ StorePointer(&vp.data, xp.data)
+ return
+ }
+}
+
+// Disable/enable preemption, implemented in runtime.
+func runtime_procPin()
+func runtime_procUnpin()
--- /dev/null
+// Copyright 2014 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 atomic_test
+
+import (
+ "math/rand"
+ "runtime"
+ "sync"
+ . "sync/atomic"
+ "testing"
+ "time"
+)
+
+func TestValue(t *testing.T) {
+ var v Value
+ if v.Load() != nil {
+ t.Fatal("initial Value is not nil")
+ }
+ v.Store(42)
+ x := v.Load()
+ if xx, ok := x.(int); !ok || xx != 42 {
+ t.Fatalf("wrong value: got %+v, want 42", x)
+ }
+ v.Store(84)
+ x = v.Load()
+ if xx, ok := x.(int); !ok || xx != 84 {
+ t.Fatalf("wrong value: got %+v, want 84", x)
+ }
+}
+
+func TestValueLarge(t *testing.T) {
+ var v Value
+ v.Store("foo")
+ x := v.Load()
+ if xx, ok := x.(string); !ok || xx != "foo" {
+ t.Fatalf("wrong value: got %+v, want foo", x)
+ }
+ v.Store("barbaz")
+ x = v.Load()
+ if xx, ok := x.(string); !ok || xx != "barbaz" {
+ t.Fatalf("wrong value: got %+v, want barbaz", x)
+ }
+}
+
+func TestValuePanic(t *testing.T) {
+ const nilErr = "sync/atomic: store of nil value into Value"
+ const badErr = "sync/atomic: store of inconsistently typed value into Value"
+ var v Value
+ func() {
+ defer func() {
+ err := recover()
+ if err != nilErr {
+ t.Fatalf("inconsistent store panic: got '%v', want '%v'", err, nilErr)
+ }
+ }()
+ v.Store(nil)
+ }()
+ v.Store(42)
+ func() {
+ defer func() {
+ err := recover()
+ if err != badErr {
+ t.Fatalf("inconsistent store panic: got '%v', want '%v'", err, badErr)
+ }
+ }()
+ v.Store("foo")
+ }()
+ func() {
+ defer func() {
+ err := recover()
+ if err != nilErr {
+ t.Fatalf("inconsistent store panic: got '%v', want '%v'", err, nilErr)
+ }
+ }()
+ v.Store(nil)
+ }()
+}
+
+func TestValueConcurrent(t *testing.T) {
+ tests := [][]interface{}{
+ {uint16(0), ^uint16(0), uint16(1 + 2<<8), uint16(3 + 4<<8)},
+ {uint32(0), ^uint32(0), uint32(1 + 2<<16), uint32(3 + 4<<16)},
+ {uint64(0), ^uint64(0), uint64(1 + 2<<32), uint64(3 + 4<<32)},
+ {complex(0, 0), complex(1, 2), complex(3, 4), complex(5, 6)},
+ }
+ p := 4 * runtime.GOMAXPROCS(0)
+ for _, test := range tests {
+ var v Value
+ done := make(chan bool)
+ for i := 0; i < p; i++ {
+ go func() {
+ r := rand.New(rand.NewSource(rand.Int63()))
+ loop:
+ for j := 0; j < 1e5; j++ {
+ x := test[r.Intn(len(test))]
+ v.Store(x)
+ x = v.Load()
+ for _, x1 := range test {
+ if x == x1 {
+ continue loop
+ }
+ }
+ t.Logf("loaded unexpected value %+v, want %+v", x, test)
+ done <- false
+ }
+ done <- true
+ }()
+ }
+ for i := 0; i < p; i++ {
+ if !<-done {
+ t.FailNow()
+ }
+ }
+ }
+}
+
+func BenchmarkValueRead(b *testing.B) {
+ var v Value
+ v.Store(new(int))
+ b.RunParallel(func(pb *testing.PB) {
+ for pb.Next() {
+ x := v.Load().(*int)
+ if *x != 0 {
+ b.Fatalf("wrong value: got %v, want 0", *x)
+ }
+ }
+ })
+}
+
+// The following example shows how to use Value for periodic program config updates
+// and propagation of the changes to worker goroutines.
+func ExampleValue_config() {
+ var config Value // holds current server configuration
+ // Create initial config value and store into config.
+ config.Store(loadConfig())
+ go func() {
+ // Reload config every 10 seconds
+ // and update config value with the new version.
+ for {
+ time.Sleep(10 * time.Second)
+ config.Store(loadConfig())
+ }
+ }()
+ // Create worker goroutines that handle incoming requests
+ // using the latest config value.
+ for i := 0; i < 10; i++ {
+ go func() {
+ for r := range requests() {
+ c := config.Load()
+ // Handle request r using config c.
+ _, _ = r, c
+ }
+ }()
+ }
+}
+
+func loadConfig() map[string]string {
+ return make(map[string]string)
+}
+
+func requests() chan int {
+ return make(chan int)
+}
+
+// The following example shows how to maintain a scalable frequently read,
+// but infrequently updated data structure using copy-on-write idiom.
+func ExampleValue_readMostly() {
+ type Map map[string]string
+ var m Value
+ m.Store(make(Map))
+ var mu sync.Mutex // used only by writers
+ // read function can be used to read the data without further synchronization
+ read := func(key string) (val string) {
+ m1 := m.Load().(Map)
+ return m1[key]
+ }
+ // insert function can be used to update the data without further synchronization
+ insert := func(key, val string) {
+ mu.Lock() // synchronize with other potential writers
+ defer mu.Unlock()
+ m1 := m.Load().(Map) // load current value of the data structure
+ m2 := make(Map) // create a new value
+ for k, v := range m1 {
+ m2[k] = v // copy all data from the current object to the new one
+ }
+ m2[key] = val // do the update that we need
+ m.Store(m2) // atomically replace the current object with the new one
+ // At this point all new readers start working with the new version.
+ // The old version will be garbage collected once the existing readers
+ // (if any) are done with it.
+ }
+ _, _ = read, insert
+}