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future.go
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future.go
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package future
import (
"sync/atomic"
"unsafe"
)
const (
stateFree uint64 = iota
stateDoing
stateDone
)
const stateDelta = 1 << 32
const (
maskCounter = 1<<32 - 1
maskState = 1<<34 - 1
)
const flagLazy uint64 = 1 << 63
type state struct {
noCopy noCopy
state uint64 // high 30 bits are flags, mid 2 bits are state, low 32 bits are waiter count.
sema uint32
val interface{}
err error
f func() (interface{}, error)
stack unsafe.Pointer // *callback
}
type callback struct {
f func(interface{}, error)
next *callback
}
type Promise struct {
state state
}
type Future struct {
state *state
}
func (s *state) set(val interface{}, err error) {
for {
st := atomic.LoadUint64(&s.state)
if !isFree(st) {
panic("promise already satisfied")
}
if atomic.CompareAndSwapUint64(&s.state, st, st+stateDelta) {
s.val = val
s.err = err
st = atomic.AddUint64(&s.state, stateDelta)
for w := st & maskCounter; w > 0; w-- {
runtime_Semrelease(&s.sema, false, 0)
}
for {
head := (*callback)(atomic.LoadPointer(&s.stack))
if head == nil {
break
}
if atomic.CompareAndSwapPointer(&s.stack, unsafe.Pointer(head), unsafe.Pointer(head.next)) {
head.f(val, err)
head.next = nil
}
}
return
}
}
}
func (s *state) get() (interface{}, error) {
if atomic.LoadUint64(&s.state)&flagLazy == flagLazy {
for {
st := atomic.LoadUint64(&s.state)
if st&flagLazy != flagLazy {
break
}
if atomic.CompareAndSwapUint64(&s.state, st, st&(^flagLazy)) {
val, err := s.f()
s.set(val, err)
return val, err
}
}
}
for {
st := atomic.LoadUint64(&s.state)
if isDone(st) {
return s.val, s.err
}
if atomic.CompareAndSwapUint64(&s.state, st, st+1) {
runtime_Semacquire(&s.sema)
if !isDone(atomic.LoadUint64(&s.state)) {
panic("sync: notified before state has done")
}
return s.val, s.err
}
}
}
func (s *state) subscribe(cb func(interface{}, error)) {
newCb := &callback{f: cb}
for {
oldCb := (*callback)(atomic.LoadPointer(&s.stack))
if isDone(atomic.LoadUint64(&s.state)) {
cb(s.val, s.err)
return
}
newCb.next = oldCb
if atomic.CompareAndSwapPointer(&s.stack, unsafe.Pointer(oldCb), unsafe.Pointer(newCb)) {
for {
// Double-check the state to ensure the callback is not missed
if isDone(atomic.LoadUint64(&s.state)) {
if atomic.CompareAndSwapPointer(&s.stack, unsafe.Pointer(newCb), unsafe.Pointer(newCb.next)) {
cb(s.val, s.err)
return
}
} else {
return
}
}
}
}
}
func NewPromise() *Promise {
return &Promise{}
}
func (p *Promise) Set(val interface{}, err error) {
p.state.set(val, err)
}
func (p *Promise) Future() *Future {
return &Future{state: &p.state}
}
func (p *Promise) Free() bool {
return isFree(atomic.LoadUint64(&p.state.state))
}
func (f *Future) Get() (interface{}, error) {
return f.state.get()
}
func (f *Future) GetOrDefault(defaultVal interface{}) interface{} {
val, err := f.state.get()
if err != nil {
return defaultVal
}
return val
}
func (f *Future) Subscribe(cb func(val interface{}, err error)) {
f.state.subscribe(cb)
}
func (f *Future) Done() bool {
return isDone(atomic.LoadUint64(&f.state.state))
}
func isFree(st uint64) bool {
return ((st & maskState) >> 32) == stateFree
}
func isDone(st uint64) bool {
return ((st & maskState) >> 32) == stateDone
}
// noCopy may be embedded into structs which must not be copied
// after the first use.
//
// See https://golang.org/issues/8005#issuecomment-190753527
// for details.
type noCopy struct{}
// Lock is a no-op used by -copylocks checker from `go vet`.
func (*noCopy) Lock() {}
func (*noCopy) Unlock() {}