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package shadow
import (
"errors"
"sync"
)
// ErrClosed is returned by any closed values.
//
// A "closed value" is when the shadow has been notified that the real
// side is complete and any blocking values will _never_ be satisfied
// in the future. In this case, this error is returned. If a value is already
// available, that is still returned.
var ErrClosed = errors.New("shadow closed")
// Value is a struct that coordinates a value between two
// parallel routines. It is similar to atomic.Value except that when
// Value is called if it isn't set it will wait for it.
//
// The Value can be closed with Close, which will cause any future
// blocking operations to return immediately with ErrClosed.
type Value struct {
lock sync.Mutex
cond *sync.Cond
value interface{}
valueSet bool
}
// Close closes the value. This can never fail. For a definition of
// "close" see the struct docs.
func (w *Value) Close() error {
w.lock.Lock()
set := w.valueSet
w.lock.Unlock()
// If we haven't set the value, set it
if !set {
w.SetValue(ErrClosed)
}
// Done
return nil
}
// Value returns the value that was set.
func (w *Value) Value() interface{} {
w.lock.Lock()
defer w.lock.Unlock()
// If we already have a value just return
for !w.valueSet {
// No value, setup the condition variable if we have to
if w.cond == nil {
w.cond = sync.NewCond(&w.lock)
}
// Wait on it
w.cond.Wait()
}
// Return the value
return w.value
}
// SetValue sets the value.
func (w *Value) SetValue(v interface{}) {
w.lock.Lock()
defer w.lock.Unlock()
// Set the value
w.valueSet = true
w.value = v
// If we have a condition, clear it
if w.cond != nil {
w.cond.Broadcast()
w.cond = nil
}
}
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