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