vendor/github.com/hashicorp/terraform/helper/shadow/value.go

   1 package shadow
   2
   3 import (
   4         "errors"
   5         "sync"
   6 )
   7
   8 // ErrClosed is returned by any closed values.
   9 //
  10 // A "closed value" is when the shadow has been notified that the real
  11 // side is complete and any blocking values will _never_ be satisfied
  12 // in the future. In this case, this error is returned. If a value is already
  13 // available, that is still returned.
  14 var ErrClosed = errors.New("shadow closed")
  15
  16 // Value is a struct that coordinates a value between two
  17 // parallel routines. It is similar to atomic.Value except that when
  18 // Value is called if it isn't set it will wait for it.
  19 //
  20 // The Value can be closed with Close, which will cause any future
  21 // blocking operations to return immediately with ErrClosed.
  22 type Value struct {
  23         lock     sync.Mutex
  24         cond     *sync.Cond
  25         value    interface{}
  26         valueSet bool
  27 }
  28
  29 func (v *Value) Lock() {
  30         v.lock.Lock()
  31 }
  32
  33 func (v *Value) Unlock() {
  34         v.lock.Unlock()
  35 }
  36
  37 // Close closes the value. This can never fail. For a definition of
  38 // "close" see the struct docs.
  39 func (w *Value) Close() error {
  40         w.lock.Lock()
  41         set := w.valueSet
  42         w.lock.Unlock()
  43
  44         // If we haven't set the value, set it
  45         if !set {
  46                 w.SetValue(ErrClosed)
  47         }
  48
  49         // Done
  50         return nil
  51 }
  52
  53 // Value returns the value that was set.
  54 func (w *Value) Value() interface{} {
  55         w.lock.Lock()
  56         defer w.lock.Unlock()
  57
  58         // If we already have a value just return
  59         for !w.valueSet {
  60                 // No value, setup the condition variable if we have to
  61                 if w.cond == nil {
  62                         w.cond = sync.NewCond(&w.lock)
  63                 }
  64
  65                 // Wait on it
  66                 w.cond.Wait()
  67         }
  68
  69         // Return the value
  70         return w.value
  71 }
  72
  73 // SetValue sets the value.
  74 func (w *Value) SetValue(v interface{}) {
  75         w.lock.Lock()
  76         defer w.lock.Unlock()
  77
  78         // Set the value
  79         w.valueSet = true
  80         w.value = v
  81
  82         // If we have a condition, clear it
  83         if w.cond != nil {
  84                 w.cond.Broadcast()
  85                 w.cond = nil
  86         }
  87 }