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 // Close closes the value. This can never fail. For a definition of
  30 // "close" see the struct docs.
  31 func (w *Value) Close() error {
  32         w.lock.Lock()
  33         set := w.valueSet
  34         w.lock.Unlock()
  35
  36         // If we haven't set the value, set it
  37         if !set {
  38                 w.SetValue(ErrClosed)
  39         }
  40
  41         // Done
  42         return nil
  43 }
  44
  45 // Value returns the value that was set.
  46 func (w *Value) Value() interface{} {
  47         w.lock.Lock()
  48         defer w.lock.Unlock()
  49
  50         // If we already have a value just return
  51         for !w.valueSet {
  52                 // No value, setup the condition variable if we have to
  53                 if w.cond == nil {
  54                         w.cond = sync.NewCond(&w.lock)
  55                 }
  56
  57                 // Wait on it
  58                 w.cond.Wait()
  59         }
  60
  61         // Return the value
  62         return w.value
  63 }
  64
  65 // SetValue sets the value.
  66 func (w *Value) SetValue(v interface{}) {
  67         w.lock.Lock()
  68         defer w.lock.Unlock()
  69
  70         // Set the value
  71         w.valueSet = true
  72         w.value = v
  73
  74         // If we have a condition, clear it
  75         if w.cond != nil {
  76                 w.cond.Broadcast()
  77                 w.cond = nil
  78         }
  79 }