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

   1 package shadow
   2
   3 import (
   4         "container/list"
   5         "sync"
   6 )
   7
   8 // OrderedValue is a struct that keeps track of a value in the order
   9 // it is set. Each time Value() is called, it will return the most recent
  10 // calls value then discard it.
  11 //
  12 // This is unlike Value that returns the same value once it is set.
  13 type OrderedValue struct {
  14         lock    sync.Mutex
  15         values  *list.List
  16         waiters *list.List
  17 }
  18
  19 // Value returns the last value that was set, or blocks until one
  20 // is received.
  21 func (w *OrderedValue) Value() interface{} {
  22         w.lock.Lock()
  23
  24         // If we have a pending value already, use it
  25         if w.values != nil && w.values.Len() > 0 {
  26                 front := w.values.Front()
  27                 w.values.Remove(front)
  28                 w.lock.Unlock()
  29                 return front.Value
  30         }
  31
  32         // No pending value, create a waiter
  33         if w.waiters == nil {
  34                 w.waiters = list.New()
  35         }
  36
  37         var val Value
  38         w.waiters.PushBack(&val)
  39         w.lock.Unlock()
  40
  41         // Return the value once we have it
  42         return val.Value()
  43 }
  44
  45 // SetValue sets the latest value.
  46 func (w *OrderedValue) SetValue(v interface{}) {
  47         w.lock.Lock()
  48         defer w.lock.Unlock()
  49
  50         // If we have a waiter, notify it
  51         if w.waiters != nil && w.waiters.Len() > 0 {
  52                 front := w.waiters.Front()
  53                 w.waiters.Remove(front)
  54
  55                 val := front.Value.(*Value)
  56                 val.SetValue(v)
  57                 return
  58         }
  59
  60         // Add it to the list of values
  61         if w.values == nil {
  62                 w.values = list.New()
  63         }
  64
  65         w.values.PushBack(v)
  66 }