github.com/wikibal01/hashicorp-terraform@v0.11.12-beta1/helper/shadow/value.go (about)

     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  }