github.com/candidpartners/terraform@v0.9.5-0.20171005231213-29f5f88820f6/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  // 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  }