libgo/go/sync/cond.go

   1 // Copyright 2011 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 package sync
   6
   7 import (
   8         "sync/atomic"
   9         "unsafe"
  10 )
  11
  12 // Cond implements a condition variable, a rendezvous point
  13 // for goroutines waiting for or announcing the occurrence
  14 // of an event.
  15 //
  16 // Each Cond has an associated Locker L (often a *Mutex or *RWMutex),
  17 // which must be held when changing the condition and
  18 // when calling the Wait method.
  19 //
  20 // A Cond must not be copied after first use.
  21 type Cond struct {
  22         noCopy noCopy
  23
  24         // L is held while observing or changing the condition
  25         L Locker
  26
  27         notify  notifyList
  28         checker copyChecker
  29 }
  30
  31 // NewCond returns a new Cond with Locker l.
  32 func NewCond(l Locker) *Cond {
  33         return &Cond{L: l}
  34 }
  35
  36 // Wait atomically unlocks c.L and suspends execution
  37 // of the calling goroutine. After later resuming execution,
  38 // Wait locks c.L before returning. Unlike in other systems,
  39 // Wait cannot return unless awoken by Broadcast or Signal.
  40 //
  41 // Because c.L is not locked when Wait first resumes, the caller
  42 // typically cannot assume that the condition is true when
  43 // Wait returns. Instead, the caller should Wait in a loop:
  44 //
  45 //    c.L.Lock()
  46 //    for !condition() {
  47 //        c.Wait()
  48 //    }
  49 //    ... make use of condition ...
  50 //    c.L.Unlock()
  51 //
  52 func (c *Cond) Wait() {
  53         c.checker.check()
  54         t := runtime_notifyListAdd(&c.notify)
  55         c.L.Unlock()
  56         runtime_notifyListWait(&c.notify, t)
  57         c.L.Lock()
  58 }
  59
  60 // Signal wakes one goroutine waiting on c, if there is any.
  61 //
  62 // It is allowed but not required for the caller to hold c.L
  63 // during the call.
  64 func (c *Cond) Signal() {
  65         c.checker.check()
  66         runtime_notifyListNotifyOne(&c.notify)
  67 }
  68
  69 // Broadcast wakes all goroutines waiting on c.
  70 //
  71 // It is allowed but not required for the caller to hold c.L
  72 // during the call.
  73 func (c *Cond) Broadcast() {
  74         c.checker.check()
  75         runtime_notifyListNotifyAll(&c.notify)
  76 }
  77
  78 // copyChecker holds back pointer to itself to detect object copying.
  79 type copyChecker uintptr
  80
  81 func (c *copyChecker) check() {
  82         if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
  83                 !atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
  84                 uintptr(*c) != uintptr(unsafe.Pointer(c)) {
  85                 panic("sync.Cond is copied")
  86         }
  87 }
  88
  89 // noCopy may be embedded into structs which must not be copied
  90 // after the first use.
  91 //
  92 // See https://golang.org/issues/8005#issuecomment-190753527
  93 // for details.
  94 type noCopy struct{}
  95
  96 // Lock is a no-op used by -copylocks checker from `go vet`.
  97 func (*noCopy) Lock()   {}
  98 func (*noCopy) Unlock() {}