diff options
Diffstat (limited to 'src/sync/cond.go')
| -rw-r--r-- | src/sync/cond.go | 117 |
1 files changed, 117 insertions, 0 deletions
diff --git a/src/sync/cond.go b/src/sync/cond.go new file mode 100644 index 0000000..cc927ad --- /dev/null +++ b/src/sync/cond.go @@ -0,0 +1,117 @@ +// Copyright 2011 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package sync + +import ( + "sync/atomic" + "unsafe" +) + +// Cond implements a condition variable, a rendezvous point +// for goroutines waiting for or announcing the occurrence +// of an event. +// +// Each Cond has an associated Locker L (often a *Mutex or *RWMutex), +// which must be held when changing the condition and +// when calling the Wait method. +// +// A Cond must not be copied after first use. +// +// In the terminology of the Go memory model, Cond arranges that +// a call to Broadcast or Signal “synchronizes before” any Wait call +// that it unblocks. +// +// For many simple use cases, users will be better off using channels than a +// Cond (Broadcast corresponds to closing a channel, and Signal corresponds to +// sending on a channel). +// +// For more on replacements for sync.Cond, see [Roberto Clapis's series on +// advanced concurrency patterns], as well as [Bryan Mills's talk on concurrency +// patterns]. +// +// [Roberto Clapis's series on advanced concurrency patterns]: https://blogtitle.github.io/categories/concurrency/ +// [Bryan Mills's talk on concurrency patterns]: https://drive.google.com/file/d/1nPdvhB0PutEJzdCq5ms6UI58dp50fcAN/view +type Cond struct { + noCopy noCopy + + // L is held while observing or changing the condition + L Locker + + notify notifyList + checker copyChecker +} + +// NewCond returns a new Cond with Locker l. +func NewCond(l Locker) *Cond { + return &Cond{L: l} +} + +// Wait atomically unlocks c.L and suspends execution +// of the calling goroutine. After later resuming execution, +// Wait locks c.L before returning. Unlike in other systems, +// Wait cannot return unless awoken by Broadcast or Signal. +// +// Because c.L is not locked while Wait is waiting, the caller +// typically cannot assume that the condition is true when +// Wait returns. Instead, the caller should Wait in a loop: +// +// c.L.Lock() +// for !condition() { +// c.Wait() +// } +// ... make use of condition ... +// c.L.Unlock() +func (c *Cond) Wait() { + c.checker.check() + t := runtime_notifyListAdd(&c.notify) + c.L.Unlock() + runtime_notifyListWait(&c.notify, t) + c.L.Lock() +} + +// Signal wakes one goroutine waiting on c, if there is any. +// +// It is allowed but not required for the caller to hold c.L +// during the call. +// +// Signal() does not affect goroutine scheduling priority; if other goroutines +// are attempting to lock c.L, they may be awoken before a "waiting" goroutine. +func (c *Cond) Signal() { + c.checker.check() + runtime_notifyListNotifyOne(&c.notify) +} + +// Broadcast wakes all goroutines waiting on c. +// +// It is allowed but not required for the caller to hold c.L +// during the call. +func (c *Cond) Broadcast() { + c.checker.check() + runtime_notifyListNotifyAll(&c.notify) +} + +// copyChecker holds back pointer to itself to detect object copying. +type copyChecker uintptr + +func (c *copyChecker) check() { + if uintptr(*c) != uintptr(unsafe.Pointer(c)) && + !atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) && + uintptr(*c) != uintptr(unsafe.Pointer(c)) { + panic("sync.Cond is copied") + } +} + +// noCopy may be added to structs which must not be copied +// after the first use. +// +// See https://golang.org/issues/8005#issuecomment-190753527 +// for details. +// +// Note that it must not be embedded, due to the Lock and Unlock methods. +type noCopy struct{} + +// Lock is a no-op used by -copylocks checker from `go vet`. +func (*noCopy) Lock() {} +func (*noCopy) Unlock() {} |