91 lines
2.2 KiB
Go
91 lines
2.2 KiB
Go
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// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package sync
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import "runtime"
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// Cond implements a condition variable, a rendezvous point
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// for goroutines waiting for or announcing the occurrence
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// of an event.
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//
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// Each Cond has an associated Locker L (often a *Mutex or *RWMutex),
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// which must be held when changing the condition and
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// when calling the Wait method.
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type Cond struct {
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L Locker // held while observing or changing the condition
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m Mutex // held to avoid internal races
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waiters int // number of goroutines blocked on Wait
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sema *uint32
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}
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// NewCond returns a new Cond with Locker l.
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func NewCond(l Locker) *Cond {
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return &Cond{L: l}
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}
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// Wait atomically unlocks c.L and suspends execution
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// of the calling goroutine. After later resuming execution,
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// Wait locks c.L before returning.
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//
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// Because L is not locked when Wait first resumes, the caller
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// typically cannot assume that the condition is true when
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// Wait returns. Instead, the caller should Wait in a loop:
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//
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// c.L.Lock()
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// for !condition() {
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// c.Wait()
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// }
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// ... make use of condition ...
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// c.L.Unlock()
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//
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func (c *Cond) Wait() {
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c.m.Lock()
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if c.sema == nil {
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c.sema = new(uint32)
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}
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s := c.sema
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c.waiters++
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c.m.Unlock()
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c.L.Unlock()
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runtime.Semacquire(s)
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c.L.Lock()
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}
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// Signal wakes one goroutine waiting on c, if there is any.
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//
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// It is allowed but not required for the caller to hold c.L
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// during the call.
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func (c *Cond) Signal() {
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c.m.Lock()
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if c.waiters > 0 {
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c.waiters--
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runtime.Semrelease(c.sema)
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}
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c.m.Unlock()
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}
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// Broadcast wakes all goroutines waiting on c.
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//
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// It is allowed but not required for the caller to hold c.L
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// during the call.
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func (c *Cond) Broadcast() {
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c.m.Lock()
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if c.waiters > 0 {
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s := c.sema
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n := c.waiters
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for i := 0; i < n; i++ {
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runtime.Semrelease(s)
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}
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// We just issued n wakeups via the semaphore s.
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// To ensure that they wake up the existing waiters
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// and not waiters that arrive after Broadcast returns,
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// clear c.sema. The next operation will allocate
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// a new one.
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c.sema = nil
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c.waiters = 0
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}
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c.m.Unlock()
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}
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