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// 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"
// A WaitGroup waits for a collection of goroutines to finish.
// The main goroutine calls Add to set the number of
// goroutines to wait for. Then each of the goroutines
// runs and calls Done when finished. At the same time,
// Wait can be used to block until all goroutines have finished.
type WaitGroup struct {
m Mutex
counter int32
waiters int32
sema *uint32
}
// WaitGroup creates a new semaphore each time the old semaphore
// is released. This is to avoid the following race:
//
// G1: Add(1)
// G1: go G2()
// G1: Wait() // Context switch after Unlock() and before Semacquire().
// G2: Done() // Release semaphore: sema == 1, waiters == 0. G1 doesn't run yet.
// G3: Wait() // Finds counter == 0, waiters == 0, doesn't block.
// G3: Add(1) // Makes counter == 1, waiters == 0.
// G3: go G4()
// G3: Wait() // G1 still hasn't run, G3 finds sema == 1, unblocked! Bug.
// Add adds delta, which may be negative, to the WaitGroup counter.
// If the counter becomes zero, all goroutines blocked on Wait() are released.
func (wg *WaitGroup) Add(delta int) {
v := atomic.AddInt32(&wg.counter, int32(delta))
if v < 0 {
panic("sync: negative WaitGroup count")
}
if v > 0 || atomic.LoadInt32(&wg.waiters) == 0 {
return
}
wg.m.Lock()
for i := int32(0); i < wg.waiters; i++ {
runtime_Semrelease(wg.sema)
}
wg.waiters = 0
wg.sema = nil
wg.m.Unlock()
}
// Done decrements the WaitGroup counter.
func (wg *WaitGroup) Done() {
wg.Add(-1)
}
// Wait blocks until the WaitGroup counter is zero.
func (wg *WaitGroup) Wait() {
if atomic.LoadInt32(&wg.counter) == 0 {
return
}
wg.m.Lock()
atomic.AddInt32(&wg.waiters, 1)
// This code is racing with the unlocked path in Add above.
// The code above modifies counter and then reads waiters.
// We must modify waiters and then read counter (the opposite order)
// to avoid missing an Add.
if atomic.LoadInt32(&wg.counter) == 0 {
atomic.AddInt32(&wg.waiters, -1)
wg.m.Unlock()
return
}
if wg.sema == nil {
wg.sema = new(uint32)
}
s := wg.sema
wg.m.Unlock()
runtime_Semacquire(s)
}
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