Source File
waitgroup.go
Belonging Package
sync
// 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 (
)
// 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.
//
// A WaitGroup must not be copied after first use.
//
// In the terminology of the Go memory model, a call to Done
// “synchronizes before” the return of any Wait call that it unblocks.
type WaitGroup struct {
noCopy noCopy
state atomic.Uint64 // high 32 bits are counter, low 32 bits are waiter count.
sema uint32
}
// Add adds delta, which may be negative, to the WaitGroup counter.
// If the counter becomes zero, all goroutines blocked on Wait are released.
// If the counter goes negative, Add panics.
//
// Note that calls with a positive delta that occur when the counter is zero
// must happen before a Wait. Calls with a negative delta, or calls with a
// positive delta that start when the counter is greater than zero, may happen
// at any time.
// Typically this means the calls to Add should execute before the statement
// creating the goroutine or other event to be waited for.
// If a WaitGroup is reused to wait for several independent sets of events,
// new Add calls must happen after all previous Wait calls have returned.
// See the WaitGroup example.
func ( *WaitGroup) ( int) {
if race.Enabled {
if < 0 {
// Synchronize decrements with Wait.
race.ReleaseMerge(unsafe.Pointer())
}
race.Disable()
defer race.Enable()
}
:= .state.Add(uint64() << 32)
:= int32( >> 32)
:= uint32()
if race.Enabled && > 0 && == int32() {
// The first increment must be synchronized with Wait.
// Need to model this as a read, because there can be
// several concurrent wg.counter transitions from 0.
race.Read(unsafe.Pointer(&.sema))
}
if < 0 {
panic("sync: negative WaitGroup counter")
}
if != 0 && > 0 && == int32() {
panic("sync: WaitGroup misuse: Add called concurrently with Wait")
}
if > 0 || == 0 {
return
}
// This goroutine has set counter to 0 when waiters > 0.
// Now there can't be concurrent mutations of state:
// - Adds must not happen concurrently with Wait,
// - Wait does not increment waiters if it sees counter == 0.
// Still do a cheap sanity check to detect WaitGroup misuse.
if .state.Load() != {
panic("sync: WaitGroup misuse: Add called concurrently with Wait")
}
// Reset waiters count to 0.
.state.Store(0)
for ; != 0; -- {
runtime_Semrelease(&.sema, false, 0)
}
}
// Done decrements the WaitGroup counter by one.
func ( *WaitGroup) () {
.Add(-1)
}
// Wait blocks until the WaitGroup counter is zero.
func ( *WaitGroup) () {
if race.Enabled {
race.Disable()
}
for {
:= .state.Load()
:= int32( >> 32)
:= uint32()
if == 0 {
// Counter is 0, no need to wait.
if race.Enabled {
race.Enable()
race.Acquire(unsafe.Pointer())
}
return
}
// Increment waiters count.
if .state.CompareAndSwap(, +1) {
if race.Enabled && == 0 {
// Wait must be synchronized with the first Add.
// Need to model this is as a write to race with the read in Add.
// As a consequence, can do the write only for the first waiter,
// otherwise concurrent Waits will race with each other.
race.Write(unsafe.Pointer(&.sema))
}
runtime_Semacquire(&.sema)
if .state.Load() != 0 {
panic("sync: WaitGroup is reused before previous Wait has returned")
}
if race.Enabled {
race.Enable()
race.Acquire(unsafe.Pointer())
}
return
}
}
}
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