syncs: add new package for extra sync types

pull/181/head
Brad Fitzpatrick 5 years ago
parent 57f220656c
commit b4d02a251a

@ -0,0 +1,66 @@
// Copyright (c) 2020 Tailscale Inc & 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 syncs contains addition sync types.
package syncs
import "sync/atomic"
// ClosedChan returns a channel that's already closed.
func ClosedChan() <-chan struct{} { return closedChan }
var closedChan = initClosedChan()
func initClosedChan() <-chan struct{} {
ch := make(chan struct{})
close(ch)
return ch
}
// WaitGroupChan is like a sync.WaitGroup, but has a chan that closes
// on completion that you can wait on. (This, you can only use the
// value once)
// Also, its zero value is not usable. Use the constructor.
type WaitGroupChan struct {
n int64 // atomic
done chan struct{} // closed on transition to zero
}
// NewWaitGroupChan returns a new single-use WaitGroupChan.
func NewWaitGroupChan() *WaitGroupChan {
return &WaitGroupChan{done: make(chan struct{})}
}
// DoneChan returns a channel that's closed on completion.
func (c *WaitGroupChan) DoneChan() <-chan struct{} { return c.done }
// Add adds delta, which may be negative, to the WaitGroupChan
// counter. If the counter becomes zero, all goroutines blocked on
// Wait or the Done chan 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.
func (c *WaitGroupChan) Add(delta int) {
n := atomic.AddInt64(&c.n, int64(delta))
if n == 0 {
close(c.done)
}
}
// Decr decrements the WaitGroup counter by one.
//
// (It is like sync.WaitGroup's Done method, but we don't use Done in
// this type, because it's ambiguous between Context.Done and
// WaitGroup.Done. So we use DoneChan and Decr instead.)
func (wg *WaitGroupChan) Decr() {
wg.Add(-1)
}
// Wait blocks until the WaitGroupChan counter is zero.
func (wg *WaitGroupChan) Wait() { <-wg.done }

@ -0,0 +1,50 @@
// Copyright (c) 2020 Tailscale Inc & 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 syncs
import "testing"
func TestWaitGroupChan(t *testing.T) {
wg := NewWaitGroupChan()
wantNotDone := func() {
t.Helper()
select {
case <-wg.DoneChan():
t.Fatal("done too early")
default:
}
}
wantDone := func() {
t.Helper()
select {
case <-wg.DoneChan():
default:
t.Fatal("expected to be done")
}
}
wg.Add(2)
wantNotDone()
wg.Decr()
wantNotDone()
wg.Decr()
wantDone()
wantDone()
}
func TestClosedChan(t *testing.T) {
ch := ClosedChan()
for i := 0; i < 2; i++ {
select {
case <-ch:
default:
t.Fatal("not closed")
}
}
}
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