[译]Concurrency Trap #2: Incomplete Work

15 func main() {
26     fmt.Println("Hello")
37     go fmt.Println("Goodbye")
48 }

1 9 // Tracker knows how to track events for the application.
210 type Tracker struct{}
311 
412 // Event records an event to a database or stream.
513 func (t *Tracker) Event(data string) {
614     time.Sleep(time.Millisecond) // Simulate network write latency.
715     log.Println(data)
816 }

 118 // App holds application state.
 219 type App struct {
 320     track Tracker
 421 }
 522 
 623 // Handle represents an example handler for the web service.
 724 func (a *App) Handle(w http.ResponseWriter, r *http.Request) {
 825 
 926     // Do some actual work.
1027 
1128     // Respond to the client.
1229     w.WriteHeader(http.StatusCreated)
1330 
1431     // Fire and Hope.
1532     // BUG: We are not managing this goroutine.
1633     go a.track.Event("this event")
1734 }

 144 // Handle represents an example handler for the web service.
 245 func (a *App) Handle(w http.ResponseWriter, r *http.Request) {
 346 
 447     // Do some actual work.
 548 
 649     // Respond to the client.
 750     w.WriteHeader(http.StatusCreated)
 851 
 952     // Track the event.
1053     a.track.Event("this event")
1154 }

 110 // Tracker knows how to track events for the application.
 211 type Tracker struct {
 312     wg sync.WaitGroup
 413 }
 514 
 615 // Event starts tracking an event. It runs asynchronously to
 716 // not block the caller. Be sure to call the Shutdown function
 817 // before the program exits so all tracked events finish.
 918 func (t *Tracker) Event(data string) {
1019 
1120     // Increment counter so Shutdown knows to wait for this event.
1221     t.wg.Add(1)
1322 
1423     // Track event in a goroutine so caller is not blocked.
1524     go func() {
1625 
1726         // Decrement counter to tell Shutdown this goroutine finished.
1827         defer t.wg.Done()
1928 
2029         time.Sleep(time.Millisecond) // Simulate network write latency.
2130         log.Println(data)
2231     }()
2332 }
2433 
2534 // Shutdown waits for all tracked events to finish processing.
2635 func (t *Tracker) Shutdown() {
2736     t.wg.Wait()
2837 }

 156 func main() {
 257 
 358     // Start a server.
 459     // Details not shown...
 560     var a App
 661 
 762     // Shut the server down.
 863     // Details not shown...
 964 
1065     // Wait for all event goroutines to finish.
1166     a.track.Shutdown()
1267 }

 136 // Shutdown waits for all tracked events to finish processing
 237 // or for the provided context to be canceled.
 338 func (t *Tracker) Shutdown(ctx context.Context) error {
 439 
 540     // Create a channel to signal when the waitgroup is finished.
 641     ch := make(chan struct{})
 742 
 843     // Create a goroutine to wait for all other goroutines to
 944     // be done then close the channel to unblock the select.
1045     go func() {
1146         t.wg.Wait()
1247         close(ch)
1348     }()
1449 
1550     // Block this function from returning. Wait for either the
1651     // waitgroup to finish or the context to expire.
1752     select {
1853     case <-ch:
1954         return nil
2055     case <-ctx.Done():
2156         return errors.New("timeout")
2257     }
2358 }

186     // Wait up to 5 seconds for all event goroutines to finish.
287     const timeout = 5 * time.Second
388     ctx, cancel := context.WithTimeout(context.Background(), timeout)
489     defer cancel()
590 
691     err := a.track.Shutdown(ctx)