eliminate deadlocks in Subscribe and AddListener

circuitbreaker.go

@@ -190,17 +190,22 @@ func NewRateBreaker(rate float64, minSamples int64) *Breaker {
 
 // Subscribe returns a channel of BreakerEvents. Whenever the breaker changes state,
 // the state will be sent over the channel. See BreakerEvent for the types of events.
+// Note that events may be dropped or not sent so clients should not rely on
+// events for program correctness.
 func (cb *Breaker) Subscribe() <-chan BreakerEvent {
 	eventReader := make(chan BreakerEvent)
 	output := make(chan BreakerEvent, 100)
-
 	go func() {
 		for v := range eventReader {
+		trySend:
 			select {
 			case output <- v:
 			default:
-				<-output
-				output <- v
+				select {
+				case <-output:
+				default:
+				}
+				goto trySend
 			}
 		}
 	}()
@@ -210,6 +215,8 @@ func (cb *Breaker) Subscribe() <-chan BreakerEvent {
 
 // AddListener adds a channel of ListenerEvents on behalf of a listener.
 // The listener channel must be buffered.
+// Note that events may be dropped or not sent so clients should not rely on
+// events for program correctness.
 func (cb *Breaker) AddListener(listener chan ListenerEvent) {
 	cb.listeners = append(cb.listeners, listener)
 }
@@ -336,7 +343,9 @@ func (cb *Breaker) Call(circuit func() error, timeout time.Duration) error {
 
 // CallContext is same as Call but if the ctx is canceled after the circuit returned an error,
 // the error will not be marked as a failure because the call was canceled intentionally.
-func (cb *Breaker) CallContext(ctx context.Context, circuit func() error, timeout time.Duration) error {
+func (cb *Breaker) CallContext(
+	ctx context.Context, circuit func() error, timeout time.Duration,
+) error {
 	var err error
 
 	if !cb.Ready() {
@@ -406,11 +415,16 @@ func (cb *Breaker) sendEvent(event BreakerEvent) {
 	}
 	for _, listener := range cb.listeners {
 		le := ListenerEvent{CB: cb, Event: event}
+	trySend:
 		select {
 		case listener <- le:
 		default:
-			<-listener
-			listener <- le
+			// The channel was full so attempt to pull off of it and send again.
+			select {
+			case <-listener:
+			default:
+			}
+			goto trySend
 		}
 	}
 }

circuitbreaker_test.go

@@ -3,6 +3,7 @@ package circuit
 import (
 	"context"
 	"fmt"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -518,3 +519,68 @@ func TestPartialSecondBackoff(t *testing.T) {
 		t.Fatalf("expected breaker to be ready after more than nextBackoff time had passed")
 	}
 }
+
+// TestNoDeadlockOnChannelSends ensures that the behavior of channel sends in
+// the face of concurrent events and consumers does not lead to deadlock.
+func TestNoDeadlockOnChannelSends(t *testing.T) {
+	const listeners = 1000
+	const subscribers = 1000
+	const resetters = 3
+	b := NewBreakerWithOptions(nil)
+	var lcs []chan ListenerEvent
+	for i := 0; i < listeners; i++ {
+		lcs = append(lcs, make(chan ListenerEvent, 1))
+		b.AddListener(lcs[i])
+	}
+	var scs []<-chan BreakerEvent
+	for i := 0; i < subscribers; i++ {
+		scs = append(scs, b.Subscribe())
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Millisecond)
+	defer cancel()
+	var wg sync.WaitGroup
+	readFromSubscribeChan := func(sc <-chan BreakerEvent) {
+		defer wg.Done()
+		for {
+			select {
+			case <-sc:
+			case <-ctx.Done():
+				return
+			}
+		}
+	}
+	readFromListenerChan := func(lc chan ListenerEvent) {
+		defer wg.Done()
+		for {
+			select {
+			case <-lc:
+			case <-ctx.Done():
+				return
+			}
+		}
+	}
+	tripAndReset := func() {
+		defer wg.Done()
+		for i := 0; true; i++ {
+			// Keep sending a bit after the other goroutine exits.
+			if i%1000 == 0 && ctx.Err() != nil {
+				return
+			}
+			b.Reset()
+			b.Trip()
+		}
+	}
+	for _, lc := range lcs {
+		wg.Add(1)
+		go readFromListenerChan(lc)
+	}
+	for _, sc := range scs {
+		wg.Add(1)
+		go readFromSubscribeChan(sc)
+	}
+	for i := 0; i < resetters; i++ {
+		wg.Add(1)
+		go tripAndReset()
+	}
+	wg.Wait()
+}