Use a ring channel to avoid blocking write of events (#2082)

* Use a ring channel to avoid blocking write of events

* Add eapache/channels dependency

vendor/github.com/eapache/channels/shared_buffer_test.go

@@ -0,0 +1,127 @@
+package channels
+
+import "testing"
+
+func TestSharedBufferSingleton(t *testing.T) {
+	buf := NewSharedBuffer(3)
+
+	ch := buf.NewChannel()
+	for i := 0; i < 5; i++ {
+		ch.In() <- (*int)(nil)
+		ch.In() <- (*int)(nil)
+		ch.In() <- (*int)(nil)
+		select {
+		case ch.In() <- (*int)(nil):
+			t.Error("Wrote to full shared-buffer")
+		default:
+		}
+
+		<-ch.Out()
+		<-ch.Out()
+		<-ch.Out()
+		select {
+		case <-ch.Out():
+			t.Error("Read from empty shared-buffer")
+		default:
+		}
+	}
+
+	ch.Close()
+	buf.Close()
+}
+
+func TestSharedBufferMultiple(t *testing.T) {
+	buf := NewSharedBuffer(3)
+
+	ch1 := buf.NewChannel()
+	ch2 := buf.NewChannel()
+
+	ch1.In() <- (*int)(nil)
+	ch1.In() <- (*int)(nil)
+	ch1.In() <- (*int)(nil)
+
+	select {
+	case ch2.In() <- (*int)(nil):
+		t.Error("Wrote to full shared-buffer")
+	case <-ch2.Out():
+		t.Error("Read from empty channel")
+	default:
+	}
+
+	<-ch1.Out()
+
+	for i := 0; i < 10; i++ {
+		ch2.In() <- (*int)(nil)
+
+		select {
+		case ch1.In() <- (*int)(nil):
+			t.Error("Wrote to full shared-buffer")
+		case ch2.In() <- (*int)(nil):
+			t.Error("Wrote to full shared-buffer")
+		default:
+		}
+
+		<-ch2.Out()
+	}
+
+	<-ch1.Out()
+	<-ch1.Out()
+
+	ch1.Close()
+	ch2.Close()
+	buf.Close()
+}
+
+func TestSharedBufferConcurrent(t *testing.T) {
+	const threads = 10
+	const iters = 200
+
+	buf := NewSharedBuffer(3)
+	done := make(chan bool)
+
+	for i := 0; i < threads; i++ {
+		go func() {
+			ch := buf.NewChannel()
+			for i := 0; i < iters; i++ {
+				ch.In() <- i
+				val := <-ch.Out()
+				if val.(int) != i {
+					t.Error("Mismatched value out of channel")
+				}
+			}
+			ch.Close()
+			done <- true
+		}()
+	}
+
+	for i := 0; i < threads; i++ {
+		<-done
+	}
+	close(done)
+	buf.Close()
+}
+
+func ExampleSharedBuffer() {
+	// never more than 3 elements in the pipeline at once
+	buf := NewSharedBuffer(3)
+
+	ch1 := buf.NewChannel()
+	ch2 := buf.NewChannel()
+
+	// or, instead of a straight pipe, implement your pipeline step
+	Pipe(ch1, ch2)
+
+	// inputs
+	go func() {
+		for i := 0; i < 20; i++ {
+			ch1.In() <- i
+		}
+		ch1.Close()
+	}()
+
+	for _ = range ch2.Out() {
+		// outputs
+	}
+
+	buf.Close()
+}