Merge pull request #493 from hashicorp/f-eval-broker-blocking-dequeue

Make a zero timeout for eval_broker.Dequeue() block

nomad/eval_broker.go

@@ -214,6 +214,7 @@ func (b *EvalBroker) enqueueLocked(eval *structs.Evaluation, queue string) {
 // Dequeue is used to perform a blocking dequeue
 func (b *EvalBroker) Dequeue(schedulers []string, timeout time.Duration) (*structs.Evaluation, string, error) {
 	var timeoutTimer *time.Timer
+	var timeoutCh <-chan time.Time
 SCAN:
 	// Scan for work
 	eval, token, err := b.scanForSchedulers(schedulers)
@@ -233,12 +234,13 @@ SCAN:
 	}
 
 	// Setup the timeout channel the first time around
-	if timeoutTimer == nil {
+	if timeoutTimer == nil && timeout != 0 {
 		timeoutTimer = time.NewTimer(timeout)
+		timeoutCh = timeoutTimer.C
 	}
 
 	// Block until we get work
-	scan := b.waitForSchedulers(schedulers, timeoutTimer.C)
+	scan := b.waitForSchedulers(schedulers, timeoutCh)
 	if scan {
 		goto SCAN
 	}

nomad/eval_broker_test.go

@@ -403,20 +403,38 @@ func TestEvalBroker_Dequeue_Timeout(t *testing.T) {
 func TestEvalBroker_Dequeue_Empty_Timeout(t *testing.T) {
 	b := testBroker(t, 0)
 	b.SetEnabled(true)
+	doneCh := make(chan struct{}, 1)
 
-	start := time.Now()
-	out, _, err := b.Dequeue(defaultSched, 0)
-	end := time.Now()
+	go func() {
+		out, _, err := b.Dequeue(defaultSched, 0)
+		if err != nil {
+			t.Fatalf("err: %v", err)
+		}
+		if out == nil {
+			t.Fatal("Expect an eval")
+		}
+		doneCh <- struct{}{}
+	}()
 
+	// Sleep for a little bit
+	select {
+	case <-time.After(5 * time.Millisecond):
+	case <-doneCh:
+		t.Fatalf("Dequeue(0) should block")
+	}
+
+	// Enqueue to unblock the dequeue.
+	eval := mock.Eval()
+	err := b.Enqueue(eval)
 	if err != nil {
 		t.Fatalf("err: %v", err)
 	}
-	if out != nil {
-		t.Fatalf("unexpected: %#v", out)
-	}
 
-	if diff := end.Sub(start); diff > 1*time.Millisecond {
-		t.Fatalf("bad: %#v", diff)
+	select {
+	case <-doneCh:
+		return
+	case <-time.After(5 * time.Millisecond):
+		t.Fatal("timeout: Dequeue(0) should return after enqueue")
 	}
 }