tor: use targets-list-length dependent timeout (#394)

Closes #288

While there, I figured it made sense to add one extra test to make sure
we can gracefully handle the case where the orchestra API gives us no
targets.

experiment/tor/tor.go

@@ -153,22 +153,30 @@ func (m *measurer) Run(
 	measurement *model.Measurement,
 	callbacks model.ExperimentCallbacks,
 ) error {
-	ctx, cancel := context.WithTimeout(ctx, 60*time.Second)
-	defer cancel()
-	// fetch experiment targets
-	clnt, err := m.newOrchestraClient(ctx, sess)
+	targets, err := m.gimmeTargets(ctx, sess)
 	if err != nil {
 		return err
 	}
-	targets, err := m.fetchTorTargets(ctx, clnt)
-	if err != nil {
-		return err
-	}
-	// run the measurement
+	ctx, cancel := context.WithTimeout(
+		ctx, 15*time.Second*time.Duration(len(targets)),
+	)
+	defer cancel()
 	m.measureTargets(ctx, sess, measurement, callbacks, targets)
 	return nil
 }
 
+func (m *measurer) gimmeTargets(
+	ctx context.Context, sess model.ExperimentSession,
+) (map[string]model.TorTarget, error) {
+	ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
+	defer cancel()
+	clnt, err := m.newOrchestraClient(ctx, sess)
+	if err != nil {
+		return nil, err
+	}
+	return m.fetchTorTargets(ctx, clnt)
+}
+
 // keytarget contains a key and the related target
 type keytarget struct {
 	key    string

experiment/tor/tor_test.go

@@ -71,6 +71,32 @@ func TestUnitMeasurerMeasureFetchTorTargetsError(t *testing.T) {
 	}
 }
 
+func TestUnitMeasurerMeasureFetchTorTargetsEmptyList(t *testing.T) {
+	measurer := newMeasurer(Config{})
+	measurer.newOrchestraClient = func(ctx context.Context, sess model.ExperimentSession) (model.ExperimentOrchestraClient, error) {
+		return new(orchestra.Client), nil
+	}
+	measurer.fetchTorTargets = func(ctx context.Context, clnt model.ExperimentOrchestraClient) (map[string]model.TorTarget, error) {
+		return nil, nil
+	}
+	measurement := new(model.Measurement)
+	err := measurer.Run(
+		context.Background(),
+		&mockable.ExperimentSession{
+			MockableLogger: log.Log,
+		},
+		measurement,
+		handler.NewPrinterCallbacks(log.Log),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tk := measurement.TestKeys.(*TestKeys)
+	if len(tk.Targets) != 0 {
+		t.Fatal("expected no targets here")
+	}
+}
+
 func TestUnitMeasurerMeasureGood(t *testing.T) {
 	measurer := newMeasurer(Config{})
 	measurer.newOrchestraClient = func(ctx context.Context, sess model.ExperimentSession) (model.ExperimentOrchestraClient, error) {