Add PromiseRejectionHandler

This just logs every time a promise gets rejected but has not reject
handler (yet).

The message is basically the one firefox gives.

The simplest variant of #2318

js/bundle.go

@@ -307,6 +307,28 @@ func (b *Bundle) instantiate(logger logrus.FieldLogger, rt *goja.Runtime, init *
 	}
 	rt.Set("__ENV", env)
 	rt.Set("__VU", vuID)
+	rt.SetPromiseRejectionTracker(func(p *goja.Promise, op goja.PromiseRejectionOperation) {
+		// TODO this only handles the case where a promise get's rejected without having a reject handler added.
+		// But the other case where for an already rejected promise (one that this call has already handled)
+		// there is now a reject handler attached is not handled.
+		//
+		// More complete implementation might need to wait for some time, the end of an iteration,
+		// some other event before logging a "rejection" that never got "handled"
+		//
+		// Read Notes on https://tc39.es/ecma262/#sec-host-promise-rejection-tracker
+		// see issue https://github.com/grafana/k6/issues/2318 for more info
+		if op == goja.PromiseRejectionReject {
+			frames := rt.CaptureCallStack(20, nil)
+			buf := &bytes.Buffer{}
+			for _, frame := range frames {
+				frame.Write(buf)
+				buf.WriteRune('\n')
+			}
+
+			logger.Warnf("Uncaught (in Promise) %s\n %s", p.Result(), buf.String())
+		}
+	})
+
 	rt.Set("console", common.Bind(rt, newConsole(logger), init.ctxPtr))
 
 	if init.compatibilityMode == lib.CompatibilityModeExtended {

js/bundle_test.go

@@ -976,3 +976,33 @@ func TestBundleMakeArchive(t *testing.T) {
 		})
 	}
 }
+
+func TestPromiseRejectionHandler(t *testing.T) {
+	t.Parallel()
+	logger := logrus.New()
+	logger.SetLevel(logrus.InfoLevel)
+	logger.Out = ioutil.Discard
+	hook := testutils.SimpleLogrusHook{
+		HookedLevels: []logrus.Level{logrus.WarnLevel, logrus.InfoLevel, logrus.ErrorLevel, logrus.FatalLevel, logrus.PanicLevel},
+	}
+	logger.AddHook(&hook)
+	_, err := getSimpleBundle(t, "/script.js", `
+		export let options = {
+			vus: 5,
+			teardownTimeout: '1s',
+		};
+		let val = true;
+		Promise.reject("here").catch((l) => console.log("caught", l));
+		export default function() {}
+	`, logger)
+	require.NoError(t, err)
+
+	entries := hook.Drain()
+	require.Len(t, entries, 2)
+	first := entries[0]
+	assert.Equal(t, logrus.WarnLevel, first.Level)
+	assert.Contains(t, first.Message, "Uncaught (in Promise) here")
+	second := entries[1]
+	assert.Equal(t, logrus.InfoLevel, second.Level)
+	assert.Contains(t, second.Message, "caught here")
+}