Fix flaky query frontend split interval unit test

[yeya24]

Signed-off-by: Ben Ye [email protected]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Fix #3084

In the previous test case, I used counter to mock HTTP handler and check how many times the handler is being called.

func counter() (*int, http.Handler) {
	count := 0
	var lock sync.Mutex
	return &count, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		lock.Lock()
		defer lock.Unlock()
		count++
	})
}

But this mock handler doesn't return a meaningful result to query frontend so it might cause other requests canceled at the same time, which causes the flaky. https://github.com/cortexproject/cortex/blob/master/pkg/querier/queryrange/limits.go#L104

So if we return promql response which is valid for query frontend, then the test case should work.

Verification

Used the function below to test it works as expected.

func TestFlaky(t *testing.T) {
	testRequest := &ThanosRequest{
		Path:  "/api/v1/query_range",
		Start: 0,
		End:   2 * hour,
		Step:  10 * seconds,
	}

	codec := NewThanosCodec(true)
	tpw, err := NewTripperWare(&fakeLimits{}, nil, codec, nil,
		time.Hour, 0, nil, log.NewNopLogger())
	testutil.Ok(t, err)

	rt, err := newFakeRoundTripper()
	testutil.Ok(t, err)
	defer rt.Close()

	for i := 0; i < 10000; i++ {
		c, handler := promqlResults(false)
		rt.setHandler(handler)

		ctx := user.InjectOrgID(context.Background(), "1")
		httpReq, err := codec.EncodeRequest(ctx, testRequest)
		testutil.Ok(t, err)

		_, err = tpw(rt).RoundTrip(httpReq)
		testutil.Ok(t, err)

		testutil.Equals(t, 2, *c)
	}
}

[kakkoyun]

lgtm. Good work, thanks for jumping on it.

[bwplotka]

Thanks!