prometheus/retrieval/scrape_test.go

// Copyright 2016 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package retrieval

import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"math"
	"net/http"
	"net/http/httptest"
	"net/url"
	"reflect"
	"strings"
	"sync"
	"testing"
	"time"

	"github.com/prometheus/common/model"
	"github.com/stretchr/testify/require"
	"golang.org/x/net/context"

	"github.com/prometheus/prometheus/config"
	"github.com/prometheus/prometheus/pkg/labels"
	"github.com/prometheus/prometheus/pkg/timestamp"
	"github.com/prometheus/prometheus/pkg/value"
	"github.com/prometheus/prometheus/storage"
)

func TestNewScrapePool(t *testing.T) {
	var (
		app = &nopAppendable{}
		cfg = &config.ScrapeConfig{}
		sp  = newScrapePool(context.Background(), cfg, app)
	)

	if a, ok := sp.appendable.(*nopAppendable); !ok || a != app {
		t.Fatalf("Wrong sample appender")
	}
	if sp.config != cfg {
		t.Fatalf("Wrong scrape config")
	}
	if sp.newLoop == nil {
		t.Fatalf("newLoop function not initialized")
	}
}

type testLoop struct {
	startFunc func(interval, timeout time.Duration, errc chan<- error)
	stopFunc  func()
}

func (l *testLoop) run(interval, timeout time.Duration, errc chan<- error) {
	l.startFunc(interval, timeout, errc)
}

func (l *testLoop) stop() {
	l.stopFunc()
}

func TestScrapePoolStop(t *testing.T) {
	sp := &scrapePool{
		targets: map[uint64]*Target{},
		loops:   map[uint64]loop{},
	}
	var mtx sync.Mutex
	stopped := map[uint64]bool{}
	numTargets := 20

	// Stopping the scrape pool must call stop() on all scrape loops,
	// clean them and the respective targets up. It must wait until each loop's
	// stop function returned before returning itself.

	for i := 0; i < numTargets; i++ {
		t := &Target{
			labels: labels.FromStrings(model.AddressLabel, fmt.Sprintf("example.com:%d", i)),
		}
		l := &testLoop{}
		l.stopFunc = func() {
			time.Sleep(time.Duration(i*20) * time.Millisecond)

			mtx.Lock()
			stopped[t.hash()] = true
			mtx.Unlock()
		}

		sp.targets[t.hash()] = t
		sp.loops[t.hash()] = l
	}

	done := make(chan struct{})
	stopTime := time.Now()

	go func() {
		sp.stop()
		close(done)
	}()

	select {
	case <-time.After(5 * time.Second):
		t.Fatalf("scrapeLoop.stop() did not return as expected")
	case <-done:
		// This should have taken at least as long as the last target slept.
		if time.Since(stopTime) < time.Duration(numTargets*20)*time.Millisecond {
			t.Fatalf("scrapeLoop.stop() exited before all targets stopped")
		}
	}

	mtx.Lock()
	if len(stopped) != numTargets {
		t.Fatalf("Expected 20 stopped loops, got %d", len(stopped))
	}
	mtx.Unlock()

	if len(sp.targets) > 0 {
		t.Fatalf("Targets were not cleared on stopping: %d left", len(sp.targets))
	}
	if len(sp.loops) > 0 {
		t.Fatalf("Loops were not cleared on stopping: %d left", len(sp.loops))
	}
}

func TestScrapePoolReload(t *testing.T) {
	var mtx sync.Mutex
	numTargets := 20

	stopped := map[uint64]bool{}

	reloadCfg := &config.ScrapeConfig{
		ScrapeInterval: model.Duration(3 * time.Second),
		ScrapeTimeout:  model.Duration(2 * time.Second),
	}
	// On starting to run, new loops created on reload check whether their preceding
	// equivalents have been stopped.
	newLoop := func(ctx context.Context, s scraper, app, reportApp func() storage.Appender) loop {
		l := &testLoop{}
		l.startFunc = func(interval, timeout time.Duration, errc chan<- error) {
			if interval != 3*time.Second {
				t.Errorf("Expected scrape interval %d but got %d", 3*time.Second, interval)
			}
			if timeout != 2*time.Second {
				t.Errorf("Expected scrape timeout %d but got %d", 2*time.Second, timeout)
			}
			mtx.Lock()
			if !stopped[s.(*targetScraper).hash()] {
				t.Errorf("Scrape loop for %v not stopped yet", s.(*targetScraper))
			}
			mtx.Unlock()
		}
		return l
	}
	sp := &scrapePool{
		appendable: &nopAppendable{},
		targets:    map[uint64]*Target{},
		loops:      map[uint64]loop{},
		newLoop:    newLoop,
	}

	// Reloading a scrape pool with a new scrape configuration must stop all scrape
	// loops and start new ones. A new loop must not be started before the preceding
	// one terminated.

	for i := 0; i < numTargets; i++ {
		t := &Target{
			labels: labels.FromStrings(model.AddressLabel, fmt.Sprintf("example.com:%d", i)),
		}
		l := &testLoop{}
		l.stopFunc = func() {
			time.Sleep(time.Duration(i*20) * time.Millisecond)

			mtx.Lock()
			stopped[t.hash()] = true
			mtx.Unlock()
		}

		sp.targets[t.hash()] = t
		sp.loops[t.hash()] = l
	}
	done := make(chan struct{})

	beforeTargets := map[uint64]*Target{}
	for h, t := range sp.targets {
		beforeTargets[h] = t
	}

	reloadTime := time.Now()

	go func() {
		sp.reload(reloadCfg)
		close(done)
	}()

	select {
	case <-time.After(5 * time.Second):
		t.Fatalf("scrapeLoop.reload() did not return as expected")
	case <-done:
		// This should have taken at least as long as the last target slept.
		if time.Since(reloadTime) < time.Duration(numTargets*20)*time.Millisecond {
			t.Fatalf("scrapeLoop.stop() exited before all targets stopped")
		}
	}

	mtx.Lock()
	if len(stopped) != numTargets {
		t.Fatalf("Expected 20 stopped loops, got %d", len(stopped))
	}
	mtx.Unlock()

	if !reflect.DeepEqual(sp.targets, beforeTargets) {
		t.Fatalf("Reloading affected target states unexpectedly")
	}
	if len(sp.loops) != numTargets {
		t.Fatalf("Expected %d loops after reload but got %d", numTargets, len(sp.loops))
	}
}

func TestScrapePoolReportAppender(t *testing.T) {
	cfg := &config.ScrapeConfig{
		MetricRelabelConfigs: []*config.RelabelConfig{
			{}, {}, {},
		},
	}
	target := newTestTarget("example.com:80", 10*time.Millisecond, nil)
	app := &nopAppendable{}

	sp := newScrapePool(context.Background(), cfg, app)

	cfg.HonorLabels = false
	wrapped := sp.reportAppender(target)

	rl, ok := wrapped.(ruleLabelsAppender)
	if !ok {
		t.Fatalf("Expected ruleLabelsAppender but got %T", wrapped)
	}
	if _, ok := rl.Appender.(nopAppender); !ok {
		t.Fatalf("Expected base appender but got %T", rl.Appender)
	}

	cfg.HonorLabels = true
	wrapped = sp.reportAppender(target)

	hl, ok := wrapped.(ruleLabelsAppender)
	if !ok {
		t.Fatalf("Expected ruleLabelsAppender but got %T", wrapped)
	}
	if _, ok := rl.Appender.(nopAppender); !ok {
		t.Fatalf("Expected base appender but got %T", hl.Appender)
	}
}

func TestScrapePoolSampleAppender(t *testing.T) {
	cfg := &config.ScrapeConfig{
		MetricRelabelConfigs: []*config.RelabelConfig{
			{}, {}, {},
		},
	}

	target := newTestTarget("example.com:80", 10*time.Millisecond, nil)
	app := &nopAppendable{}

	sp := newScrapePool(context.Background(), cfg, app)

	cfg.HonorLabels = false
	wrapped := sp.sampleAppender(target)

	rl, ok := wrapped.(ruleLabelsAppender)
	if !ok {
		t.Fatalf("Expected ruleLabelsAppender but got %T", wrapped)
	}
	re, ok := rl.Appender.(relabelAppender)
	if !ok {
		t.Fatalf("Expected relabelAppender but got %T", rl.Appender)
	}
	if _, ok := re.Appender.(nopAppender); !ok {
		t.Fatalf("Expected base appender but got %T", re.Appender)
	}

	cfg.HonorLabels = true
	cfg.SampleLimit = 100
	wrapped = sp.sampleAppender(target)

	hl, ok := wrapped.(honorLabelsAppender)
	if !ok {
		t.Fatalf("Expected honorLabelsAppender but got %T", wrapped)
	}
	re, ok = hl.Appender.(relabelAppender)
	if !ok {
		t.Fatalf("Expected relabelAppender but got %T", hl.Appender)
	}
	lm, ok := re.Appender.(*limitAppender)
	if !ok {
		t.Fatalf("Expected limitAppender but got %T", lm.Appender)
	}
	if _, ok := lm.Appender.(nopAppender); !ok {
		t.Fatalf("Expected base appender but got %T", re.Appender)
	}
}

func TestScrapeLoopStop(t *testing.T) {
	scraper := &testScraper{}
	sl := newScrapeLoop(context.Background(), scraper, nil, nil)

	// The scrape pool synchronizes on stopping scrape loops. However, new scrape
	// loops are started asynchronously. Thus it's possible, that a loop is stopped
	// again before having started properly.
	// Stopping not-yet-started loops must block until the run method was called and exited.
	// The run method must exit immediately.

	stopDone := make(chan struct{})
	go func() {
		sl.stop()
		close(stopDone)
	}()

	select {
	case <-stopDone:
		t.Fatalf("Stopping terminated before run exited successfully")
	case <-time.After(500 * time.Millisecond):
	}

	// Running the scrape loop must exit before calling the scraper even once.
	scraper.scrapeFunc = func(context.Context, io.Writer) error {
		t.Fatalf("scraper was called for terminated scrape loop")
		return nil
	}

	runDone := make(chan struct{})
	go func() {
		sl.run(1, 0, nil)
		close(runDone)
	}()

	select {
	case <-runDone:
	case <-time.After(1 * time.Second):
		t.Fatalf("Running terminated scrape loop did not exit")
	}

	select {
	case <-stopDone:
	case <-time.After(1 * time.Second):
		t.Fatalf("Stopping did not terminate after running exited")
	}
}

func TestScrapeLoopRun(t *testing.T) {
	var (
		signal = make(chan struct{})
		errc   = make(chan error)

		scraper   = &testScraper{}
		app       = func() storage.Appender { return &nopAppender{} }
		reportApp = func() storage.Appender { return &nopAppender{} }
	)
	defer close(signal)

	ctx, cancel := context.WithCancel(context.Background())
	sl := newScrapeLoop(ctx, scraper, app, reportApp)

	// The loop must terminate during the initial offset if the context
	// is canceled.
	scraper.offsetDur = time.Hour

	go func() {
		sl.run(time.Second, time.Hour, errc)
		signal <- struct{}{}
	}()

	// Wait to make sure we are actually waiting on the offset.
	time.Sleep(1 * time.Second)

	cancel()
	select {
	case <-signal:
	case <-time.After(5 * time.Second):
		t.Fatalf("Cancelation during initial offset failed")
	case err := <-errc:
		t.Fatalf("Unexpected error: %s", err)
	}

	// The provided timeout must cause cancelation of the context passed down to the
	// scraper. The scraper has to respect the context.
	scraper.offsetDur = 0

	block := make(chan struct{})
	scraper.scrapeFunc = func(ctx context.Context, _ io.Writer) error {
		select {
		case <-block:
		case <-ctx.Done():
			return ctx.Err()
		}
		return nil
	}

	ctx, cancel = context.WithCancel(context.Background())
	sl = newScrapeLoop(ctx, scraper, app, reportApp)

	go func() {
		sl.run(time.Second, 100*time.Millisecond, errc)
		signal <- struct{}{}
	}()

	select {
	case err := <-errc:
		if err != context.DeadlineExceeded {
			t.Fatalf("Expected timeout error but got: %s", err)
		}
	case <-time.After(3 * time.Second):
		t.Fatalf("Expected timeout error but got none")
	}

	// We already caught the timeout error and are certainly in the loop.
	// Let the scrapes returns immediately to cause no further timeout errors
	// and check whether canceling the parent context terminates the loop.
	close(block)
	cancel()

	select {
	case <-signal:
		// Loop terminated as expected.
	case err := <-errc:
		t.Fatalf("Unexpected error: %s", err)
	case <-time.After(3 * time.Second):
		t.Fatalf("Loop did not terminate on context cancelation")
	}
}

func TestScrapeLoopRunCreatesStaleMarkersOnFailedScrape(t *testing.T) {
	appender := &collectResultAppender{}
	var (
		signal = make(chan struct{})

		scraper    = &testScraper{}
		app        = func() storage.Appender { return appender }
		reportApp  = func() storage.Appender { return &nopAppender{} }
		numScrapes = 0
	)
	defer close(signal)

	ctx, cancel := context.WithCancel(context.Background())
	sl := newScrapeLoop(ctx, scraper, app, reportApp)

	// Succeed once, several failures, then stop.
	scraper.scrapeFunc = func(ctx context.Context, w io.Writer) error {
		numScrapes += 1
		if numScrapes == 1 {
			w.Write([]byte("metric_a 42\n"))
			return nil
		} else if numScrapes == 5 {
			cancel()
		}
		return fmt.Errorf("Scrape failed.")
	}

	go func() {
		sl.run(10*time.Millisecond, time.Hour, nil)
		signal <- struct{}{}
	}()

	select {
	case <-signal:
	case <-time.After(5 * time.Second):
		t.Fatalf("Scrape wasn't stopped.")
	}

	if len(appender.result) != 2 {
		t.Fatalf("Appended samples not as expected. Wanted: %d samples Got: %d", 2, len(appender.result))
	}
	if appender.result[0].v != 42.0 {
		t.Fatalf("Appended first sample not as expected. Wanted: %f Got: %f", appender.result[0], 42)
	}
	if !value.IsStaleNaN(appender.result[1].v) {
		t.Fatalf("Appended second sample not as expected. Wanted: stale NaN Got: %x", math.Float64bits(appender.result[1].v))
	}
}

func TestScrapeLoopRunCreatesStaleMarkersOnParseFailure(t *testing.T) {
	appender := &collectResultAppender{}
	var (
		signal = make(chan struct{})

		scraper    = &testScraper{}
		app        = func() storage.Appender { return appender }
		reportApp  = func() storage.Appender { return &nopAppender{} }
		numScrapes = 0
	)
	defer close(signal)

	ctx, cancel := context.WithCancel(context.Background())
	sl := newScrapeLoop(ctx, scraper, app, reportApp)

	// Succeed once, several failures, then stop.
	scraper.scrapeFunc = func(ctx context.Context, w io.Writer) error {
		numScrapes += 1
		if numScrapes == 1 {
			w.Write([]byte("metric_a 42\n"))
			return nil
		} else if numScrapes == 2 {
			w.Write([]byte("7&-\n"))
			return nil
		} else if numScrapes == 3 {
			cancel()
		}
		return fmt.Errorf("Scrape failed.")
	}

	go func() {
		sl.run(10*time.Millisecond, time.Hour, nil)
		signal <- struct{}{}
	}()

	select {
	case <-signal:
	case <-time.After(5 * time.Second):
		t.Fatalf("Scrape wasn't stopped.")
	}

	if len(appender.result) != 2 {
		t.Fatalf("Appended samples not as expected. Wanted: %d samples Got: %d", 2, len(appender.result))
	}
	if appender.result[0].v != 42.0 {
		t.Fatalf("Appended first sample not as expected. Wanted: %f Got: %f", appender.result[0], 42)
	}
	if !value.IsStaleNaN(appender.result[1].v) {
		t.Fatalf("Appended second sample not as expected. Wanted: stale NaN Got: %x", math.Float64bits(appender.result[1].v))
	}
}

func TestScrapeLoopAppend(t *testing.T) {
	app := &collectResultAppender{}
	sl := &scrapeLoop{
		appender:       func() storage.Appender { return app },
		reportAppender: func() storage.Appender { return nopAppender{} },
		refCache:       map[string]uint64{},
		lsetCache:      map[uint64]lsetCacheEntry{},
	}

	now := time.Now()
	_, _, err := sl.append([]byte("metric_a 1\nmetric_b NaN\n"), now)
	if err != nil {
		t.Fatalf("Unexpected append error: %s", err)
	}

	ingestedNaN := math.Float64bits(app.result[1].v)
	if ingestedNaN != value.NormalNaN {
		t.Fatalf("Appended NaN samples wasn't as expected. Wanted: %x Got: %x", value.NormalNaN, ingestedNaN)
	}

	// DeepEqual will report NaNs as being different, so replace with a different value.
	app.result[1].v = 42
	want := []sample{
		{
			metric: labels.FromStrings(model.MetricNameLabel, "metric_a"),
			t:      timestamp.FromTime(now),
			v:      1,
		},
		{
			metric: labels.FromStrings(model.MetricNameLabel, "metric_b"),
			t:      timestamp.FromTime(now),
			v:      42,
		},
	}
	if !reflect.DeepEqual(want, app.result) {
		t.Fatalf("Appended samples not as expected. Wanted: %+v Got: %+v", want, app.result)
	}
}

func TestScrapeLoopAppendStaleness(t *testing.T) {
	app := &collectResultAppender{}
	sl := &scrapeLoop{
		appender:       func() storage.Appender { return app },
		reportAppender: func() storage.Appender { return nopAppender{} },
		refCache:       map[string]uint64{},
		lsetCache:      map[uint64]lsetCacheEntry{},
	}

	now := time.Now()
	_, _, err := sl.append([]byte("metric_a 1\n"), now)
	if err != nil {
		t.Fatalf("Unexpected append error: %s", err)
	}
	_, _, err = sl.append([]byte(""), now.Add(time.Second))
	if err != nil {
		t.Fatalf("Unexpected append error: %s", err)
	}

	ingestedNaN := math.Float64bits(app.result[1].v)
	if ingestedNaN != value.StaleNaN {
		t.Fatalf("Appended stale sample wasn't as expected. Wanted: %x Got: %x", value.StaleNaN, ingestedNaN)
	}

	// DeepEqual will report NaNs as being different, so replace with a different value.
	app.result[1].v = 42
	want := []sample{
		{
			metric: labels.FromStrings(model.MetricNameLabel, "metric_a"),
			t:      timestamp.FromTime(now),
			v:      1,
		},
		{
			metric: labels.FromStrings(model.MetricNameLabel, "metric_a"),
			t:      timestamp.FromTime(now.Add(time.Second)),
			v:      42,
		},
	}
	if !reflect.DeepEqual(want, app.result) {
		t.Fatalf("Appended samples not as expected. Wanted: %+v Got: %+v", want, app.result)
	}

}

func TestScrapeLoopAppendNoStalenessIfTimestamp(t *testing.T) {
	app := &collectResultAppender{}
	sl := &scrapeLoop{
		appender:       func() storage.Appender { return app },
		reportAppender: func() storage.Appender { return nopAppender{} },
		refCache:       map[string]uint64{},
		lsetCache:      map[uint64]lsetCacheEntry{},
	}

	now := time.Now()
	_, _, err := sl.append([]byte("metric_a 1 1000\n"), now)
	if err != nil {
		t.Fatalf("Unexpected append error: %s", err)
	}
	_, _, err = sl.append([]byte(""), now.Add(time.Second))
	if err != nil {
		t.Fatalf("Unexpected append error: %s", err)
	}

	want := []sample{
		{
			metric: labels.FromStrings(model.MetricNameLabel, "metric_a"),
			t:      1000,
			v:      1,
		},
	}
	if !reflect.DeepEqual(want, app.result) {
		t.Fatalf("Appended samples not as expected. Wanted: %+v Got: %+v", want, app.result)
	}

}

type errorAppender struct {
	collectResultAppender
}

func (app *errorAppender) Add(lset labels.Labels, t int64, v float64) (uint64, error) {
	if lset.Get(model.MetricNameLabel) == "out_of_order" {
		return 0, storage.ErrOutOfOrderSample
	} else if lset.Get(model.MetricNameLabel) == "amend" {
		return 0, storage.ErrDuplicateSampleForTimestamp
	}
	return app.collectResultAppender.Add(lset, t, v)
}

func (app *errorAppender) AddFast(ref uint64, t int64, v float64) error {
	return app.collectResultAppender.AddFast(ref, t, v)
}

func TestScrapeLoopAppendGracefullyIfAmendOrOutOfOrder(t *testing.T) {
	app := &errorAppender{}
	sl := &scrapeLoop{
		appender:       func() storage.Appender { return app },
		reportAppender: func() storage.Appender { return nopAppender{} },
		refCache:       map[string]uint64{},
		lsetCache:      map[uint64]lsetCacheEntry{},
	}

	now := time.Unix(1, 0)
	_, _, err := sl.append([]byte("out_of_order 1\namend 1\nnormal 1\n"), now)
	if err != nil {
		t.Fatalf("Unexpected append error: %s", err)
	}
	want := []sample{
		{
			metric: labels.FromStrings(model.MetricNameLabel, "normal"),
			t:      timestamp.FromTime(now),
			v:      1,
		},
	}
	if !reflect.DeepEqual(want, app.result) {
		t.Fatalf("Appended samples not as expected. Wanted: %+v Got: %+v", want, app.result)
	}

}

func TestTargetScraperScrapeOK(t *testing.T) {
	const (
		configTimeout   = 1500 * time.Millisecond
		expectedTimeout = "1.500000"
	)

	server := httptest.NewServer(
		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			timeout := r.Header.Get("X-Prometheus-Scrape-Timeout-Seconds")
			if timeout != expectedTimeout {
				t.Errorf("Scrape timeout did not match expected timeout")
				t.Errorf("Expected: %v", expectedTimeout)
				t.Fatalf("Got: %v", timeout)
			}

			w.Header().Set("Content-Type", `text/plain; version=0.0.4`)
			w.Write([]byte("metric_a 1\nmetric_b 2\n"))
		}),
	)
	defer server.Close()

	serverURL, err := url.Parse(server.URL)
	if err != nil {
		panic(err)
	}

	ts := &targetScraper{
		Target: &Target{
			labels: labels.FromStrings(
				model.SchemeLabel, serverURL.Scheme,
				model.AddressLabel, serverURL.Host,
			),
		},
		client:  http.DefaultClient,
		timeout: configTimeout,
	}
	var buf bytes.Buffer

	if err := ts.scrape(context.Background(), &buf); err != nil {
		t.Fatalf("Unexpected scrape error: %s", err)
	}
	require.Equal(t, "metric_a 1\nmetric_b 2\n", buf.String())
}

func TestTargetScrapeScrapeCancel(t *testing.T) {
	block := make(chan struct{})

	server := httptest.NewServer(
		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			<-block
		}),
	)
	defer server.Close()

	serverURL, err := url.Parse(server.URL)
	if err != nil {
		panic(err)
	}

	ts := &targetScraper{
		Target: &Target{
			labels: labels.FromStrings(
				model.SchemeLabel, serverURL.Scheme,
				model.AddressLabel, serverURL.Host,
			),
		},
		client: http.DefaultClient,
	}
	ctx, cancel := context.WithCancel(context.Background())

	errc := make(chan error)

	go func() {
		time.Sleep(1 * time.Second)
		cancel()
	}()

	go func() {
		if err := ts.scrape(ctx, ioutil.Discard); err != context.Canceled {
			errc <- fmt.Errorf("Expected context cancelation error but got: %s", err)
		}
		close(errc)
	}()

	select {
	case <-time.After(5 * time.Second):
		t.Fatalf("Scrape function did not return unexpectedly")
	case err := <-errc:
		if err != nil {
			t.Fatalf(err.Error())
		}
	}
	// If this is closed in a defer above the function the test server
	// does not terminate and the test doens't complete.
	close(block)
}

func TestTargetScrapeScrapeNotFound(t *testing.T) {
	server := httptest.NewServer(
		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			w.WriteHeader(http.StatusNotFound)
		}),
	)
	defer server.Close()

	serverURL, err := url.Parse(server.URL)
	if err != nil {
		panic(err)
	}

	ts := &targetScraper{
		Target: &Target{
			labels: labels.FromStrings(
				model.SchemeLabel, serverURL.Scheme,
				model.AddressLabel, serverURL.Host,
			),
		},
		client: http.DefaultClient,
	}

	if err := ts.scrape(context.Background(), ioutil.Discard); !strings.Contains(err.Error(), "404") {
		t.Fatalf("Expected \"404 NotFound\" error but got: %s", err)
	}
}

// testScraper implements the scraper interface and allows setting values
// returned by its methods. It also allows setting a custom scrape function.
type testScraper struct {
	offsetDur time.Duration

	lastStart    time.Time
	lastDuration time.Duration
	lastError    error

	scrapeErr  error
	scrapeFunc func(context.Context, io.Writer) error
}

func (ts *testScraper) offset(interval time.Duration) time.Duration {
	return ts.offsetDur
}

func (ts *testScraper) report(start time.Time, duration time.Duration, err error) {
	ts.lastStart = start
	ts.lastDuration = duration
	ts.lastError = err
}

func (ts *testScraper) scrape(ctx context.Context, w io.Writer) error {
	if ts.scrapeFunc != nil {
		return ts.scrapeFunc(ctx, w)
	}
	return ts.scrapeErr
}