Scrape: Test that deduplicated targets are started (#7975)

This PR test that de-duplicated targets are actually started.

It is a unit test for this line of code:
072b9649a3/scrape/scrape.go (L457)
which is working and necessary but was not tested yet.

It also tests that scrapes are started in the normal way, in the targets
limit test.

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

scrape/scrape_test.go

@@ -142,9 +142,14 @@ type testLoop struct {
 	stopFunc     func()
 	forcedErr    error
 	forcedErrMtx sync.Mutex
+	runOnce      bool
 }
 
 func (l *testLoop) run(interval, timeout time.Duration, errc chan<- error) {
+	if l.runOnce {
+		panic("loop must be started only once")
+	}
+	l.runOnce = true
 	l.startFunc(interval, timeout, errc)
 }
 
@@ -316,12 +321,16 @@ func TestScrapePoolReload(t *testing.T) {
 }
 
 func TestScrapePoolTargetLimit(t *testing.T) {
+	var wg sync.WaitGroup
 	// On starting to run, new loops created on reload check whether their preceding
 	// equivalents have been stopped.
 	newLoop := func(opts scrapeLoopOptions) loop {
+		wg.Add(1)
 		l := &testLoop{
-			startFunc: func(interval, timeout time.Duration, errc chan<- error) {},
-			stopFunc:  func() {},
+			startFunc: func(interval, timeout time.Duration, errc chan<- error) {
+				wg.Done()
+			},
+			stopFunc: func() {},
 		}
 		return l
 	}
@@ -361,6 +370,13 @@ func TestScrapePoolTargetLimit(t *testing.T) {
 		sp.Sync(tgs[:n])
 	}
 
+	validateIsRunning := func() {
+		wg.Wait()
+		for _, l := range sp.loops {
+			testutil.Assert(t, l.(*testLoop).runOnce, "loop should be running")
+		}
+	}
+
 	validateErrorMessage := func(shouldErr bool) {
 		for _, l := range sp.loops {
 			lerr := l.(*testLoop).getForcedError()
@@ -375,29 +391,54 @@ func TestScrapePoolTargetLimit(t *testing.T) {
 
 	reloadWithLimit(0)
 	loadTargets(50)
+	validateIsRunning()
 
 	// Simulate an initial config with a limit.
 	sp.config.TargetLimit = 30
 	limit = 30
 	loadTargets(50)
+	validateIsRunning()
 	validateErrorMessage(true)
 
 	reloadWithLimit(50)
+	validateIsRunning()
 	validateErrorMessage(false)
 
 	reloadWithLimit(40)
+	validateIsRunning()
 	validateErrorMessage(true)
 
 	loadTargets(30)
+	validateIsRunning()
 	validateErrorMessage(false)
 
 	loadTargets(40)
+	validateIsRunning()
 	validateErrorMessage(false)
 
 	loadTargets(41)
+	validateIsRunning()
 	validateErrorMessage(true)
 
 	reloadWithLimit(51)
+	validateIsRunning()
+	validateErrorMessage(false)
+
+	tgs = append(tgs,
+		&targetgroup.Group{
+			Targets: []model.LabelSet{
+				{model.AddressLabel: model.LabelValue("127.0.0.1:1090")},
+			},
+		},
+		&targetgroup.Group{
+			Targets: []model.LabelSet{
+				{model.AddressLabel: model.LabelValue("127.0.0.1:1090")},
+			},
+		},
+	)
+
+	sp.Sync(tgs)
+	validateIsRunning()
 	validateErrorMessage(false)
 }
 
@@ -476,6 +517,54 @@ func TestScrapePoolRaces(t *testing.T) {
 	sp.stop()
 }
 
+func TestScrapePoolScrapeLoopsStarted(t *testing.T) {
+	var wg sync.WaitGroup
+	newLoop := func(opts scrapeLoopOptions) loop {
+		wg.Add(1)
+		l := &testLoop{
+			startFunc: func(interval, timeout time.Duration, errc chan<- error) {
+				wg.Done()
+			},
+			stopFunc: func() {},
+		}
+		return l
+	}
+	sp := &scrapePool{
+		appendable:    &nopAppendable{},
+		activeTargets: map[uint64]*Target{},
+		loops:         map[uint64]loop{},
+		newLoop:       newLoop,
+		logger:        nil,
+		client:        http.DefaultClient,
+	}
+
+	tgs := []*targetgroup.Group{
+		{
+			Targets: []model.LabelSet{
+				{model.AddressLabel: model.LabelValue("127.0.0.1:9090")},
+			},
+		},
+		{
+			Targets: []model.LabelSet{
+				{model.AddressLabel: model.LabelValue("127.0.0.1:9090")},
+			},
+		},
+	}
+
+	testutil.Ok(t, sp.reload(&config.ScrapeConfig{
+		ScrapeInterval: model.Duration(3 * time.Second),
+		ScrapeTimeout:  model.Duration(2 * time.Second),
+	}))
+	sp.Sync(tgs)
+
+	testutil.Equals(t, 1, len(sp.loops))
+
+	wg.Wait()
+	for _, l := range sp.loops {
+		testutil.Assert(t, l.(*testLoop).runOnce, "loop should be running")
+	}
+}
+
 func TestScrapeLoopStopBeforeRun(t *testing.T) {
 	scraper := &testScraper{}