scrape: Introduce an offsetSeed option for deterministic scrape offset calculation and utilize it in tests

Signed-off-by: Ridwan Sharif <ridwanmsharif@google.com>

scrape/manager.go

@@ -149,8 +149,9 @@ type Options struct {
 	// because of an early startup scrape.
 	InitialScrapeOffset time.Duration
 
-	// private option for testability.
+	// private options for testability.
 	skipJitterOffsetting bool
+	offsetSeed           uint64
 }
 
 // Manager maintains a set of scrape pools and manages start/stop cycles
@@ -269,6 +270,11 @@ func (m *Manager) reload() {
 
 // setOffsetSeed calculates a global offsetSeed per server relying on extra label set.
 func (m *Manager) setOffsetSeed(labels labels.Labels) error {
+	if m.opts.offsetSeed != 0 {
+		m.offsetSeed = m.opts.offsetSeed
+		return nil
+	}
+
 	h := fnv.New64a()
 	hostname, err := osutil.GetFQDN()
 	if err != nil {

scrape/scrape_test.go

@@ -6823,6 +6823,7 @@ func TestScrapeOffsetDistribution(t *testing.T) {
 
 		app := teststorage.NewAppendable()
 		opts := &Options{
+			offsetSeed: 1,
 			HTTPClientOptions: []config_util.HTTPClientOption{
 				config_util.WithDialContextFunc(func(ctx context.Context, _, _ string) (net.Conn, error) {
 					srvConn, cliConn := net.Pipe()
@@ -6868,20 +6869,13 @@ func TestScrapeOffsetDistribution(t *testing.T) {
 
 		scrapeManager.reload()
 
-		time.Sleep(22 * time.Second)
+		numScrapes := 4
+		time.Sleep((time.Duration(numScrapes) * interval) + time.Second)
 		synctest.Wait()
 
 		scrapeManager.Stop()
 
-		maxScrapes := 0
-		for _, times := range scrapeTimes {
-			if len(times) > maxScrapes {
-				maxScrapes = len(times)
-			}
-		}
-		require.Positive(t, maxScrapes, "Expected at least one scrape")
-
-		for i := 0; i < maxScrapes; i++ {
+		for i := range numScrapes {
 			uniqueTimes := make(map[time.Duration]struct{})
 			for _, times := range scrapeTimes {
 				if i < len(times) {