Return an error if no data is available yet instead of 0.

pkg/autoscaler/aggregation/bucketing.go

@@ -52,6 +52,14 @@ func (t *TimedFloat64Buckets) Record(time time.Time, name string, value float64)
 	bucket.Record(name, value)
 }
 
+// IsEmpty returns whether or not there are no values currently stored.
+func (t *TimedFloat64Buckets) IsEmpty() bool {
+	t.bucketsMutex.RLock()
+	defer t.bucketsMutex.RUnlock()
+
+	return len(t.buckets) == 0
+}
+
 // ForEachBucket calls the given Accumulator function for each bucket.
 func (t *TimedFloat64Buckets) ForEachBucket(accs ...Accumulator) {
 	t.bucketsMutex.RLock()

pkg/autoscaler/aggregation/bucketing_test.go

@@ -85,8 +85,8 @@ func TestTimedFloat64Buckets(t *testing.T) {
 			if !cmp.Equal(tt.want, got) {
 				t.Errorf("Unexpected values (-want +got): %v", cmp.Diff(tt.want, got))
 			}
-			if got := len(buckets.buckets); len(tt.want) == 0 && got != 0 {
-				t.Errorf("len(buckets) = %d, want 0", got)
+			if len(tt.want) == 0 && !buckets.IsEmpty() {
+				t.Error("IsEmpty() = false, want true")
 			}
 		})
 	}

pkg/autoscaler/collector.go

@@ -18,6 +18,7 @@ package autoscaler
 
 import (
 	"context"
+	"errors"
 	"sync"
 	"time"
 
@@ -200,8 +201,7 @@ func (c *MetricCollector) StableAndPanicConcurrency(key string) (float64, float6
 		return 0, 0, k8serrors.NewNotFound(kpa.Resource("Metrics"), key)
 	}
 
-	stable, panic := collection.stableAndPanicConcurrency(time.Now())
-	return stable, panic, nil
+	return collection.stableAndPanicConcurrency(time.Now())
 }
 
 // collection represents the collection of metrics for one specific entity.
@@ -274,18 +274,23 @@ func (c *collection) record(stat Stat) {
 
 // stableAndPanicConcurrency calculates both stable and panic concurrency based on the
 // current stats.
-func (c *collection) stableAndPanicConcurrency(now time.Time) (float64, float64) {
+func (c *collection) stableAndPanicConcurrency(now time.Time) (float64, float64, error) {
 	spec := c.currentMetric().Spec
 
 	c.buckets.RemoveOlderThan(now.Add(-spec.StableWindow))
+
+	if c.buckets.IsEmpty() {
+		return 0, 0, errors.New("no data available")
+	}
+
 	panicAverage := aggregation.Average{}
 	stableAverage := aggregation.Average{}
 	c.buckets.ForEachBucket(
 		aggregation.YoungerThan(now.Add(-spec.PanicWindow), panicAverage.Accumulate),
 		stableAverage.Accumulate, // No need to add a YoungerThan condition as we already deleted all outdated stats above.
 	)
 
-	return stableAverage.Value(), panicAverage.Value()
+	return stableAverage.Value(), panicAverage.Value(), nil
 }
 
 // close stops collecting metrics, stops the scraper.

pkg/autoscaler/collector_test.go

@@ -173,9 +173,15 @@ func TestMetricCollectorRecord(t *testing.T) {
 	factory := scraperFactory(scraper, nil)
 
 	coll := NewMetricCollector(factory, logger)
+
+	// Freshly created collection does not contain any metrics and should return an error.
 	coll.Create(ctx, defaultMetric)
-	coll.Record(metricKey, stat)
+	if _, _, err := coll.StableAndPanicConcurrency(metricKey); err == nil {
+		t.Error("StableAndPanicConcurrency() = nil, wanted an error")
+	}
 
+	// After adding a stat the concurrencies are calculated correctly.
+	coll.Record(metricKey, stat)
 	if stable, panic, _ := coll.StableAndPanicConcurrency(metricKey); stable != panic && stable != want {
 		t.Errorf("StableAndPanicConcurrency() = %v, %v; want %v (for both)", stable, panic, want)
 	}