In histogram_quantile merge buckets with equivalent le values (#5158)

This makes things generally more resilient, and will
help with OpenMetrics transitions (and inconsistencies).

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

promql/quantile.go

@@ -75,16 +75,18 @@ func bucketQuantile(q float64, buckets buckets) float64 {
 	if q > 1 {
 		return math.Inf(+1)
 	}
-	if len(buckets) < 2 {
-		return math.NaN()
-	}
 	sort.Sort(buckets)
 	if !math.IsInf(buckets[len(buckets)-1].upperBound, +1) {
 		return math.NaN()
 	}
 
+	buckets = coalesceBuckets(buckets)
 	ensureMonotonic(buckets)
 
+	if len(buckets) < 2 {
+		return math.NaN()
+	}
+
 	rank := q * buckets[len(buckets)-1].count
 	b := sort.Search(len(buckets)-1, func(i int) bool { return buckets[i].count >= rank })
 
@@ -107,6 +109,25 @@ func bucketQuantile(q float64, buckets buckets) float64 {
 	return bucketStart + (bucketEnd-bucketStart)*(rank/count)
 }
 
+// coalesceBuckets merges buckets with the same upper bound.
+//
+// The input buckets must be sorted.
+func coalesceBuckets(buckets buckets) buckets {
+	last := buckets[0]
+	i := 0
+	for _, b := range buckets[1:] {
+		if b.upperBound == last.upperBound {
+			last.count += b.count
+		} else {
+			buckets[i] = last
+			last = b
+			i++
+		}
+	}
+	buckets[i] = last
+	return buckets[:i+1]
+}
+
 // The assumption that bucket counts increase monotonically with increasing
 // upperBound may be violated during:
 //

promql/testdata/histograms.test

@@ -31,6 +31,15 @@ load 5m
 	request_duration_seconds_bucket{job="job2", instance="ins2", le="0.2"}	0+7x10
 	request_duration_seconds_bucket{job="job2", instance="ins2", le="+Inf"}	0+9x10
 
+# Different le representations in one histogram.
+load 5m
+	mixed_bucket{job="job1", instance="ins1", le="0.1"}	0+1x10
+	mixed_bucket{job="job1", instance="ins1", le="0.2"}	0+1x10
+	mixed_bucket{job="job1", instance="ins1", le="2e-1"}	0+1x10
+	mixed_bucket{job="job1", instance="ins1", le="2.0e-1"}	0+1x10
+	mixed_bucket{job="job1", instance="ins1", le="+Inf"}	0+4x10
+	mixed_bucket{job="job1", instance="ins2", le="+inf"}	0+0x10
+	mixed_bucket{job="job1", instance="ins2", le="+Inf"}	0+0x10
 
 # Quantile too low.
 eval instant at 50m histogram_quantile(-0.1, testhistogram_bucket)
@@ -157,3 +166,16 @@ load 5m
 # Nonmonotonic buckets
 eval instant at 50m histogram_quantile(0.99, nonmonotonic_bucket)
     {} 0.989875
+
+# Buckets with different representations of the same upper bound.
+eval instant at 50m histogram_quantile(0.5, rate(mixed_bucket[5m]))
+	{instance="ins1", job="job1"} 0.15
+	{instance="ins2", job="job1"} NaN
+
+eval instant at 50m histogram_quantile(0.75, rate(mixed_bucket[5m]))
+	{instance="ins1", job="job1"} 0.2
+	{instance="ins2", job="job1"} NaN
+
+eval instant at 50m histogram_quantile(1, rate(mixed_bucket[5m]))
+	{instance="ins1", job="job1"} 0.2
+	{instance="ins2", job="job1"} NaN