promql: Fix limiting of extrapolation to negative values

promql/functions.go

@@ -128,37 +128,36 @@ func extrapolatedRate(vals []parser.Value, args parser.Expressions, enh *EvalNod
 	sampledInterval := float64(lastT-firstT) / 1000
 	averageDurationBetweenSamples := sampledInterval / float64(numSamplesMinusOne)
 
-	// TODO(beorn7): Do this for histograms, too.
+	// If the first/last samples are close to the boundaries of the range,
+	// extrapolate the result. This is as we expect that another sample
+	// will exist given the spacing between samples we've seen thus far,
+	// with an allowance for noise.
+	extrapolationThreshold := averageDurationBetweenSamples * 1.1
+	extrapolateToInterval := sampledInterval
+
+	if durationToStart >= extrapolationThreshold {
+		durationToStart = averageDurationBetweenSamples / 2
+	}
 	if isCounter && resultFloat > 0 && len(samples.Floats) > 0 && samples.Floats[0].F >= 0 {
 		// Counters cannot be negative. If we have any slope at all
 		// (i.e. resultFloat went up), we can extrapolate the zero point
 		// of the counter. If the duration to the zero point is shorter
 		// than the durationToStart, we take the zero point as the start
 		// of the series, thereby avoiding extrapolation to negative
 		// counter values.
+		// TODO(beorn7): Do this for histograms, too.
 		durationToZero := sampledInterval * (samples.Floats[0].F / resultFloat)
 		if durationToZero < durationToStart {
 			durationToStart = durationToZero
 		}
 	}
+	extrapolateToInterval += durationToStart
 
-	// If the first/last samples are close to the boundaries of the range,
-	// extrapolate the result. This is as we expect that another sample
-	// will exist given the spacing between samples we've seen thus far,
-	// with an allowance for noise.
-	extrapolationThreshold := averageDurationBetweenSamples * 1.1
-	extrapolateToInterval := sampledInterval
-
-	if durationToStart < extrapolationThreshold {
-		extrapolateToInterval += durationToStart
-	} else {
-		extrapolateToInterval += averageDurationBetweenSamples / 2
-	}
-	if durationToEnd < extrapolationThreshold {
-		extrapolateToInterval += durationToEnd
-	} else {
-		extrapolateToInterval += averageDurationBetweenSamples / 2
+	if durationToEnd >= extrapolationThreshold {
+		durationToEnd = averageDurationBetweenSamples / 2
 	}
+	extrapolateToInterval += durationToEnd
+
 	factor := extrapolateToInterval / sampledInterval
 	if isRate {
 		factor /= ms.Range.Seconds()

promql/testdata/functions.test

@@ -71,15 +71,28 @@ clear
 load 5m
 	http_requests{path="/foo"}	0+10x10
 	http_requests{path="/bar"}	0+10x5 0+10x5
+	http_requests{path="/dings"}   10+10x10
+	http_requests{path="/bumms"}    1+10x10
 
 # Tests for increase().
 eval instant at 50m increase(http_requests[50m])
-	{path="/foo"} 100
-	{path="/bar"}  90
-
+	{path="/foo"}   100
+	{path="/bar"}    90
+	{path="/dings"} 100
+	{path="/bumms"} 100
+
+# "foo" and "bar" are already at value 0 at t=0, so no extrapolation
+# happens. "dings" has value 10 at t=0 and would reach 0 at t=-5m. The
+# normal extrapolation by half a sample interval only goes to
+# t=-2m30s, so that's not yet reaching a negative value and therefore
+# chosen. However, "bumms" has value 1 at t=0 and would reach 0 at
+# t=-30s. Here the extrapolation to t=-2m30s would reach a negative
+# value, and therefore the extrapolation happens only by 30s.
 eval instant at 50m increase(http_requests[100m])
-	{path="/foo"} 100
-	{path="/bar"}  90
+	{path="/foo"}   100
+	{path="/bar"}    90
+	{path="/dings"} 105
+	{path="/bumms"} 101
 
 clear
 
@@ -133,13 +146,15 @@ load 4m
 	testcounter_zero_cutoff{start="4m"}	240+240x10
 	testcounter_zero_cutoff{start="5m"}	300+240x10
 
-# Zero cutoff for left-side extrapolation.
+# Zero cutoff for left-side extrapolation happens until we
+# reach half a sampling interval (2m). Beyond that, we only
+# extrapolate by half a sampling interval.
 eval instant at 10m rate(testcounter_zero_cutoff[20m])
 	{start="0m"} 0.5
 	{start="1m"} 0.55
 	{start="2m"} 0.6
-	{start="3m"} 0.65
-	{start="4m"} 0.7
+	{start="3m"} 0.6
+	{start="4m"} 0.6
 	{start="5m"} 0.6
 
 # Normal half-interval cutoff for left-side extrapolation.