feat: linreg => slope 0 for const values (#9728)

* feat: linreg => slope 0 for const values

Signed-off-by: mtfoley <mtfoley.mae@gmail.com>

promql/functions.go

@@ -716,8 +716,16 @@ func linearRegression(samples []Point, interceptTime int64) (slope, intercept fl
 		sumY, cY   float64
 		sumXY, cXY float64
 		sumX2, cX2 float64
+		initY      float64
+		constY     bool
 	)
-	for _, sample := range samples {
+	initY = samples[0].V
+	constY = true
+	for i, sample := range samples {
+		// Set constY to false if any new y values are encountered.
+		if constY && i > 0 && sample.V != initY {
+			constY = false
+		}
 		n += 1.0
 		x := float64(sample.T-interceptTime) / 1e3
 		sumX, cX = kahanSumInc(x, sumX, cX)
@@ -725,7 +733,12 @@ func linearRegression(samples []Point, interceptTime int64) (slope, intercept fl
 		sumXY, cXY = kahanSumInc(x*sample.V, sumXY, cXY)
 		sumX2, cX2 = kahanSumInc(x*x, sumX2, cX2)
 	}
-
+	if constY {
+		if math.IsInf(initY, 0) {
+			return math.NaN(), math.NaN()
+		}
+		return 0, initY
+	}
 	sumX = sumX + cX
 	sumY = sumY + cY
 	sumXY = sumXY + cXY

promql/functions_test.go

@@ -44,8 +44,14 @@ func TestDeriv(t *testing.T) {
 	a := storage.Appender(context.Background())
 
 	metric := labels.FromStrings("__name__", "foo")
-	a.Append(0, metric, 1493712816939, 1.0)
+	start := 1493712816939
-	a.Append(0, metric, 1493712846939, 1.0)
+	interval := 30 * 1000
+	// Introduce some timestamp jitter to test 0 slope case.
+	// https://github.com/prometheus/prometheus/issues/7180
+	for i := 0; i < 15; i++ {
+		jitter := 12 * i % 2
+		a.Append(0, metric, int64(start+interval*i+jitter), 1)
+	}
 
 	require.NoError(t, a.Commit())