From 639b14e8ab8f5a3e2780744c43b191a6e4bdd9de Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@gmail.com>
Date: Thu, 15 Aug 2019 23:29:59 +0300
Subject: [PATCH] app/vmselect/promql: properly handle corner cases for rollup
 functions

---
 app/vmselect/promql/rollup.go      | 20 +++++++++++++++-----
 app/vmselect/promql/rollup_test.go | 17 ++++++++++++++---
 2 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/app/vmselect/promql/rollup.go b/app/vmselect/promql/rollup.go
index a627c209e..16df4755c 100644
--- a/app/vmselect/promql/rollup.go
+++ b/app/vmselect/promql/rollup.go
@@ -615,10 +615,15 @@ func rollupDelta(rfa *rollupFuncArg) float64 {
 		if len(values) == 0 {
 			return nan
 		}
+		if len(values) == 1 {
+			// Assume that the previous non-existing value was 0.
+			return values[0]
+		}
 		prevValue = values[0]
 		values = values[1:]
 	}
 	if len(values) == 0 {
+		// Assume that the value didn't change on the given interval.
 		return 0
 	}
 	return values[len(values)-1] - prevValue
@@ -632,6 +637,7 @@ func rollupIdelta(rfa *rollupFuncArg) float64 {
 		if math.IsNaN(rfa.prevValue) {
 			return nan
 		}
+		// Assume that the value didn't change on the given interval.
 		return 0
 	}
 	lastValue := values[len(values)-1]
@@ -639,7 +645,8 @@ func rollupIdelta(rfa *rollupFuncArg) float64 {
 	if len(values) == 0 {
 		prevValue := rfa.prevValue
 		if math.IsNaN(prevValue) {
-			return 0
+			// Assume that the previous non-existing value was 0.
+			return lastValue
 		}
 		return lastValue - prevValue
 	}
@@ -661,7 +668,8 @@ func rollupDerivFast(rfa *rollupFuncArg) float64 {
 	prevValue := rfa.prevValue
 	prevTimestamp := rfa.prevTimestamp
 	if math.IsNaN(prevValue) {
-		if len(values) == 0 {
+		if len(values) < 2 {
+			// It is impossible to calculate derivative on 0 or 1 values.
 			return nan
 		}
 		prevValue = values[0]
@@ -670,6 +678,7 @@ func rollupDerivFast(rfa *rollupFuncArg) float64 {
 		timestamps = timestamps[1:]
 	}
 	if len(values) == 0 {
+		// Assume that the value didn't change on the given interval.
 		return 0
 	}
 	vEnd := values[len(values)-1]
@@ -684,11 +693,12 @@ func rollupIderiv(rfa *rollupFuncArg) float64 {
 	// before calling rollup funcs.
 	values := rfa.values
 	timestamps := rfa.timestamps
-	if len(values) == 0 {
-		if math.IsNaN(rfa.prevValue) {
+	if len(values) < 2 {
+		if len(values) == 0 || math.IsNaN(rfa.prevValue) {
+			// It is impossible to calculate derivative on 0 or 1 values.
 			return nan
 		}
-		return 0
+		return (values[0] - rfa.prevValue) / float64(timestamps[0]-rfa.prevTimestamp)
 	}
 	vEnd := values[len(values)-1]
 	tEnd := timestamps[len(timestamps)-1]
diff --git a/app/vmselect/promql/rollup_test.go b/app/vmselect/promql/rollup_test.go
index fcf598b63..337d63458 100644
--- a/app/vmselect/promql/rollup_test.go
+++ b/app/vmselect/promql/rollup_test.go
@@ -45,8 +45,19 @@ func TestRollupIderivDuplicateTimestamps(t *testing.T) {
 		timestamps: []int64{100},
 	}
 	n = rollupIderiv(rfa)
-	if n != 0 {
-		t.Fatalf("unexpected value; got %v; want %v", n, 0)
+	if !math.IsNaN(n) {
+		t.Fatalf("unexpected value; got %v; want %v", n, nan)
+	}
+
+	rfa = &rollupFuncArg{
+		prevTimestamp: 90,
+		prevValue:     10,
+		values:        []float64{15},
+		timestamps:    []int64{100},
+	}
+	n = rollupIderiv(rfa)
+	if n != 0.5 {
+		t.Fatalf("unexpected value; got %v; want %v", n, 0.5)
 	}
 
 	rfa = &rollupFuncArg{
@@ -569,7 +580,7 @@ func TestRollupFuncsNoWindow(t *testing.T) {
 		}
 		rc.Timestamps = getTimestamps(rc.Start, rc.End, rc.Step)
 		values := rc.Do(nil, testValues, testTimestamps)
-		valuesExpected := []float64{0, 33, -87, 0}
+		valuesExpected := []float64{123, 33, -87, 0}
 		timestampsExpected := []int64{10, 50, 90, 130}
 		testRowsEqual(t, values, rc.Timestamps, valuesExpected, timestampsExpected)
 	})