assertion: address NaN issue

When a timeseries is constant zero, the computations were previously resulting
in NaNs, which we don't want here.

Author: tbg

pkg/kv/kvserver/asim/assertion/assert.go

@@ -128,10 +128,28 @@ func (sa SteadyStateAssertion) Assert(
 		max, _ := stats.Max(trimmedStoreStats)
 		min, _ := stats.Min(trimmedStoreStats)
 
-		maxMean := math.Abs(max/mean - 1)
-		minMean := math.Abs(min/mean - 1)
+		var maxMean, minMean float64
+		if mean == 0 {
+			if min == 0 || max == 0 {
+				// If the min is zero, all datapoints are nonnegative, so for the mean
+				// to be zero, they must all be zero. If the max is zero, vice versa.
+				// Define 0/0=1 to capture that they're equal, which is what matters
+				// here (0/0 defaults to NaN in Go, but we don't want that here).
+				maxMean = 0
+				minMean = 0
+			} else {
+				// The datapoints cross zero, and their mean is zero, for example
+				// [-1, 1]. The values here would also result from the "regular"
+				// computation below, but it doesn't hurt to be explicit.
+				minMean = math.Inf(1)
+				maxMean = math.Inf(1)
+			}
+		} else {
+			maxMean = math.Abs(max/mean - 1)
+			minMean = math.Abs(min/mean - 1)
+		}
 
-		if sa.Threshold.isViolated(maxMean) || sa.Threshold.isViolated(minMean) {
+		if sa.Threshold.isViolated(maxMean) || sa.Threshold.isViolated(minMean) || math.IsNaN(maxMean) || math.IsNaN(minMean) {
 			if holds {
 				fmt.Fprintf(&buf, "  %s\n", sa)
 				holds = false

pkg/kv/kvserver/asim/tests/testdata/non_rand/example_multi_store.txt

@@ -19,7 +19,6 @@ assertion stat=leases type=steady ticks=6 exact_bound=0
 ----
 asserting: |leases(t)/mean_{T}(leases) - 1| = 0.00 ∀ t∈T and each store (T=last 6 ticks)
 
-# TODO(tbg): fix the NaN issue here.
 eval duration=5m seed=42 metrics=(leases) cfgs=(sma-count,mma-only)
 ----
 leases#1: first: [s1=8, s2=3, s3=0, s4=0, s5=0, s6=0, s7=0, s8=0, s9=1, s10=0, s11=1, s12=0, s13=1, s14=0] (stddev=2.10, mean=1.00, sum=14)
@@ -36,9 +35,3 @@ failed assertion sample 1
 	max/mean=4.00 tick=3
 	max/mean=4.00 tick=4
 	max/mean=4.00 tick=5
-  steady state stat=leases threshold=(=0.00) ticks=6
-	store=3 min/mean=NaN max/mean=NaN
-	store=4 min/mean=NaN max/mean=NaN
-	store=5 min/mean=NaN max/mean=NaN
-	store=7 min/mean=NaN max/mean=NaN
-	store=8 min/mean=NaN max/mean=NaN