Fix edge case with spread scoring

scheduler/spread.go

@@ -214,7 +214,12 @@ func evenSpreadScoreBoost(pset *propertySet, option *structs.Node) float64 {
 	} else if minCount == maxCount {
 		// Maximum possible penalty when the distribution is even
 		return -1.0
+	} else if minCount == 0 {
+		// Current attribute count is equal to min and both are zero. This means no allocations
+		// were placed for this attribute value yet. Should get the maximum possible boost.
+		return 1.0
 	}
+
 	// Penalty based on delta from max value
 	delta := int(maxCount - minCount)
 	deltaBoost = float64(delta) / float64(minCount)

scheduler/spread_test.go

@@ -1,6 +1,7 @@
 package scheduler
 
 import (
+	"math"
 	"testing"
 
 	"fmt"
@@ -311,6 +312,7 @@ func TestSpreadIterator_EvenSpread(t *testing.T) {
 	}
 	for _, rn := range out {
 		require.Equal(t, fmt.Sprintf("%.3f", expectedScores[rn.Node.Datacenter]), fmt.Sprintf("%.3f", rn.FinalScore))
+
 	}
 
 	// Update the plan to add allocs to nodes in dc1
@@ -543,3 +545,26 @@ func TestSpreadIterator_MaxPenalty(t *testing.T) {
 	}
 
 }
+
+func Test_evenSpreadScoreBoost(t *testing.T) {
+	pset := &propertySet{
+		existingValues: map[string]uint64{},
+		proposedValues: map[string]uint64{
+			"dc2": 1,
+			"dc1": 1,
+			"dc3": 1,
+		},
+		clearedValues: map[string]uint64{
+			"dc2": 1,
+			"dc3": 1,
+		},
+		targetAttribute: "${node.datacenter}",
+	}
+
+	opt := &structs.Node{
+		Datacenter: "dc2",
+	}
+	boost := evenSpreadScoreBoost(pset, opt)
+	require.False(t, math.IsInf(boost, 1))
+	require.Equal(t, 1.0, boost)
+}