HPA Controller: Check for 0-sum request value

In certain conditions in which the set of metrics returned by Heapster
is completely disjoint from the set of pods returned by the API server,
we can have a request sum of zero, which can cause a panic (due to
division by zero).  This checks for that condition.

Fixes #39680

pkg/controller/podautoscaler/metrics/utilization.go

@@ -16,6 +16,10 @@ limitations under the License.
 
 package metrics
 
+import (
+	"fmt"
+)
+
 // GetResourceUtilizationRatio takes in a set of metrics, a set of matching requests,
 // and a target utilization percentage, and calcuates the the ratio of
 // desired to actual utilization (returning that and the actual utilization)
@@ -34,6 +38,12 @@ func GetResourceUtilizationRatio(metrics PodResourceInfo, requests map[string]in
 		requestsTotal += request
 	}
 
+	// if the set of requests is completely disjoint from the set of metrics,
+	// then we could have an issue where the requests total is zero
+	if requestsTotal == 0 {
+		return 0, 0, fmt.Errorf("no metrics returned matched known pods")
+	}
+
 	currentUtilization := int32((metricsTotal * 100) / requestsTotal)
 
 	return float64(currentUtilization) / float64(targetUtilization), currentUtilization, nil