kubectl: set maxUnavailable to 1 if both fenceposts resolve to zero

Due to rounding down for maxUnavailable, we may end up with rolling updates
that have zero surge and unavailable pods something that 1) is not allowed
as per validation, 2) blocks updates. If we end up in such a situation
set maxUnavailable to 1 on the theory that surge might not work due to
quota.

pkg/util/deployment/deployment.go

@@ -466,3 +466,33 @@ func WaitForObservedDeployment(getDeploymentFunc func() (*extensions.Deployment,
 		return deployment.Status.ObservedGeneration >= desiredGeneration, nil
 	})
 }
+
+// ResolveFenceposts resolves both maxSurge and maxUnavailable. This needs to happen in one
+// step. For example:
+//
+// 2 desired, max unavailable 1%, surge 0% - should scale old(-1), then new(+1), then old(-1), then new(+1)
+// 1 desired, max unavailable 1%, surge 0% - should scale old(-1), then new(+1)
+// 2 desired, max unavailable 25%, surge 1% - should scale new(+1), then old(-1), then new(+1), then old(-1)
+// 1 desired, max unavailable 25%, surge 1% - should scale new(+1), then old(-1)
+// 2 desired, max unavailable 0%, surge 1% - should scale new(+1), then old(-1), then new(+1), then old(-1)
+// 1 desired, max unavailable 0%, surge 1% - should scale new(+1), then old(-1)
+func ResolveFenceposts(maxSurge, maxUnavailable *intstrutil.IntOrString, desired int) (int, int, error) {
+	surge, err := intstrutil.GetValueFromIntOrPercent(maxSurge, desired, true)
+	if err != nil {
+		return 0, 0, err
+	}
+	unavailable, err := intstrutil.GetValueFromIntOrPercent(maxUnavailable, desired, false)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	if surge == 0 && unavailable == 0 {
+		// Validation should never allow the user to explicitly use zero values for both maxSurge
+		// maxUnavailable. Due to rounding down maxUnavailable though, it may resolve to zero.
+		// If both fenceposts resolve to zero, then we should set maxUnavailable to 1 on the
+		// theory that surge might not work due to quota.
+		unavailable = 1
+	}
+
+	return surge, unavailable, nil
+}