Patch in the VirtualService is not working when there is only one named route

[jeanmorais]

Checklist:

• I've included steps to reproduce the bug.
• I've included the version of argo rollouts.

Describe the bug

When the VirtualService has only 1 HTTPRoute, with the defined name, and this name is not defined in Rollout (trafficRouting.istio.virtualService.routes), the patch is not performed.

To Reproduce

apiVersion: argoproj.io/v1alpha1
kind: Rollout
metadata:
  labels:
  name: canary-sample-app
  namespace: sample-canary
spec:
  replicas: 3
  selector:
    matchLabels:
      app: canary-sample-app
  strategy:
    canary:
      canaryMetadata:
        labels:
          canary: "true"
      maxSurge: 25%
      maxUnavailable: 0
      stableMetadata:
        labels:
          canary: "false"
      steps:
      - pause: {}
      - setWeight: 30
      - pause: {}
      - setWeight: 70
      - pause: {}
      trafficRouting:
        istio:
          destinationRule:
            canarySubsetName: canary
            name: canary-sample-app
            stableSubsetName: stable
          virtualService:
            name: canary-sample-app
---
apiVersion: networking.istio.io/v1beta1
kind: VirtualService
metadata:
  name: canary-sample-app
  namespace: sample-canary
spec:
  gateways:
  - canary-sample-app
  hosts:
  - canary-sample-app.domain.com
  http:
  - match:
    - uri:
        prefix: /
    name: default
    route:
    - destination:
        host: canary-sample-app
        port:
          number: 80
        subset: stable
      weight: 100
    - destination:
        host: canary-sample-app
        port:
          number: 80
        subset: canary
      weight: 0

The doc mentions that (optional if there is a single route in VirtualService, required otherwise):

      trafficRouting:
        istio:
          virtualService:
            name: rollout-vsvc   # required
            routes:
            - primary            # optional if there is a single route in VirtualService, required otherwise

Ref: https://argo-rollouts.readthedocs.io/en/stable/features/traffic-management/istio/#host-level-traffic-splitting

But it doesn't happen here:

argo-rollouts/rollout/trafficrouting/istio/istio.go

Lines 903 to 913 in 50300e5

	// getHttpRouteIndexesToPatch returns array indices of the httpRoutes which need to be patched when updating weights
	func getHttpRouteIndexesToPatch(routeNames []string, httpRoutes []VirtualServiceHTTPRoute) ([]int, error) {
	//We have no routes listed in spec.strategy.canary.trafficRouting.istio.virtualService.routes so find index
	//of the first empty named route
	if len(routeNames) == 0 {
	for i, route := range httpRoutes {
	if route.Name == "" {
	return []int{i}, nil
	}
	}
	}

Maybe, the code should first check if the VirtualService has only one route, and then return it (I've tested it and it worked):

// something like this
	if len(routeNames) == 0 {
		if len(httpRoutes) == 1 {
			return []int{0}, nil
		}
		
		for i, route := range httpRoutes {
			if route.Name == "" {
				return []int{i}, nil
			}
		}
	}

Could you confirm if this is enough? I can open a PR with this fix.

Expected behavior

The weight field changed according to the promotion of the steps.

Version

v1.7.2

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