Fluxctl should search for pod labels added by helm chart

The Flux Helm chart does not have a name label on it, but we still want the flux instance
to be discoverable by fluxctl.

This change makes it so that if fluxctl does not find a pod by the labels
`name in (flux,fluxd,weave-flux-agent)`, it searches for `app=flux`.

If it does not find either label, it returns a nice message:

```
$ ./fluxctl --k8s-fwd-ns lol list-controllers
Error: Flux pod not found for labels in namespace lol:
  name in (flux,fluxd,weave-flux-agent)
  app=flux
Run 'fluxctl list-controllers --help' for usage.
$
```

Fixes #1242

cmd/fluxctl/portforward.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/justinbarrick/go-k8s-portforward"
+	"github.com/pkg/errors"
+
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+)
+
+// Attempt to create PortForwards to fluxes that match the label selectors until a Flux
+// is found or an error is returned.
+func tryPortforwards(ns string, selectors ...metav1.LabelSelector) (p *portforward.PortForward, err error) {
+	message := fmt.Sprintf("Flux pod not found for labels in namespace %s:", ns)
+
+	for _, selector := range selectors {
+		p, err = tryPortforward(ns, selector)
+		if err == nil {
+			return
+		}
+
+		if ! strings.Contains(err.Error(), "Could not find pod for selector") {
+			return
+		} else {
+			message = fmt.Sprintf("%s\n  %s", message, metav1.FormatLabelSelector(&selector))
+		}
+	}
+
+	if err != nil {
+		err = errors.New(message)
+	}
+
+	return
+}
+
+// Attempt to create a portforward in the namespace for the provided LabelSelector
+func tryPortforward(ns string, selector metav1.LabelSelector) (*portforward.PortForward, error) {
+	portforwarder, err := portforward.NewPortForwarder(ns, selector, 3030)
+	if err != nil {
+		return portforwarder, err
+	}
+
+	err = portforwarder.Start()
+	if err != nil {
+		return portforwarder, err
+	}
+
+	return portforwarder, nil
+}

cmd/fluxctl/root_cmd.go

@@ -7,15 +7,13 @@ import (
 	"os"
 	"strings"
 
-	"github.com/justinbarrick/go-k8s-portforward"
 	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
 
 	"github.com/weaveworks/flux/api"
 	transport "github.com/weaveworks/flux/http"
 	"github.com/weaveworks/flux/http/client"
-
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 )
 
@@ -103,22 +101,21 @@ func (opts *rootOpts) PersistentPreRunE(cmd *cobra.Command, _ []string) error {
 	}
 
 	if opts.URL == "" {
-		portforwarder, err := portforward.NewPortForwarder(opts.Namespace, metav1.LabelSelector{
+		portforwarder, err := tryPortforwards(opts.Namespace, metav1.LabelSelector{
 			MatchExpressions: []metav1.LabelSelectorRequirement{
 				metav1.LabelSelectorRequirement{
 					Key:      "name",
 					Operator: metav1.LabelSelectorOpIn,
 					Values:   []string{"flux", "fluxd", "weave-flux-agent"},
 				},
 			},
-		}, 3030)
-		if err != nil {
-			return errors.Wrap(err, "initializing port forwarder")
-		}
-
-		err = portforwarder.Start()
+		}, metav1.LabelSelector{
+			MatchLabels: map[string]string{
+				"app": "flux",
+			},
+		})
 		if err != nil {
-			return errors.Wrap(err, "creating port forward")
+			return err
 		}
 
 		opts.URL = fmt.Sprintf("http://127.0.0.1:%d/api/flux", portforwarder.ListenPort)