ps: Avoid error when the storage is uninitialized

Running ignite ps for the first time on a new machine fails with:
```
FATA[0000] open /var/lib/firecracker/vm: no such file or directory
```

This change handles the error returned when the backend storage is
uninitialized. NewPsOptions() returns an empty list of VMs and allows
the ps execution to continue without any error.

cmd/ignite/run/ps.go

@@ -3,6 +3,7 @@ package run
 import (
 	"bytes"
 	"fmt"
+	"strings"
 	"text/template"
 
 	api "github.com/weaveworks/ignite/pkg/apis/ignite"
@@ -27,6 +28,11 @@ type PsOptions struct {
 func (pf *PsFlags) NewPsOptions() (po *PsOptions, err error) {
 	po = &PsOptions{PsFlags: pf}
 	po.allVMs, err = providers.Client.VMs().FindAll(filter.NewVMFilterAll("", po.All))
+	// If the storage is uninitialized, avoid failure and continue with empty
+	// VM list.
+	if err != nil && strings.Contains(err.Error(), "no such file or directory") {
+		err = nil
+	}
 	return
 }
 

cmd/ignite/run/ps_test.go

@@ -11,8 +11,14 @@ import (
 	"time"
 
 	api "github.com/weaveworks/ignite/pkg/apis/ignite"
+	"github.com/weaveworks/ignite/pkg/apis/ignite/scheme"
 	meta "github.com/weaveworks/ignite/pkg/apis/meta/v1alpha1"
+	"github.com/weaveworks/ignite/pkg/client"
+	"github.com/weaveworks/ignite/pkg/providers"
 	"github.com/weaveworks/libgitops/pkg/runtime"
+	"github.com/weaveworks/libgitops/pkg/storage"
+	"github.com/weaveworks/libgitops/pkg/storage/cache"
+	"gotest.tools/assert"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 
 	"github.com/weaveworks/ignite/pkg/util"
@@ -167,3 +173,23 @@ func TestPs(t *testing.T) {
 	}
 
 }
+
+// TestNewPsOptionsStorageNotExists tests that no error is returned if the
+// storage directory doesn't exist and NewPsOptions() succeeds with empty VM
+// list.
+func TestNewPsOptionsStorageNotExists(t *testing.T) {
+	// Path to a directory that doesn't exist.
+	dir := "/tmp/ignite-fake-storage-path"
+	storage := cache.NewCache(
+		storage.NewGenericStorage(
+			storage.NewGenericRawStorage(dir), scheme.Serializer))
+
+	// Create ignite client with the storage.
+	providers.Client = client.NewClient(storage)
+
+	// Create a new PsOptions and check result.
+	pf := &PsFlags{}
+	po, err := pf.NewPsOptions()
+	assert.NilError(t, err)
+	assert.Equal(t, 0, len(po.allVMs))
+}