refactored overlay class

overlay.GetOverlay(name) returns now an error if the overlay doesn't
exist. This is the most canonical way to act if there is no overlay.

internal/app/wwctl/overlay/create/main_test.go

@@ -0,0 +1,63 @@
+package create
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/warewulf/warewulf/internal/pkg/testenv"
+	"github.com/warewulf/warewulf/internal/pkg/warewulfd"
+)
+
+func Test_OverlayCreate(t *testing.T) {
+	tests := []struct {
+		name        string
+		overlayName string
+		setup       func(t *testing.T, overlayDir string)
+		wantErr     bool
+	}{
+		{
+			name:        "simple create",
+			overlayName: "test-overlay",
+			setup:       func(t *testing.T, overlayDir string) {},
+			wantErr:     false,
+		},
+		{
+			name:        "create existing overlay",
+			overlayName: "test-overlay-exists",
+			setup: func(t *testing.T, overlayDir string) {
+				err := os.MkdirAll(overlayDir, 0755)
+				assert.NoError(t, err)
+			},
+			wantErr: true,
+		},
+	}
+
+	warewulfd.SetNoDaemon()
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			env := testenv.New(t)
+			conf := env.Configure()
+
+			overlayDir := filepath.Join(conf.Paths.WWOverlaydir, tt.overlayName)
+			tt.setup(t, overlayDir)
+
+			baseCmd := GetCommand()
+			baseCmd.SetArgs([]string{tt.overlayName})
+			buf := new(bytes.Buffer)
+			baseCmd.SetOut(buf)
+			baseCmd.SetErr(buf)
+			err := baseCmd.Execute()
+
+			if tt.wantErr {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+				_, err := os.Stat(overlayDir)
+				assert.NoError(t, err, "overlay directory should have been created")
+			}
+		})
+	}
+}