making tarball flag global and adding error checking

cmd/dmverity-vhd/README.md

@@ -37,5 +37,5 @@ dmverity-vhd --docker roothash -i alpine:3.12
 Compute root hashes with tarball:
 
 ```bash
-dmverity-vhd roothash --tarball /path/to/tarball.tar -i alpine:3.12
+dmverity-vhd --tarball /path/to/tarball.tar roothash -i alpine:3.12
 ```

cmd/dmverity-vhd/main.go

@@ -64,6 +64,26 @@ func main() {
 			Name:  dockerFlag + ",d",
 			Usage: "Optional: use local docker daemon",
 		},
+		cli.StringFlag{
+			Name:  tarballFlag + ",t",
+			Usage: "Optional: path to tarball containing image info",
+		},
+	}
+
+	// error check to see if the `docker` and `tarball` flags are both being used
+	// without having access to the `cli.Context` struct
+	localDockerUsed := false
+	tarballUsed := false
+	for i := 0; i < len(os.Args); i++ {
+		if os.Args[i] == "-d" || os.Args[i] == "-"+dockerFlag {
+			localDockerUsed = true
+		} else if os.Args[i] == "-t" || os.Args[i] == "-"+tarballFlag {
+			tarballUsed = true
+		}
+	}
+	if localDockerUsed && tarballUsed {
+		_, _ = fmt.Fprintln(os.Stderr, "cannot use both docker and tarball for image source")
+		os.Exit(1)
 	}
 
 	if err := app.Run(os.Args); err != nil {
@@ -74,7 +94,7 @@ func main() {
 
 func fetchImageLayers(ctx *cli.Context) (layers []v1.Layer, err error) {
 	image := ctx.String(imageFlag)
-	tarballPath := ctx.String(tarballFlag)
+	tarballPath := ctx.GlobalString(tarballFlag)
 	ref, err := name.ParseReference(image)
 	if err != nil {
 		return nil, errors.Wrapf(err, "failed to parse image reference: %s", image)
@@ -229,10 +249,6 @@ var rootHashVHDCommand = cli.Command{
 			Usage:    "Required: container image reference",
 			Required: true,
 		},
-		cli.StringFlag{
-			Name:  tarballFlag + ",t",
-			Usage: "Optional: path to tarball containing image info",
-		},
 		cli.StringFlag{
 			Name:  usernameFlag + ",u",
 			Usage: "Optional: custom registry username",