targets: offline-update: Check if dest dir already has data

This commit prevents the execution of "targets offlile-update" using a
destination directory that already contains update data (i.e.,
"ostree_repo" or "apps" subdirectories). There is no check for the tuf
subdirectory, since the metadata in that subdir is updated by default,
and there is no conflict.
It is possible to override the default behavior by using a new
--allow-multiple-targets option.

Signed-off-by: Andre Detsch <andre.detsch@foundries.io>

subcommands/targets/offline-update.go

@@ -29,11 +29,12 @@ type (
 )
 
 var (
-	ouTag       string
-	ouProd      bool
-	ouExpiresIn int
-	ouTufOnly   bool
-	ouNoApps    bool
+	ouTag                  string
+	ouProd                 bool
+	ouExpiresIn            int
+	ouTufOnly              bool
+	ouNoApps               bool
+	ouAllowMultipleTargets bool
 )
 
 func init() {
@@ -62,6 +63,8 @@ func init() {
 		"Fetch only TUF metadata")
 	offlineUpdateCmd.Flags().BoolVarP(&ouNoApps, "no-apps", "", false,
 		"Skip fetching Target Apps")
+	offlineUpdateCmd.Flags().BoolVarP(&ouAllowMultipleTargets, "allow-multiple-targets", "", false,
+		"Allow multiple targets to be stored in the same <dst> directory")
 }
 
 func doOfflineUpdate(cmd *cobra.Command, args []string) {
@@ -84,6 +87,14 @@ func doOfflineUpdate(cmd *cobra.Command, args []string) {
 	fmt.Println("Successfully refreshed and downloaded TUF metadata")
 
 	if !ouTufOnly {
+		if !isDstDirClean(dstDir) {
+			if !ouAllowMultipleTargets {
+				subcommands.DieNotNil(errors.New(`Destination directory already has update data.
+Provide a clean destination directory or re-run with --allow-multiple-targets to add a new target to a directory which already has update data.
+Notice that multiple targets in the same directory is only supported in LmP >= v92.`))
+			}
+		}
+
 		fmt.Printf("Downloading an ostree repo from the Target's OE build %d...\n", ti.ostreeVersion)
 		subcommands.DieNotNil(downloadOstree(factory, ti.ostreeVersion, ti.hardwareID, dstDir), "Failed to download Target's ostree repo:")
 		if !ouNoApps {
@@ -152,6 +163,17 @@ func getTargetInfo(factory string, targetName string) (*ouTargetInfo, error) {
 	return &info, nil
 }
 
+func isDstDirClean(dstDir string) bool {
+	for _, subDir := range []string{"ostree_repo", "apps"} {
+		fullPath := path.Join(dstDir, subDir)
+		if _, err := os.Stat(fullPath); !os.IsNotExist(err) {
+			fmt.Println(fullPath + " already exists")
+			return false
+		}
+	}
+	return true
+}
+
 func downloadTufRepo(factory string, target string, tag string, prod bool, expiresIn int, dstDir string) error {
 	// v1 - auto-generated by tuf_keyserver (default, on Factory creation);
 	// v2 - auto-generated by ota-lite to take keys online (default, on Factory creation);