builder: prepend stargz root entry automatically

Signed-off-by: Yan Song <yansong.ys@antfin.com>

Author: imeoer

src/bin/nydus-image/builder/stargz.rs

@@ -298,7 +298,7 @@ impl TocIndex {
     fn load(path: &Path) -> Result<TocIndex> {
         let index_file = File::open(path)
             .with_context(|| format!("failed to open stargz index file {:?}", path))?;
-        let toc_index: TocIndex = serde_json::from_reader(index_file)
+        let mut toc_index: TocIndex = serde_json::from_reader(index_file)
             .with_context(|| format!("invalid stargz index file {:?}", path))?;
 
         if toc_index.version != 1 {
@@ -308,6 +308,15 @@ impl TocIndex {
             )));
         }
 
+        // Append root directory entry if not exists.
+        if !toc_index.entries.is_empty() && toc_index.entries[0].name()? != PathBuf::from("/") {
+            let root_entry = TocEntry {
+                toc_type: String::from("dir"),
+                ..Default::default()
+            };
+            toc_index.entries.insert(0, root_entry);
+        }
+
         Ok(toc_index)
     }
 }

tests/texture/stargz/stargz.index-lower.json

@@ -1,15 +1,6 @@
 {
 	"version": 1,
 	"entries": [
-		{
-			"name": "",
-			"type": "dir",
-			"modtime": "2020-09-10T07:40:29Z",
-			"mode": 493,
-			"userName": "root",
-			"groupName": "root",
-			"NumLink": 0
-		},
 		{
 			"name": "root-large",
 			"type": "reg",

tests/texture/stargz/stargz.index-upper.json

@@ -1,15 +1,6 @@
 {
 	"version": 1,
 	"entries": [
-		{
-			"name": "",
-			"type": "dir",
-			"modtime": "2020-09-10T06:43:07Z",
-			"mode": 493,
-			"userName": "root",
-			"groupName": "root",
-			"NumLink": 0
-		},
 		{
 			"name": "sub/",
 			"type": "dir",