Keystore 2.0 legacy_blob.rs: Fix list non existing user.

In when listing the keys for a user an empty list should be returned
instead of a system error if the user did not exists in the legacy
databse.

Test: atest keystore2_test
Change-Id: Ic9d4822dfe002adf2728b7f84e3e122a5bd1db24
diff --git a/keystore2/src/legacy_blob.rs b/keystore2/src/legacy_blob.rs
index 65e6818..a3e440b 100644
--- a/keystore2/src/legacy_blob.rs
+++ b/keystore2/src/legacy_blob.rs
@@ -801,10 +801,18 @@
     /// encoded with UID prefix.
     fn list_user(&self, user_id: u32) -> Result<Vec<String>> {
         let path = self.make_user_path_name(user_id);
-        let dir =
-            Self::with_retry_interrupted(|| fs::read_dir(path.as_path())).with_context(|| {
-                format!("In list_user: Failed to open legacy blob database. {:?}", path)
-            })?;
+        let dir = match Self::with_retry_interrupted(|| fs::read_dir(path.as_path())) {
+            Ok(dir) => dir,
+            Err(e) => match e.kind() {
+                ErrorKind::NotFound => return Ok(Default::default()),
+                _ => {
+                    return Err(e).context(format!(
+                        "In list_user: Failed to open legacy blob database. {:?}",
+                        path
+                    ))
+                }
+            },
+        };
         let mut result: Vec<String> = Vec::new();
         for entry in dir {
             let file_name = entry.context("In list_user: Trying to access dir entry")?.file_name();
@@ -1352,4 +1360,14 @@
 
         Ok(())
     }
+
+    #[test]
+    fn list_non_existing_user() -> Result<()> {
+        let temp_dir = TempDir::new("list_non_existing_user")?;
+        let legacy_blob_loader = LegacyBlobLoader::new(temp_dir.path());
+
+        assert!(legacy_blob_loader.list_user(20)?.is_empty());
+
+        Ok(())
+    }
 }