Basic check of user data structure for errors

virtweb_backend/src/utils/cloud_init_utils.rs

@@ -35,6 +35,27 @@ pub struct CloudInitConfig {
 }
 
 impl CloudInitConfig {
+    /// Check cloud init configuration
+    pub fn check_error(&self) -> Option<String> {
+        if !self.user_data.is_empty() {
+            // Check YAML content
+            if let Err(e) = serde_yml::from_str::<serde_json::Value>(&self.user_data) {
+                return Some(format!(
+                    "user data is an invalid YAML file! Deserialization error: {e}"
+                ));
+            }
+
+            // Check first line
+            if !self.user_data.starts_with("#cloud-config\n") {
+                return Some(
+                    "user data file MUST start with '#cloud-config' as first line!".to_string(),
+                );
+            }
+        }
+
+        None
+    }
+
     /// Generate disk image for nocloud usage
     pub fn generate_nocloud_disk(&self) -> anyhow::Result<Vec<u8>> {
         let temp_path = tempfile::tempdir_in(&AppConfig::get().temp_dir)?;