Fix agama config show before a product is selected (#2066)

## Problem

- #2057 
- https://trello.com/c/I7rdVd7D

## Solution

~Return an empty JSON object (Ruby Hash) instead of null/nil~

Adapt the Rust client to deal with the `null` and omit the section.

## Testing

- Tested manually: 
  - `./testing-using-container.sh` and in its shell, `agama config show`
  - Web UI still works normally
- Added a unit test

## Debugging

[`git bisect`](https://git-scm.com/docs/git-bisect) saved me

## Screenshots

No

rust/agama-lib/src/storage/client.rs

@@ -148,7 +148,7 @@ impl<'a> StorageClient<'a> {
     pub async fn set_config(&self, settings: StorageSettings) -> Result<u32, ServiceError> {
         Ok(self
             .storage_proxy
-            .set_config(serde_json::to_string(&settings).unwrap().as_str())
+            .set_config(serde_json::to_string(&settings)?.as_str())
             .await?)
     }
 
@@ -158,9 +158,9 @@ impl<'a> StorageClient<'a> {
     }
 
     /// Get the storage config according to the JSON schema
-    pub async fn get_config(&self) -> Result<StorageSettings, ServiceError> {
+    pub async fn get_config(&self) -> Result<Option<StorageSettings>, ServiceError> {
         let serialized_settings = self.storage_proxy.get_config().await?;
-        let settings = serde_json::from_str(serialized_settings.as_str()).unwrap();
+        let settings = serde_json::from_str(serialized_settings.as_str())?;
         Ok(settings)
     }
 
@@ -230,7 +230,7 @@ impl<'a> StorageClient<'a> {
         &'b self,
         object: &'b DBusObject,
         name: &str,
-    ) -> Option<&HashMap<String, OwnedValue>> {
+    ) -> Option<&'b HashMap<String, OwnedValue>> {
         let interface: OwnedInterfaceName = InterfaceName::from_str_unchecked(name).into();
         let interfaces = &object.1;
         interfaces.get(&interface)

rust/agama-lib/src/storage/http_client.rs

@@ -32,7 +32,7 @@ impl StorageHTTPClient {
         Self { client: base }
     }
 
-    pub async fn get_config(&self) -> Result<StorageSettings, ServiceError> {
+    pub async fn get_config(&self) -> Result<Option<StorageSettings>, ServiceError> {
         self.client.get("/storage/config").await
     }
 

rust/agama-lib/src/storage/store.rs

@@ -37,7 +37,7 @@ impl StorageStore {
         })
     }
 
-    pub async fn load(&self) -> Result<StorageSettings, ServiceError> {
+    pub async fn load(&self) -> Result<Option<StorageSettings>, ServiceError> {
         self.storage_client.get_config().await
     }
 
@@ -80,7 +80,9 @@ mod test {
         let url = server.url("/api");
 
         let store = storage_store(url);
-        let settings = store.load().await?;
+        let opt_settings = store.load().await?;
+        assert!(opt_settings.is_some());
+        let settings = opt_settings.unwrap();
 
         // main assertion
         assert_eq!(settings.storage.unwrap().get(), r#"{ "some": "stuff" }"#);
@@ -91,6 +93,28 @@ mod test {
         Ok(())
     }
 
+    #[test]
+    async fn test_getting_storage_null() -> Result<(), Box<dyn Error>> {
+        let server = MockServer::start();
+        let storage_mock = server.mock(|when, then| {
+            when.method(GET).path("/api/storage/config");
+            then.status(200)
+                .header("content-type", "application/json")
+                .body("null");
+        });
+        let url = server.url("/api");
+
+        let store = storage_store(url);
+        let opt_settings = store.load().await?;
+
+        // main assertion
+        assert!(opt_settings.is_none());
+
+        // Ensure the specified mock was called exactly one time (or fail with a detailed error description).
+        storage_mock.assert();
+        Ok(())
+    }
+
     #[test]
     async fn test_setting_storage_ok() -> Result<(), Box<dyn Error>> {
         let server = MockServer::start();

rust/agama-lib/src/store.rs

@@ -80,9 +80,10 @@ impl Store {
             ..Default::default()
         };
 
-        let storage_settings = self.storage.load().await?;
-        settings.storage = storage_settings.storage;
-        settings.storage_autoyast = storage_settings.storage_autoyast;
+        if let Some(storage_settings) = self.storage.load().await? {
+            settings.storage = storage_settings.storage;
+            settings.storage_autoyast = storage_settings.storage_autoyast;
+        }
 
         // TODO: use try_join here
         Ok(settings)

rust/agama-server/src/storage/web.rs

@@ -154,7 +154,9 @@ pub async fn storage_service(dbus: zbus::Connection) -> Result<Router, ServiceEr
         (status = 400, description = "The D-Bus service could not perform the action")
     )
 )]
-async fn get_config(State(state): State<StorageState<'_>>) -> Result<Json<StorageSettings>, Error> {
+async fn get_config(
+    State(state): State<StorageState<'_>>,
+) -> Result<Json<Option<StorageSettings>>, Error> {
     // StorageSettings is just a wrapper over serde_json::value::RawValue
     let settings = state.client.get_config().await.map_err(Error::Service)?;
     Ok(Json(settings))