add query to obtain the chain ID of a user (#4817)

## Motivation

Making debugging easier

## Proposal

add query to obtain the chain ID of a user

## Test Plan

CI

## Release Plan

- These changes should be backported to the latest `devnet` branch, then
    - be released in a FAUCET hotfix.

Author: ma2bd

linera-faucet/server/src/lib.rs

@@ -180,6 +180,7 @@ mod tests;
 pub struct QueryRoot<C: ClientContext> {
     client: ChainClient<C::Environment>,
     genesis_config: Arc<GenesisConfig>,
+    faucet_storage: Arc<FaucetDatabase>,
 }
 
 /// The root GraphQL mutation type.
@@ -265,6 +266,26 @@ where
     async fn current_committee(&self) -> Result<Committee, Error> {
         Ok(self.client.local_committee().await?)
     }
+
+    /// Find the existing a chain with the given authentication key, if any.
+    async fn chain_id(&self, owner: AccountOwner) -> Result<ChainId, Error> {
+        // Check if this owner already has a chain.
+        #[cfg(with_metrics)]
+        let db_start_time = std::time::Instant::now();
+
+        let chain_id = self
+            .faucet_storage
+            .get_chain_id(&owner)
+            .await
+            .map_err(|e| Error::new(e.to_string()))?;
+
+        #[cfg(with_metrics)]
+        metrics::DATABASE_OPERATION_LATENCY
+            .with_label_values(&["get_chain_id"])
+            .observe(db_start_time.elapsed().as_secs_f64() * 1000.0);
+
+        chain_id.ok_or(Error::new("This user has no chain yet"))
+    }
 }
 
 #[async_graphql::Object(cache_control(no_cache))]
@@ -896,6 +917,7 @@ where
         let query_root = QueryRoot {
             genesis_config: Arc::clone(&self.genesis_config),
             client: self.client.clone(),
+            faucet_storage: Arc::clone(&self.faucet_storage),
         };
         Schema::build(query_root, mutation_root, EmptySubscription).finish()
     }

linera-faucet/server/src/tests.rs

@@ -399,6 +399,38 @@ async fn test_faucet_persistence() {
     // Create a new faucet instance with the same database path (simulating restart)
     let faucet_storage_2 = Arc::new(FaucetDatabase::new(&storage_path).await.unwrap());
 
+    // Test the chain_id query API through the database for owners that have claimed chains
+    let queried_chain_1 = faucet_storage_2
+        .get_chain_id(&test_owner_1)
+        .await
+        .expect("Query should succeed for owner 1")
+        .expect("Owner 1 should have a chain ID");
+    assert_eq!(
+        chain_1_id, queried_chain_1,
+        "Query should return correct chain ID for owner 1"
+    );
+
+    let queried_chain_2 = faucet_storage_2
+        .get_chain_id(&test_owner_2)
+        .await
+        .expect("Query should succeed for owner 2")
+        .expect("Owner 2 should have a chain ID");
+    assert_eq!(
+        chain_2_id, queried_chain_2,
+        "Query should return correct chain ID for owner 2"
+    );
+
+    // Test the chain_id query for an owner that hasn't claimed a chain yet
+    let test_owner_new = AccountPublicKey::test_key(99).into();
+    let result_new = faucet_storage_2
+        .get_chain_id(&test_owner_new)
+        .await
+        .expect("Query should succeed even for non-existent owner");
+    assert!(
+        result_new.is_none(),
+        "Query should return None for owner that hasn't claimed a chain"
+    );
+
     // Set up the new MutationRoot instance
     let pending_requests_2 = Arc::new(Mutex::new(VecDeque::new()));
     let request_notifier_2 = Arc::new(Notify::new());