Add client_current_leader API

Closes #97.

async-raft/src/raft.rs

@@ -123,6 +123,15 @@ impl<D: AppData, R: AppDataResponse, N: RaftNetwork<D>, S: RaftStorage<D, R>> Ra
         Ok(rx.await.map_err(|_| RaftError::ShuttingDown).and_then(|res| res)?)
     }
 
+    /// Get the id of the current leader from this Raft node.
+    ///
+    /// Noted that it is the responsibility of the application to verify the leader by calling
+    /// [`client_read`] or [`client_write`].
+    #[tracing::instrument(level = "debug", skip(self))]
+    pub async fn client_current_leader(&self) -> Option<NodeId> {
+        self.metrics().borrow().current_leader
+    }
+
     /// Check to ensure this node is still the cluster leader, in order to guard against stale reads (§8).
     ///
     /// The actual read operation itself is up to the application, this method just ensures that

async-raft/tests/client_current_leader.rs

@@ -0,0 +1,51 @@
+mod fixtures;
+
+use std::sync::Arc;
+use std::time::Duration;
+
+use anyhow::Result;
+use async_raft::Config;
+use tokio::time::delay_for;
+
+use fixtures::RaftRouter;
+
+/// Client current leader tests.
+///
+/// What does this test do?
+///
+/// - create a stable 3-node cluster.
+/// - call the client_current_leader interface on the all nodes, and assert success.
+///
+/// RUST_LOG=async_raft,memstore,client_reads=trace cargo test -p async-raft --test client_current_leader
+#[tokio::test(core_threads = 4)]
+async fn client_current_leader() -> Result<()> {
+    fixtures::init_tracing();
+
+    // Setup test dependencies.
+    let config = Arc::new(Config::build("test".into()).validate().expect("failed to build Raft config"));
+    let router = Arc::new(RaftRouter::new(config.clone()));
+    router.new_raft_node(0).await;
+    router.new_raft_node(1).await;
+    router.new_raft_node(2).await;
+
+    // Assert all nodes are in non-voter state & have no entries.
+    delay_for(Duration::from_secs(10)).await;
+    router.assert_pristine_cluster().await;
+
+    // Initialize the cluster, then assert that a stable cluster was formed & held.
+    tracing::info!("--- initializing cluster");
+    router.initialize_from_single_node(0).await?;
+    delay_for(Duration::from_secs(10)).await;
+    router.assert_stable_cluster(Some(1), Some(1)).await;
+
+    // Get the ID of the leader, and assert that client_read succeeds.
+    let leader = router.leader().await.expect("leader not found");
+    assert_eq!(leader, 0, "expected leader to be node 0, got {}", leader);
+
+    for i in 0..3 {
+        let leader = router.client_current_leader(i).await;
+        assert_eq!(leader, Some(0), "expected leader to be node 0, got {:?}", leader);
+    }
+
+    Ok(())
+}

async-raft/tests/fixtures/mod.rs

@@ -158,6 +158,13 @@ impl RaftRouter {
         }
     }
 
+    /// Request the current leader from the target node.
+    pub async fn client_current_leader(&self, target: NodeId) -> Option<NodeId> {
+        let rt = self.routing_table.read().await;
+        let node = rt.get(&target).unwrap_or_else(|| panic!("node with ID {} does not exist", target));
+        node.0.client_current_leader().await
+    }
+
     /// Send multiple client requests to the target node, causing test failure on error.
     pub async fn client_request_many(&self, target: NodeId, client_id: &str, count: usize) {
         for idx in 0..count {