chore(apollo_l1_endpoint_monitor): extract RPC call to const and add tests

crates/apollo_l1_endpoint_monitor/Cargo.toml

@@ -13,6 +13,8 @@ tracing.workspace = true
 url = { workspace = true, features = ["serde"] }
 
 [dev-dependencies]
+mockito.workspace = true
+tokio.workspace = true
 
 [lints]
 workspace = true

crates/apollo_l1_endpoint_monitor/src/l1_endpoint_monitor_tests.rs

@@ -0,0 +1,138 @@
+use mockito::{Matcher, Server, ServerGuard};
+use url::Url;
+
+use crate::monitor::{
+    HEALTH_CHECK_RPC_METHOD, L1EndpointMonitor, L1EndpointMonitorConfig, L1EndpointMonitorError,
+};
+
+// Unreachable localhost endpoints for simulating failures.
+// Using localhost to prevent IO (so don't switch to example.com in order to avoid port issues).
+// Using these ports since they are not well-used ports in unix and privileged (<1024),
+// so unless the user runs as root and binds them explicitly, they should be closed.
+const BAD_ENDPOINT_1: &str = "http://localhost:1";
+const BAD_ENDPOINT_2: &str = "http://localhost:2";
+
+// Helper to assert the active URL and current index in one call.
+async fn check_get_active_l1_endpoint_success(
+    monitor: &mut L1EndpointMonitor,
+    expected_returned_url: &Url,
+    expected_index_of_returned_url: usize,
+) {
+    let active = monitor.get_active_l1_endpoint().await.unwrap();
+    assert_eq!(&active, expected_returned_url);
+    assert_eq!(monitor.current_l1_endpoint_index, expected_index_of_returned_url);
+}
+
+fn url(url: &str) -> Url {
+    Url::parse(url).unwrap()
+}
+
+/// Used to mock an L1 endpoint, like infura.
+/// This can be replaced by Anvil, but for unit tests it isn't worth the large overhead Anvil
+/// entails, given that we only need a valid HTTP response from the given url to test the API.
+pub struct MockL1Endpoint {
+    pub url: Url,
+    pub endpoint: ServerGuard,
+}
+
+async fn mock_working_l1_endpoint() -> MockL1Endpoint {
+    // Very simple mock is all we need _for now_: create a thin http server that expect a single
+    // call to the given API and return a valid response. Note that the validity of the response
+    // is coupled with the RPC method used. Server is dropped when the guard drops.
+    let mut server_guard = Server::new_async().await;
+    server_guard
+        .mock("POST", "/")
+        // Catch this specific RPC method.
+        .match_body(Matcher::PartialJsonString(format!(
+            r#"{{ "method": "{}"}}"#,
+            HEALTH_CHECK_RPC_METHOD
+        )))
+        .with_status(200)
+        // Return 2_u64 as a valid response for the method.
+        .with_body(r#"{"jsonrpc":"2.0","id":1,"result":"0x2"}"#)
+        .create_async()
+        .await;
+
+    let url = Url::parse(&server_guard.url()).unwrap();
+    MockL1Endpoint { url, endpoint: server_guard }
+}
+
+#[tokio::test]
+async fn non_responsive_skips_to_next() {
+    // Setup.
+    let endpoint = mock_working_l1_endpoint().await;
+    let good_endpoint = endpoint.url.clone();
+
+    let mut monitor = L1EndpointMonitor {
+        current_l1_endpoint_index: 0,
+        config: L1EndpointMonitorConfig {
+            ordered_l1_endpoint_urls: vec![url(BAD_ENDPOINT_1), good_endpoint.clone()],
+        },
+    };
+
+    // Test.
+    check_get_active_l1_endpoint_success(&mut monitor, &good_endpoint, 1).await;
+}
+
+#[tokio::test]
+async fn current_endpoint_still_works() {
+    // Setup.
+    let endpoint = mock_working_l1_endpoint().await;
+    let good_endpoint = endpoint.url.clone();
+
+    let mut monitor = L1EndpointMonitor {
+        current_l1_endpoint_index: 1,
+        config: L1EndpointMonitorConfig {
+            ordered_l1_endpoint_urls: vec![
+                url(BAD_ENDPOINT_1),
+                good_endpoint.clone(),
+                url(BAD_ENDPOINT_2),
+            ],
+        },
+    };
+
+    // Test.
+    check_get_active_l1_endpoint_success(&mut monitor, &good_endpoint, 1).await;
+}
+
+#[tokio::test]
+async fn wrap_around_success() {
+    // Setup.
+    let endpoint = mock_working_l1_endpoint().await;
+    let good_url = endpoint.url.clone();
+
+    let mut monitor = L1EndpointMonitor {
+        current_l1_endpoint_index: 2,
+        config: L1EndpointMonitorConfig {
+            ordered_l1_endpoint_urls: vec![
+                url(BAD_ENDPOINT_1),
+                good_url.clone(),
+                url(BAD_ENDPOINT_2),
+            ],
+        },
+    };
+
+    // Test.
+    check_get_active_l1_endpoint_success(&mut monitor, &good_url, 1).await;
+}
+
+#[tokio::test]
+async fn all_down_fails() {
+    // Setup.
+    let mut monitor = L1EndpointMonitor {
+        current_l1_endpoint_index: 0,
+        config: L1EndpointMonitorConfig {
+            ordered_l1_endpoint_urls: vec![url(BAD_ENDPOINT_1), url(BAD_ENDPOINT_2)],
+        },
+    };
+
+    // Test.
+    let result = monitor.get_active_l1_endpoint().await;
+    assert_eq!(result, Err(L1EndpointMonitorError::NoActiveL1Endpoint));
+    assert_eq!(monitor.current_l1_endpoint_index, 0);
+}
+
+#[test]
+#[ignore = "Enable once we add a constructor to the monitor (soon) which asserts \
+            correct index and returns an error otherwise"]
+fn initialized_with_index_out_of_bounds() {}

crates/apollo_l1_endpoint_monitor/src/monitor.rs

@@ -1,11 +1,21 @@
+use alloy::primitives::U64;
 use alloy::providers::{Provider, ProviderBuilder};
 use serde::{Deserialize, Serialize};
 use thiserror::Error;
 use tracing::{error, warn};
 use url::Url;
 
+#[cfg(test)]
+#[path = "l1_endpoint_monitor_tests.rs"]
+pub mod l1_endpoint_monitor_tests;
+
 type L1EndpointMonitorResult<T> = Result<T, L1EndpointMonitorError>;
 
+/// The JSON-RPC method used to check L1 endpoint health.
+// Note: is this fast enough? Alternatively, we can just check connectivity, but we already hit
+// a bug in infura where the connectivity was fine, but get_block_number() failed.
+pub const HEALTH_CHECK_RPC_METHOD: &str = "eth_blockNumber";
+
 #[derive(Debug, Clone)]
 pub struct L1EndpointMonitor {
     pub current_l1_endpoint_index: usize,
@@ -47,13 +57,14 @@ impl L1EndpointMonitor {
         &self.config.ordered_l1_endpoint_urls[index]
     }
 
+    /// Check if the L1 endpoint is operational by sending a carefully-chosen request to it.
+    // note: Using a raw request instead of just alloy API (like `get_block_number()`) to improve
+    // high-level readability (through a dedicated const) and to improve testability.
     async fn is_operational(&self, l1_endpoint_index: usize) -> bool {
         let l1_endpoint_url = self.get_node_url(l1_endpoint_index);
         let l1_client = ProviderBuilder::new().on_http(l1_endpoint_url.clone());
-        // Is this fast enough? we can use something to just check connectivity, but a recent infura
-        // bug failed on this API even though connectivity was fine. Besides, this API is called for
-        // most of our operations anyway.
-        l1_client.get_block_number().await.is_ok()
+        // Note: response type annotation is coupled with the rpc method used.
+        l1_client.client().request_noparams::<U64>(HEALTH_CHECK_RPC_METHOD).await.is_ok()
     }
 }