chore(rpc): improve rpc server launch error diagnostic (#1979)

crates/rpc/rpc-builder/tests/it/startup.rs

@@ -0,0 +1,39 @@
+//! Startup tests
+use crate::utils::{launch_http, launch_ws, test_rpc_builder};
+use reth_rpc_builder::{
+    error::{RpcError, ServerKind},
+    RethRpcModule, RpcServerConfig, TransportRpcModuleConfig,
+};
+use std::io;
+
+fn is_addr_in_use_kind(err: RpcError, kind: ServerKind) -> bool {
+    match err {
+        RpcError::AddressAlreadyInUse { kind: k, error } => {
+            k == kind && error.kind() == io::ErrorKind::AddrInUse
+        }
+        _ => false,
+    }
+}
+
+#[tokio::test(flavor = "multi_thread")]
+async fn test_http_addr_in_use() {
+    let handle = launch_http(vec![RethRpcModule::Admin]).await;
+    let addr = handle.http_local_addr().unwrap();
+    let builder = test_rpc_builder();
+    let server = builder.build(TransportRpcModuleConfig::set_http(vec![RethRpcModule::Admin]));
+    let result = server
+        .start_server(RpcServerConfig::http(Default::default()).with_http_address(addr))
+        .await;
+    assert!(is_addr_in_use_kind(result.unwrap_err(), ServerKind::Http(addr)));
+}
+
+#[tokio::test(flavor = "multi_thread")]
+async fn test_ws_addr_in_use() {
+    let handle = launch_ws(vec![RethRpcModule::Admin]).await;
+    let addr = handle.ws_local_addr().unwrap();
+    let builder = test_rpc_builder();
+    let server = builder.build(TransportRpcModuleConfig::set_ws(vec![RethRpcModule::Admin]));
+    let result =
+        server.start_server(RpcServerConfig::ws(Default::default()).with_ws_address(addr)).await;
+    assert!(is_addr_in_use_kind(result.unwrap_err(), ServerKind::WS(addr)));
+}