Added logic to cancel pending oauth login to free up localhost port (#3217)

This PR addresses an issue that several users have reported. If the
local oauth login server in one codex instance is left running (e.g. the
user abandons the oauth flow), a subsequent codex instance will receive
an error when attempting to log in because the localhost port is already
in use by the dangling web server from the first instance.

This PR adds a cancelation mechanism that the second instance can use to
abort the first login attempt and free up the port.

codex-rs/login/tests/suite/login_server_e2e.rs

@@ -1,7 +1,9 @@
 #![allow(clippy::unwrap_used)]
+use std::io;
 use std::net::SocketAddr;
 use std::net::TcpListener;
 use std::thread;
+use std::time::Duration;
 
 use base64::Engine;
 use codex_login::ServerOptions;
@@ -177,3 +179,67 @@ async fn creates_missing_codex_home_dir() {
         "auth.json should be created even if parent dir was missing"
     );
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+async fn cancels_previous_login_server_when_port_is_in_use() {
+    if std::env::var(CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR).is_ok() {
+        println!(
+            "Skipping test because it cannot execute when network is disabled in a Codex sandbox."
+        );
+        return;
+    }
+
+    let (issuer_addr, _issuer_handle) = start_mock_issuer();
+    let issuer = format!("http://{}:{}", issuer_addr.ip(), issuer_addr.port());
+
+    let first_tmp = tempdir().unwrap();
+    let first_codex_home = first_tmp.path().to_path_buf();
+
+    let first_opts = ServerOptions {
+        codex_home: first_codex_home,
+        client_id: codex_login::CLIENT_ID.to_string(),
+        issuer: issuer.clone(),
+        port: 0,
+        open_browser: false,
+        force_state: Some("cancel_state".to_string()),
+        originator: "test_originator".to_string(),
+    };
+
+    let first_server = run_login_server(first_opts).unwrap();
+    let login_port = first_server.actual_port;
+    let first_server_task = tokio::spawn(async move { first_server.block_until_done().await });
+
+    tokio::time::sleep(Duration::from_millis(100)).await;
+
+    let second_tmp = tempdir().unwrap();
+    let second_codex_home = second_tmp.path().to_path_buf();
+
+    let second_opts = ServerOptions {
+        codex_home: second_codex_home,
+        client_id: codex_login::CLIENT_ID.to_string(),
+        issuer,
+        port: login_port,
+        open_browser: false,
+        force_state: Some("cancel_state_2".to_string()),
+        originator: "test_originator".to_string(),
+    };
+
+    let second_server = run_login_server(second_opts).unwrap();
+    assert_eq!(second_server.actual_port, login_port);
+
+    let cancel_result = first_server_task
+        .await
+        .expect("first login server task panicked")
+        .expect_err("login server should report cancellation");
+    assert_eq!(cancel_result.kind(), io::ErrorKind::Interrupted);
+
+    let client = reqwest::Client::new();
+    let cancel_url = format!("http://127.0.0.1:{login_port}/cancel");
+    let resp = client.get(cancel_url).send().await.unwrap();
+    assert!(resp.status().is_success());
+
+    second_server
+        .block_until_done()
+        .await
+        .expect_err("second login server should report cancellation");
+}