feat(app): better external browser Modrinth login flow (#4033)

* fix(app-frontend): do not emit exceptions when no loaders are available

* refactor(app): simplify Microsoft login code without functional changes

* feat(app): external browser auth flow for Modrinth account login

* chore: address Clippy lint

* chore(app/oauth_utils): simplify `handle_reply` error handling according to review

* chore(app-lib): simplify `Url` usage out of MC auth module

apps/app/src/api/oauth_utils/auth_code_reply.rs

@@ -0,0 +1,159 @@
+//! A minimal OAuth 2.0 authorization code grant flow redirection/reply loopback URI HTTP
+//! server implementation, compliant with [RFC 6749]'s authorization code grant flow and
+//! [RFC 8252]'s best current practices for OAuth 2.0 in native apps.
+//!
+//! This server is needed for the step 4 of the OAuth authentication dance represented in
+//! figure 1 of [RFC 8252].
+//!
+//! Further reading: https://www.oauth.com/oauth2-servers/oauth-native-apps/redirect-urls-for-native-apps/
+//!
+//! [RFC 6749]: https://datatracker.ietf.org/doc/html/rfc6749
+//! [RFC 8252]: https://datatracker.ietf.org/doc/html/rfc8252
+
+use std::{
+    net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr},
+    sync::{LazyLock, Mutex},
+    time::Duration,
+};
+
+use hyper::body::Incoming;
+use hyper_util::rt::{TokioIo, TokioTimer};
+use theseus::ErrorKind;
+use tokio::{
+    net::TcpListener,
+    sync::{broadcast, oneshot},
+};
+
+static SERVER_SHUTDOWN: LazyLock<broadcast::Sender<()>> =
+    LazyLock::new(|| broadcast::channel(1024).0);
+
+/// Starts a temporary HTTP server to receive OAuth 2.0 authorization code grant flow redirects
+/// on a loopback interface with an ephemeral port. The caller can know the bound socket address
+/// by listening on the counterpart channel for `listen_socket_tx`.
+///
+/// If the server is stopped before receiving an authorization code, `Ok(None)` is returned.
+pub async fn listen(
+    listen_socket_tx: oneshot::Sender<Result<SocketAddr, theseus::Error>>,
+) -> Result<Option<String>, theseus::Error> {
+    // IPv4 is tried first for the best compatibility and performance with most systems.
+    // IPv6 is also tried in case IPv4 is not available. Resolving "localhost" is avoided
+    // to prevent failures deriving from improper name resolution setup. Any available
+    // ephemeral port is used to prevent conflicts with other services. This is all as per
+    // RFC 8252's recommendations
+    const ANY_LOOPBACK_SOCKET: &[SocketAddr] = &[
+        SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 0),
+        SocketAddr::new(IpAddr::V6(Ipv6Addr::LOCALHOST), 0),
+    ];
+
+    let listener = match TcpListener::bind(ANY_LOOPBACK_SOCKET).await {
+        Ok(listener) => {
+            listen_socket_tx
+                .send(listener.local_addr().map_err(|e| {
+                    ErrorKind::OtherError(format!(
+                        "Failed to get auth code reply socket address: {e}"
+                    ))
+                    .into()
+                }))
+                .ok();
+
+            listener
+        }
+        Err(e) => {
+            let error_msg =
+                format!("Failed to bind auth code reply socket: {e}");
+
+            listen_socket_tx
+                .send(Err(ErrorKind::OtherError(error_msg.clone()).into()))
+                .ok();
+
+            return Err(ErrorKind::OtherError(error_msg).into());
+        }
+    };
+
+    let mut auth_code = Mutex::new(None);
+    let mut shutdown_notification = SERVER_SHUTDOWN.subscribe();
+
+    while auth_code.get_mut().unwrap().is_none() {
+        let client_socket = tokio::select! {
+            biased;
+            _ = shutdown_notification.recv() => {
+                break;
+            }
+            conn_accept_result = listener.accept() => {
+                match conn_accept_result {
+                    Ok((socket, _)) => socket,
+                    Err(e) => {
+                        tracing::warn!("Failed to accept auth code reply: {e}");
+                        continue;
+                    }
+                }
+            }
+        };
+
+        if let Err(e) = hyper::server::conn::http1::Builder::new()
+            .keep_alive(false)
+            .header_read_timeout(Duration::from_secs(5))
+            .timer(TokioTimer::new())
+            .auto_date_header(false)
+            .serve_connection(
+                TokioIo::new(client_socket),
+                hyper::service::service_fn(|req| handle_reply(req, &auth_code)),
+            )
+            .await
+        {
+            tracing::warn!("Failed to handle auth code reply: {e}");
+        }
+    }
+
+    Ok(auth_code.into_inner().unwrap())
+}
+
+/// Stops any active OAuth 2.0 authorization code grant flow reply listening HTTP servers.
+pub fn stop_listeners() {
+    SERVER_SHUTDOWN.send(()).ok();
+}
+
+async fn handle_reply(
+    req: hyper::Request<Incoming>,
+    auth_code_out: &Mutex<Option<String>>,
+) -> Result<hyper::Response<String>, hyper::http::Error> {
+    if req.method() != hyper::Method::GET {
+        return hyper::Response::builder()
+            .status(hyper::StatusCode::METHOD_NOT_ALLOWED)
+            .header("Allow", "GET")
+            .body("".into());
+    }
+
+    // The authorization code is guaranteed to be sent as a "code" query parameter
+    // in the request URI query string as per RFC 6749 § 4.1.2
+    let auth_code = req.uri().query().and_then(|query_string| {
+        query_string
+            .split('&')
+            .filter_map(|query_pair| query_pair.split_once('='))
+            .find_map(|(key, value)| (key == "code").then_some(value))
+    });
+
+    let response = if let Some(auth_code) = auth_code {
+        *auth_code_out.lock().unwrap() = Some(auth_code.to_string());
+
+        hyper::Response::builder()
+            .status(hyper::StatusCode::OK)
+            .header("Content-Type", "text/html;charset=utf-8")
+            .body(
+                include_str!("auth_code_reply/page.html")
+                    .replace("{{title}}", "Success")
+                    .replace("{{message}}", "You have successfully signed in! You can close this page now."),
+            )
+    } else {
+        hyper::Response::builder()
+            .status(hyper::StatusCode::BAD_REQUEST)
+            .header("Content-Type", "text/html;charset=utf-8")
+            .body(
+                include_str!("auth_code_reply/page.html")
+                    .replace("{{title}}", "Error")
+                    .replace("{{message}}", "Authorization code not found. Please try signing in again."),
+            )
+    }?;
+
+    Ok(response)
+}