Sign out through the proxy when the proxy signed you in

Sign out cleared ipx's cookies and showed its password page, while
Cloudflare Access still vouched for the person: nothing was signed out,
and the page looked like the wrong login. /api/me now says, for someone
the proxy signed in, where to go instead ([web] sign_out_url, which is
/cdn-cgi/access/logout behind Access), and /login sends anyone the proxy
vouches for on to their feeds. The header check both use is one function.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TAC7sLVqfKmY6rsTLXzNgk

src/web.rs

@@ -91,23 +91,8 @@ async fn auth(State(state): State<WebState>, mut req: Request, next: Next) -> Re
     let cfg = state.ctx.cfg();
     let token = cfg.web.token.clone();
 
-    let peer = req
-        .extensions()
-        .get::<axum::extract::ConnectInfo<std::net::SocketAddr>>()
-        .map(|c| c.0.ip().to_string())
-        .unwrap_or_default();
-
-    // 1. A header, but only from a hop we were told to believe. Anyone able to reach the
-    //    port could otherwise send it and be whoever they liked.
-    let vouched = (!cfg.web.trusted_header.is_empty()
-        && cfg.web.trusted_proxies.iter().any(|p| p == &peer))
-        .then(|| {
-            req.headers()
-                .get(&cfg.web.trusted_header)
-                .and_then(|v| v.to_str().ok())
-                .and_then(crate::auth::name_from_header)
-        })
-        .flatten();
+    // 1. A header, but only from a hop we were told to believe.
+    let vouched = vouched_name(&cfg, &req);
 
     let mut set_cookie: Option<String> = None;
     let mut user = None;
@@ -139,6 +124,7 @@ async fn auth(State(state): State<WebState>, mut req: Request, next: Next) -> Re
             let _ = state.ctx.db.signed_in(u.id);
         }
     }
+    let by_proxy = user.is_some();
 
     // 2. A session cookie from signing in here.
     if user.is_none() {
@@ -189,6 +175,7 @@ async fn auth(State(state): State<WebState>, mut req: Request, next: Next) -> Re
     };
 
     req.extensions_mut().insert(user);
+    req.extensions_mut().insert(Proxied(by_proxy));
     let mut resp = next.run(req).await;
     if let Some(c) = set_cookie {
         if let Ok(v) = header::HeaderValue::from_str(&c) {
@@ -200,6 +187,29 @@ async fn auth(State(state): State<WebState>, mut req: Request, next: Next) -> Re
 
 const SESSION_COOKIE: &str = "ipx_session";
 
+/// Whether the proxy signed this request in, rather than a session or the token: signing out
+/// has to go through the proxy then, or its next request signs the person straight back in.
+#[derive(Clone, Copy)]
+struct Proxied(bool);
+
+/// The name the proxy vouches for, when this request came from one of `trusted_proxies` and
+/// carries `trusted_header`. Anyone able to reach the port could otherwise send the header and
+/// be whoever they liked.
+fn vouched_name(cfg: &crate::config::Config, req: &Request) -> Option<String> {
+    let peer = req
+        .extensions()
+        .get::<axum::extract::ConnectInfo<std::net::SocketAddr>>()
+        .map(|c| c.0.ip().to_string())
+        .unwrap_or_default();
+    if cfg.web.trusted_header.is_empty() || !cfg.web.trusted_proxies.iter().any(|p| p == &peer) {
+        return None;
+    }
+    req.headers()
+        .get(&cfg.web.trusted_header)
+        .and_then(|v| v.to_str().ok())
+        .and_then(crate::auth::name_from_header)
+}
+
 /// Handlers take `User` to say they need one; the auth layer put it there, and nothing
 /// reaches a handler without passing through it.
 impl<S: Send + Sync> axum::extract::FromRequestParts<S> for crate::db::User {
@@ -293,8 +303,15 @@ async fn logout(State(state): State<WebState>, req: Request) -> Response {
     resp
 }
 
-async fn me(user: crate::db::User) -> Json<serde_json::Value> {
-    Json(serde_json::json!({ "name": user.name, "admin": user.is_admin }))
+/// Who is signed in, and, for someone the proxy signed in, where Sign out should send them.
+async fn me(
+    State(state): State<WebState>,
+    user: crate::db::User,
+    axum::Extension(Proxied(by_proxy)): axum::Extension<Proxied>,
+) -> Json<serde_json::Value> {
+    let url = state.ctx.cfg().web.sign_out_url.clone();
+    let sign_out = (by_proxy && !url.is_empty()).then_some(url);
+    Json(serde_json::json!({ "name": user.name, "admin": user.is_admin, "sign_out": sign_out }))
 }
 
 // ---- accounts: admin only ----
@@ -417,8 +434,13 @@ async fn remove_user(
     Ok(StatusCode::NO_CONTENT)
 }
 
-async fn login_page() -> Html<&'static str> {
-    Html(include_str!("../web/login.html"))
+/// The password form, except for someone the proxy vouches for: they are signed in already, and
+/// the form only made it look as if they were not.
+async fn login_page(State(state): State<WebState>, req: Request) -> Response {
+    if vouched_name(&state.ctx.cfg(), &req).is_some() {
+        return Redirect::to("/").into_response();
+    }
+    Html(include_str!("../web/login.html")).into_response()
 }
 
 /// The 2004 icon, served once for both pages rather than inlined as base64 into each. The