auth: no password prompt over the dashboard's own requests

Opening a dashboard through its login page showed a 401 on /auth/login
and the browser's password prompt. The page asked for its token by the
browser's password first (POST /auth/login), with none to send, and the
401 carried a Basic challenge, which a browser answers with its prompt.

The page now asks by its session first (POST /auth/session), then by
the browser's password or the proxy's. And a 401 to a page's own
requests (Sec-Fetch-Dest: empty: fetch, EventSource) carries no
challenge: a page has its login of its own. Navigations and clients
without the header (curl) are still challenged.

2 files changed, 10 insertions, 5 deletions

diff --git a/src/api/auth.rs b/src/api/auth.rs
index f28aa22..acf9d46 100644
--- a/src/api/auth.rs
+++ b/src/api/auth.rs
@@ -316,15 +316,20 @@ pub(crate) async fn basic_auth(
             )
                 .into_response();
         }
-        // A challenge only where basic authentication can answer it.
+        // A challenge only where basic authentication can answer it, and not to a page's
+        // own requests (`Sec-Fetch-Dest: empty`: fetch, EventSource): the browser would
+        // ask for a password over the page, which has its own login (`/login`).
+        let from_a_page = headers
+            .get("sec-fetch-dest")
+            .is_some_and(|dest| dest == "empty");
         return match state.users {
-            Some(_) => (
+            Some(_) if !from_a_page => (
                 StatusCode::UNAUTHORIZED,
                 [(WWW_AUTHENTICATE, r#"Basic realm="Restricted""#)],
                 "Unauthorized",
             )
                 .into_response(),
-            None => (StatusCode::UNAUTHORIZED, "Unauthorized").into_response(),
+            _ => (StatusCode::UNAUTHORIZED, "Unauthorized").into_response(),
         };
     };
 
diff --git a/src/api/dashboard/dashboard.html b/src/api/dashboard/dashboard.html
index b80e0d2..0f31d7c 100644
--- a/src/api/dashboard/dashboard.html
+++ b/src/api/dashboard/dashboard.html
@@ -2314,10 +2314,10 @@ async function obtainTokens() {
   clearTimeout(tokenTimer);
   try {
     // Refreshed as the session (`POST /auth/session`), whose cookie lets the page in;
-    // the first: by the browser's password or the proxy's, else by that cookie.
+    // the first: by that cookie, else by the browser's password or the proxy's.
     token = token && token.expiresAt > Date.now() + 60000
       ? await tokenCall("../auth/session", token.token)
-      : await tokenCall("../auth/login").catch(() => tokenCall("../auth/session"));
+      : await tokenCall("../auth/session").catch(() => tokenCall("../auth/login"));
     narrowToken = await tokenCall("../auth/refresh", token.token, { limit: READ_MEMBERS });
     tokenWhy = null;
     const life = Math.min(token.expiresAt, narrowToken.expiresAt) - Date.now();