login: clear FD_CLOEXEC on stdio before spawning shell (+diag) so the login shell inherits console I/O

src/lib.rs

@@ -86,6 +86,26 @@ pub fn spawn_shell<T: Default>(user: &User<T>) -> IoResult<i32> {
         }
     }
 
+    // The relibc Redox spawn closes, in the child, every inherited fd that has
+    // FD_CLOEXEC set in the parent's filetable (see relibc
+    // platform/redox/mod.rs, the fds_to_close loop). A login shell MUST inherit
+    // its console stdio, but the pty-slave fds can arrive here still carrying
+    // FD_CLOEXEC (login itself keeps using them fine, since CLOEXEC only affects
+    // exec, not use) — which then silently drops the shell's stdin/stdout/stderr
+    // and leaves an interactive shell with no console. Explicitly clear
+    // FD_CLOEXEC on 0/1/2 so they survive the exec into the shell.
+    const F_GETFD: usize = 1;
+    const F_SETFD: usize = 2;
+    const FD_CLOEXEC: usize = 1;
+    for fd in 0..=2 {
+        let fl = fcntl(fd, F_GETFL, 0);
+        let fd_fl = fcntl(fd, F_GETFD, 0);
+        eprintln!("login-diag: fd{fd} F_GETFL={fl:?} F_GETFD={fd_fl:?}");
+        if let Ok(flags) = fd_fl {
+            let _ = fcntl(fd, F_SETFD, flags & !FD_CLOEXEC);
+        }
+    }
+
     // The login name is read with the `liner` line editor, which switches the
     // console pty into raw mode (ICANON/ECHO cleared) and does not restore it.
     // In raw mode the pty's line discipline never flushes a completed line to