epoll: retry EPOLL_CTL_ADD/MOD on transient EINVAL/EBADF (Redox)

Registering an fd with /scheme/event can transiently fail with
EINVAL/EBADF while the scheme backing the fd is still starting up or the
fd table is churning during early boot (e.g. a tokio/mio reactor built by
a daemon that races ahead of ipcd's uds_stream scheme). Without recovery,
tokio's Runtime::build aborts the whole runtime with "Invalid argument
(os error 22)", killing every zbus daemon (sessiond, polkit, udisks,
upower) and any tokio program including the login shell.

Retry the /scheme/event registration write with a 5ms backoff, up to ~2s,
on EINVAL/EBADF; a genuinely permanent failure still surfaces after the
bounded window. Redox-only. This is the registration-site counterpart to
the existing park-path recovery in the P0-epoll-redox-recovery tokio patch.

src/platform/redox/epoll.rs

@@ -57,16 +57,43 @@ impl PalEpoll for Sys {
     ) -> Result<(), Errno> {
         match op {
             EPOLL_CTL_ADD | EPOLL_CTL_MOD => {
-                Sys::write(
-                    epfd,
-                    &Event {
-                        id: fd as usize,
-                        flags: unsafe { epoll_to_event_flags((*event).events) },
-                        // NOTE: Danger when using something smaller than 64-bit
-                        // systems. If this is needed, use a box or something
-                        data: unsafe { (*event).data.u64 as usize },
-                    },
-                )?;
+                let ev = Event {
+                    id: fd as usize,
+                    flags: unsafe { epoll_to_event_flags((*event).events) },
+                    // NOTE: Danger when using something smaller than 64-bit
+                    // systems. If this is needed, use a box or something
+                    data: unsafe { (*event).data.u64 as usize },
+                };
+                // Registering an fd with /scheme/event can transiently fail
+                // with EINVAL/EBADF while the scheme backing `fd` is still
+                // starting up or the fd-table is churning during early boot
+                // (e.g. a tokio/mio reactor built by a daemon that races ahead
+                // of ipcd's uds_stream scheme). Without recovery, tokio's
+                // Runtime::build aborts the whole runtime, killing every
+                // zbus-based daemon and any tokio program (including the login
+                // shell). Retry with a short backoff so the registration
+                // succeeds once the scheme settles; a genuinely permanent
+                // failure still surfaces after the bounded window. This mirrors
+                // the park-path recovery in the P0-epoll-redox-recovery tokio
+                // patch, but at the registration site the patch cannot reach.
+                let mut attempts: u32 = 0;
+                loop {
+                    match Sys::write(epfd, &ev) {
+                        Ok(_) => break,
+                        Err(Errno(err))
+                            if (err == EINVAL || err == EBADF) && attempts < 400 =>
+                        {
+                            attempts += 1;
+                            let req = syscall::data::TimeSpec {
+                                tv_sec: 0,
+                                tv_nsec: 5_000_000, // 5ms; up to ~2s total
+                            };
+                            let mut rem = syscall::data::TimeSpec::default();
+                            let _ = syscall::nanosleep(&req, &mut rem);
+                        }
+                        Err(err) => return Err(err),
+                    }
+                }
             }
             EPOLL_CTL_DEL => {
                 Sys::write(