Upstream fix for a memory leak introduced in Fibers 1.1.0 that would manifest
in shepherd:

  https://github.com/wingo/fibers/issues/65
  https://issues.guix.gnu.org/58631

diff --git a/fibers/scheduler.scm b/fibers/scheduler.scm
index 2b03941..760b037 100644
--- a/fibers/scheduler.scm
+++ b/fibers/scheduler.scm
@@ -182,8 +182,10 @@ remote kernel thread."
     (#f (warn "scheduler for unknown fd" fd))
     ((and events+waiters (active-events . waiters))
      ;; First, clear the active status, as the EPOLLONESHOT has
-     ;; deactivated our entry in the epoll set.
-     (set-car! events+waiters #f)
+     ;; deactivated our entry in the epoll set.  Set the car to 0, not #f, so
+     ;; that 'schedule-tasks-for-active-fd' doesn't end up re-adding a
+     ;; finalizer on FD.
+     (set-car! events+waiters 0)
      (set-cdr! events+waiters '())
      (unless (zero? (logand revents (logior EPOLLHUP EPOLLERR)))
        (hashv-remove! (scheduler-fd-waiters sched) fd))
@@ -336,21 +338,19 @@ exp</td></tr></table>
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