mon/MDSMonitor: send reply to beacons with state=DNE

During shutdown, the MDS sends a `MSG_MDS_BEACON` with
`MDSMap::STATE_DNE` (in `MDSDaemon::suicide()`) and then waits for a
`MSG_MDS_BEACON` reply from the MON.

The MON, however, suppresses replies to `STATE_DNE`; in
`MDSMonitor::preprocess_beacon()`, it returns early on `STATE_DNE` and
`MDSMonitor::prepare_beacon()` silently evicts the dying MDS without
any reply.

This delays the MDS shutdown until the MDS times out.

Since `MDSDaemon::suicide()` has code to wait for a beacon reply, I
figure that the MON reply was suppressed accidently, therefore I
suggest adding it.

Fixes: https://tracker.ceph.com/issues/68761
Signed-off-by: Max Kellermann <[email protected]>

Author: MaxKellermann

src/mon/MDSMonitor.cc

@@ -758,6 +758,14 @@ bool MDSMonitor::prepare_beacon(MonOpRequestRef op)
 
     if (state == MDSMap::STATE_DNE) {
       dout(1) << __func__ << ": DNE from " << info << dendl;
+
+      /* send a beacon reply so MDSDaemon::suicide() finishes the
+         Beacon::send_and_wait() call */
+      auto beacon = make_message<MMDSBeacon>(mon.monmap->fsid,
+          m->get_global_id(), m->get_name(), get_fsmap().get_epoch(),
+          m->get_state(), m->get_seq(), CEPH_FEATURES_SUPPORTED_DEFAULT);
+      mon.send_reply(op, beacon.detach());
+
       goto evict;
     }