Fix "can't get SQE, will fall out of sync with EPOLLET" when overflowing the ring

OSDs shouldn't crash or hang with long iodepths anymore

README.md

@@ -357,9 +357,6 @@ and calculate disk offsets almost by hand. This will be fixed in near future.
 
 ## Known Problems
 
-- OSDs may currently crash with "can't get SQE, will fall out of sync with EPOLLET"
-  if you try to load them with very long iodepths because io_uring queue (ring) is limited
-  and OSDs don't check if it fills up.
 - Object deletion requests may currently lead to 'incomplete' objects if your OSDs crash during
   deletion because proper handling of object cleanup in a cluster should be "three-phase"
   and it's currently not implemented. Inode removal tool currently can't handle unclean

msgr_send.cpp

@@ -122,9 +122,6 @@ bool osd_messenger_t::try_send(osd_client_t *cl)
     {
         return true;
     }
-    cl->write_msg.msg_iov = cl->send_list.data();
-    cl->write_msg.msg_iovlen = cl->send_list.size();
-    cl->refs++;
     if (ringloop && !use_sync_send_recv)
     {
         io_uring_sqe* sqe = ringloop->get_sqe();
@@ -132,12 +129,18 @@ bool osd_messenger_t::try_send(osd_client_t *cl)
         {
             return false;
         }
+        cl->write_msg.msg_iov = cl->send_list.data();
+        cl->write_msg.msg_iovlen = cl->send_list.size();
+        cl->refs++;
         ring_data_t* data = ((ring_data_t*)sqe->user_data);
         data->callback = [this, cl](ring_data_t *data) { handle_send(data->res, cl); };
         my_uring_prep_sendmsg(sqe, peer_fd, &cl->write_msg, 0);
     }
     else
     {
+        cl->write_msg.msg_iov = cl->send_list.data();
+        cl->write_msg.msg_iovlen = cl->send_list.size();
+        cl->refs++;
         int result = sendmsg(peer_fd, &cl->write_msg, MSG_NOSIGNAL);
         if (result < 0)
         {

ringloop.cpp

@@ -66,10 +66,18 @@ void ring_loop_t::loop()
         struct ring_data_t *d = (struct ring_data_t*)cqe->user_data;
         if (d->callback)
         {
-            d->res = cqe->res;
-            d->callback(d);
+            // First free ring_data item, then call the callback
+            // so it has at least 1 free slot for the next event
+            // which is required for EPOLLET to function properly
+            struct ring_data_t dl;
+            dl.iov = d->iov;
+            dl.res = cqe->res;
+            dl.callback.swap(d->callback);
+            free_ring_data[free_ring_data_ptr++] = d - ring_datas;
+            dl.callback(&dl);
         }
-        free_ring_data[free_ring_data_ptr++] = d - ring_datas;
+        else
+            free_ring_data[free_ring_data_ptr++] = d - ring_datas;
         io_uring_cqe_seen(&ring, cqe);
     }
     while (get_sqe_queue.size() > 0)