Currently, there are 2 ways of sharing a backlog with other nodes: through
a gossip mechanism, and with responses to replica writes. In gossip, we
check each second if the backlog changed, and if it did we update other
nodes with it. However if the backlog for this node changed on another
node with a write response, the gossiped backlog is currently not updated,
so if after the response the backlog goes back to the value from the previous
gossip round, it will not get sent and the other node will stay with an
outdated backlog - this can be observed in the following scenario:
1. Cluster starts, all nodes gossip their empty view update backlog to one another
2. On node N, `view_update_backlog_broker` (the backlog gossiper) performs an iteration of its backlog update loop, sees no change (backlog has been empty since the start), schedules the next iteration after 1s
3. Within the next 1s, coordinator (different than N) sends a write to N causing a remote view update (which we do not wait for). As a result, node N replies immediately with an increased view update backlog, which is then noted by the coordinator.
4. Still within the 1s, node N finishes the view update in the background, dropping its view update backlog to 0.
5. In the next and following iterations of `view_update_backlog_broker` on N, backlog is empty, as it was in step 2, so no change is seen and no update is sent due to the check
```
auto backlog = _sp.local().get_view_update_backlog();
if (backlog_published && *backlog_published == backlog) {
sleep_abortable(gms::gossiper::INTERVAL, _as).get();
continue;
}
```
After this scenario happens, the coordinator keeps an information about an increased view update backlog on N even though it's actually already empty
This patch fixes the issue this by notifying the gossip that a different backlog
was sent in a response, causing it to send an unchanged backlog to other
nodes in the following gossip round.
Fixes: https://github.com/scylladb/scylladb/issues/18461
Similarly to https://github.com/scylladb/scylladb/pull/18646, without admission control (https://github.com/scylladb/scylladb/pull/18334), this patch doesn't affect much, so I'm marking it as backport/none
Tests: manual. Currently this patch only affects the length of MV flow control delay, which is not reliable to base a test on. A proper test will be added when MV admission control is added, so we'll be able to base the test on rejected requests
Closesscylladb/scylladb#18663
* github.com:scylladb/scylladb:
mv: gossip the same backlog if a different backlog was sent in a response
node_update_backlog: divide adding and fetching backlogs
e18aeb2486