r/consensus: allow raft to flush on empty append entries requests

When linearizable barrier is requested we want follower to flush its log
to make sure that all possible entries are committed with traditional
raft semantics. Added handling of flushing log on the follower if leader
requested it and append entries request is empty.

Signed-off-by: Michal Maslanka <michal@redpanda.com>

src/v/raft/consensus.cc

@@ -1899,19 +1899,25 @@ consensus::do_append_entries(append_entries_request&& r) {
             return ss::make_ready_future<append_entries_reply>(
               std::move(reply));
         }
+        auto f = ss::now();
+        if (r.is_flush_required() && lstats.dirty_offset > _flushed_offset) {
+            f = flush_log();
+        }
         auto last_visible = std::min(
           lstats.dirty_offset, request_metadata.last_visible_index);
         // on the follower leader control visibility of entries in the log
         maybe_update_last_visible_index(last_visible);
         _last_leader_visible_offset = std::max(
           request_metadata.last_visible_index, _last_leader_visible_offset);
-        return maybe_update_follower_commit_idx(
-                 model::offset(request_metadata.commit_index))
-          .then([reply = std::move(reply)]() mutable {
-              reply.result = append_entries_reply::status::success;
-              return ss::make_ready_future<append_entries_reply>(
-                std::move(reply));
-          });
+        return f.then([this, reply, request_metadata] {
+            return maybe_update_follower_commit_idx(
+                     model::offset(request_metadata.commit_index))
+              .then([this, reply]() mutable {
+                  reply.last_flushed_log_index = _flushed_offset;
+                  reply.result = append_entries_reply::status::success;
+                  return reply;
+              });
+        });
     }
 
     // section 3