cluster: drop redundant set_status config updates

If a follower isn't seeing controller log updates
promptly, it may issue many set_status RPCs while
it's waiting.  The controller leader should not
turn all of these into log writes: if the status
of the node already matches what it is reporting,
then do not write anything.

Fixes redpanda-data#4923

src/v/cluster/config_manager.h

@@ -71,6 +71,14 @@ class config_manager final {
 
     config_version get_version() const noexcept { return _seen_version; }
 
+    bool needs_update(const config_status& new_status) {
+        if (auto s = status.find(new_status.node); s != status.end()) {
+            return s->second != new_status;
+        } else {
+            return true;
+        }
+    }
+
 private:
     void merge_apply_result(
       config_status&,

src/v/cluster/fwd.h

@@ -34,6 +34,7 @@ class security_frontend;
 class controller_api;
 class members_frontend;
 class config_frontend;
+class config_manager;
 class members_backend;
 class data_policy_frontend;
 class tx_gateway;

src/v/cluster/service.cc

@@ -43,6 +43,7 @@ service::service(
   ss::sharded<controller_api>& api,
   ss::sharded<members_frontend>& members_frontend,
   ss::sharded<config_frontend>& config_frontend,
+  ss::sharded<config_manager>& config_manager,
   ss::sharded<feature_manager>& feature_manager,
   ss::sharded<feature_table>& feature_table,
   ss::sharded<health_monitor_frontend>& hm_frontend,
@@ -55,6 +56,7 @@ service::service(
   , _api(api)
   , _members_frontend(members_frontend)
   , _config_frontend(config_frontend)
+  , _config_manager(config_manager)
   , _feature_manager(feature_manager)
   , _feature_table(feature_table)
   , _hm_frontend(hm_frontend)
@@ -355,6 +357,10 @@ service::hello(hello_request&& req, rpc::streaming_context&) {
 
 ss::future<config_status_reply>
 service::config_status(config_status_request&& req, rpc::streaming_context&) {
+    if (!_config_manager.local().needs_update(req.status)) {
+        co_return config_status_reply{.error = errc::success};
+    }
+
     auto ec = co_await _config_frontend.local().set_status(
       req.status,
       config::shard_local_cfg().replicate_append_timeout_ms()

src/v/cluster/service.h

@@ -33,6 +33,7 @@ class service : public controller_service {
       ss::sharded<controller_api>&,
       ss::sharded<members_frontend>&,
       ss::sharded<config_frontend>&,
+      ss::sharded<config_manager>&,
       ss::sharded<feature_manager>&,
       ss::sharded<feature_table>&,
       ss::sharded<health_monitor_frontend>&,
@@ -130,6 +131,7 @@ class service : public controller_service {
     ss::sharded<controller_api>& _api;
     ss::sharded<members_frontend>& _members_frontend;
     ss::sharded<config_frontend>& _config_frontend;
+    ss::sharded<config_manager>& _config_manager;
     ss::sharded<feature_manager>& _feature_manager;
     ss::sharded<feature_table>& _feature_table;
     ss::sharded<health_monitor_frontend>& _hm_frontend;

src/v/redpanda/application.cc

@@ -1204,6 +1204,7 @@ void application::start_redpanda(::stop_signal& app_signal) {
             std::ref(controller->get_api()),
             std::ref(controller->get_members_frontend()),
             std::ref(controller->get_config_frontend()),
+            std::ref(controller->get_config_manager()),
             std::ref(controller->get_feature_manager()),
             std::ref(controller->get_feature_table()),
             std::ref(controller->get_health_monitor()),