Merge pull request redpanda-data#20802 from pgellert/quotas/follow-up…

…-changes

Quotas: address post merge code review comments

src/v/cluster/client_quota_serde.h

@@ -170,13 +170,13 @@ struct entity_value
 struct entity_value_diff
   : serde::
       envelope<entity_value_diff, serde::version<0>, serde::compat_version<0>> {
-    enum key : int8_t {
+    enum class key {
         producer_byte_rate = 0,
         consumer_byte_rate,
         controller_mutation_rate,
     };
 
-    enum operation : int8_t {
+    enum class operation {
         upsert = 0,
         remove,
     };

src/v/cluster/tests/client_quota_store_test.cc

@@ -142,7 +142,7 @@ BOOST_AUTO_TEST_CASE(quota_store_snapshot_delta) {
             .key = key1,
             .diff = {
               .entries = {
-                entity_value_diff::entry(entity_value_diff::remove, entity_value_diff::key::producer_byte_rate, 0),
+                entity_value_diff::entry(entity_value_diff::operation::remove, entity_value_diff::key::producer_byte_rate, 0),
               },
             },
           },

src/v/kafka/server/quota_manager.h

@@ -69,6 +69,16 @@ class quota_manager : public ss::peering_sharded_service<quota_manager> {
         std::optional<atomic_token_bucket> pm_rate;
     };
 
+    // Note: the use of std::shared_ptr<> is generally discouraged in the
+    // redpanda codebase because of the overhead of the atomic reference count
+    // and because seastar's memory model expects data to be deallocated on the
+    // same shard as where it was allocated and sharing std::shared_ptr<>'s
+    // across cores makes it easy to have the deallocation be on a different
+    // shard. The reason why it is acceptable to use a std::shared_ptr<> here is
+    // because (1) we're always allocating and deallocating the objects inside
+    // std::shared_ptr<> on shard 0 and (2) we need a way to keep track of a
+    // reference count across multiple shards, which is exactly what
+    // std::shared_ptr<> is meant for.
     using local_map_t
       = chunked_hash_map<tracker_key, std::shared_ptr<client_quota>>;