c/controller_backend: use std::list to store deltas

Deltas were previously stored as a vector per `ntp`. Deltas access
pattern (iteration, inserting and popping elements from back and from
the end) makes it perfect candidate for `std::list` usage. The
`std::list` doesn't use contiguous allocation so will not account for
the memory fragmentation.

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

src/v/cluster/controller_backend.cc

@@ -46,6 +46,7 @@
 #include <absl/container/flat_hash_map.h>
 #include <absl/container/flat_hash_set.h>
 #include <absl/container/node_hash_map.h>
+#include <fmt/ranges.h>
 
 #include <algorithm>
 #include <exception>
@@ -589,7 +590,6 @@ controller_backend::deltas_t calculate_bootstrap_deltas(
 
     // *it is not included
     auto start = it.base();
-    result_delta.reserve(std::distance(start, deltas.end()));
     std::move(start, deltas.end(), std::back_inserter(result_delta));
     return result_delta;
 }
@@ -2023,7 +2023,11 @@ ss::future<> controller_backend::delete_partition(
 std::vector<controller_backend::delta_metadata>
 controller_backend::list_ntp_deltas(const model::ntp& ntp) const {
     if (auto it = _topic_deltas.find(ntp); it != _topic_deltas.end()) {
-        return it->second;
+        std::vector<controller_backend::delta_metadata> ret;
+        ret.reserve(it->second.size());
+        std::copy(
+          it->second.begin(), it->second.end(), std::back_inserter(ret));
+        return ret;
     }
 
     return {};

src/v/cluster/controller_backend.h

@@ -231,7 +231,7 @@ class controller_backend
         friend std::ostream& operator<<(std::ostream&, const delta_metadata&);
     };
 
-    using deltas_t = std::vector<delta_metadata>;
+    using deltas_t = std::list<delta_metadata>;
     using results_t = std::vector<std::error_code>;
     controller_backend(
       ss::sharded<cluster::topic_table>&,
@@ -437,6 +437,6 @@ class controller_backend
     ss::metrics::metric_groups _metrics;
 };
 
-std::vector<controller_backend::delta_metadata> calculate_bootstrap_deltas(
-  model::node_id self, const std::vector<controller_backend::delta_metadata>&);
+std::list<controller_backend::delta_metadata> calculate_bootstrap_deltas(
+  model::node_id self, const std::list<controller_backend::delta_metadata>&);
 } // namespace cluster

src/v/cluster/tests/controller_backend_test.cc

@@ -31,7 +31,7 @@ make_assignment(std::vector<model::broker_shard> replicas) {
 using op_t = cluster::topic_table::delta::op_type;
 using delta_t = cluster::topic_table::delta;
 using meta_t = cluster::controller_backend::delta_metadata;
-using deltas_t = std::vector<meta_t>;
+using deltas_t = std::list<meta_t>;
 
 meta_t make_delta(
   int64_t o,
@@ -142,11 +142,14 @@ SEASTAR_THREAD_TEST_CASE(update_including_current_node) {
       current_node, std::move(d_1));
 
     BOOST_REQUIRE_EQUAL(deltas.size(), 3);
-    BOOST_REQUIRE_EQUAL(deltas[0].delta.offset, recreate_current.delta.offset);
     BOOST_REQUIRE_EQUAL(
-      deltas[1].delta.offset, update_with_current.delta.offset);
+      deltas.begin()->delta.offset, recreate_current.delta.offset);
     BOOST_REQUIRE_EQUAL(
-      deltas[2].delta.offset, finish_update_with_current.delta.offset);
+      std::next(deltas.begin())->delta.offset,
+      update_with_current.delta.offset);
+    BOOST_REQUIRE_EQUAL(
+      std::next(deltas.begin(), 2)->delta.offset,
+      finish_update_with_current.delta.offset);
 }
 
 SEASTAR_THREAD_TEST_CASE(update_excluding_current_node) {
@@ -196,9 +199,10 @@ SEASTAR_THREAD_TEST_CASE(move_back_to_current_node) {
 
     BOOST_REQUIRE_EQUAL(deltas.size(), 2);
     BOOST_REQUIRE_EQUAL(
-      deltas[0].delta.offset, update_with_current_2.delta.offset);
+      deltas.begin()->delta.offset, update_with_current_2.delta.offset);
     BOOST_REQUIRE_EQUAL(
-      deltas[1].delta.offset, finish_update_with_current_2.delta.offset);
+      std::next(deltas.begin())->delta.offset,
+      finish_update_with_current_2.delta.offset);
 }
 
 SEASTAR_THREAD_TEST_CASE(move_back_to_current_node_not_finished) {
@@ -218,5 +222,5 @@ SEASTAR_THREAD_TEST_CASE(move_back_to_current_node_not_finished) {
 
     BOOST_REQUIRE_EQUAL(deltas.size(), 1);
     BOOST_REQUIRE_EQUAL(
-      deltas[0].delta.offset, update_with_current_2.delta.offset);
+      deltas.begin()->delta.offset, update_with_current_2.delta.offset);
 }