task_manager.hh: replace boost ranges with std ranges

Standardize on one range library to reduce dependency load.

Unfortunately, std::views::concat (the replacement for boost::join),
is C++26 only. We use two separate inserts to the result vector to
compensate, and rationalize it by saying that boost::join() is likely
slow due to the need for type-erasure.

Closes scylladb/scylladb#21834

service/storage_service.cc

@@ -64,6 +64,7 @@
 #include "version.hh"
 #include "dht/range_streamer.hh"
 #include <boost/range/algorithm.hpp>
+#include <boost/range/join.hpp>
 #include "transport/server.hh"
 #include <seastar/core/rwlock.hh>
 #include "db/batchlog_manager.hh"

service/task_manager_module.cc

@@ -13,6 +13,7 @@
 #include "tasks/task_handler.hh"
 #include "tasks/virtual_task_hint.hh"
 #include <seastar/coroutine/maybe_yield.hh>
+#include <boost/range/adaptor/transformed.hpp>
 
 namespace service {
 

service/topology_coordinator.cc

@@ -60,6 +60,8 @@
 
 #include "service/topology_coordinator.hh"
 
+#include <boost/range/join.hpp>
+
 using token = dht::token;
 using inet_address = gms::inet_address;
 

sstables_loader.cc

@@ -27,6 +27,8 @@
 #include <cfloat>
 #include <algorithm>
 
+#include <boost/range/adaptor/transformed.hpp>
+
 static logging::logger llog("sstables_loader");
 
 namespace {

tasks/task_manager.hh

@@ -9,9 +9,7 @@
 #pragma once
 
 #include <list>
-#include <boost/range/adaptor/map.hpp>
-#include <boost/range/algorithm/transform.hpp>
-#include <boost/range/join.hpp>
+#include <ranges>
 #include <seastar/core/on_internal_error.hh>
 #include <seastar/core/gate.hh>
 #include <seastar/core/rwlock.hh>
@@ -172,13 +170,16 @@ public:
                     std::function<std::optional<Res>(const task_essentials&)> map_finished_children) const {
                 auto shared_holder = co_await _lock.hold_read_lock();
 
-                co_return boost::join(
-                            _children | boost::adaptors::map_values | boost::adaptors::transformed(map_children),
-                            _finished_children | boost::adaptors::transformed(map_finished_children)
-                        )
-                        | std::views::filter([] (const auto& task) { return bool(task); })
-                        | std::views::transform([] (const auto& res) { return res.value(); })
-                        | std::ranges::to<std::vector<Res>>();
+                auto deopt = std::views::filter([] (const auto& task) { return bool(task); })
+                        | std::views::transform([] (const auto& res) { return res.value(); });
+
+                auto kids = _children | std::views::values | std::views::transform(map_children) | deopt;
+                auto finished_kids = _finished_children | std::views::transform(map_finished_children) | deopt;
+                std::vector<Res> result;
+                // Want to use insert_range(), but libstd++ hasn't implemented it yet.
+                result.insert(result.end(), kids.begin(), kids.end());
+                result.insert(result.end(), finished_kids.begin(), finished_kids.end());
+                co_return result;
             }
         };