cluster: best-effort quiesce of archiver on leadership transfer

This substantially reduces the probability of leaving orphaned
objects in the object store when partitions change leadership
under load, e.g. during upgrades or leader balancing.

This fixes a test failure that indirectly detects orphan
objects by checking that topic deletion clears all objects.

Fixes redpanda-data#8496

src/v/cluster/partition.cc

@@ -20,6 +20,8 @@
 #include "prometheus/prometheus_sanitize.h"
 #include "raft/types.h"
 
+#include <seastar/util/defer.hh>
+
 namespace cluster {
 
 static bool is_id_allocator_topic(model::ntp ntp) {
@@ -601,7 +603,8 @@ ss::future<std::error_code>
 partition::transfer_leadership(std::optional<model::node_id> target) {
     vlog(
       clusterlog.debug,
-      "Transferring leadership to {}",
+      "Transferring {} leadership to {}",
+      ntp(),
       target.value_or(model::node_id{-1}));
 
     // Some state machines need a preparatory phase to efficiently transfer
@@ -613,6 +616,23 @@ partition::transfer_leadership(std::optional<model::node_id> target) {
     } else if (_tm_stm) {
         stm_prepare_lock = co_await _tm_stm->prepare_transfer_leadership();
     }
+
+    std::optional<ss::deferred_action<std::function<void()>>> complete_archiver;
+    if (_archiver) {
+        complete_archiver.emplace(
+          [a = &(*_archiver)]() { a->complete_transfer_leadership(); });
+        bool archiver_clean = co_await _archiver->prepare_transfer_leadership(
+          5s);
+        if (!archiver_clean) {
+            vlog(
+              clusterlog.error,
+              "Timed out waiting for {} uploads to complete before "
+              "transferring "
+              "leadership: proceeding anyway",
+              ntp());
+        }
+    }
+
     co_return co_await _raft->do_transfer_leadership(target);
 }