r/tests: always use current leader offsets in linearizable barrier test

Previously the linearizable barrier test might use out of date leader
offsets to assert if the barrier was executed correctly. This made the
test flaky especially for slow debug builds where leadership can change
throughout the test.

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

src/v/raft/tests/append_entries_test.cc

@@ -24,6 +24,7 @@
 #include "storage/kvstore.h"
 #include "storage/record_batch_builder.h"
 #include "storage/tests/utils/disk_log_builder.h"
+#include "storage/types.h"
 #include "test_utils/async.h"
 
 #include <system_error>
@@ -747,18 +748,28 @@ FIXTURE_TEST(test_linarizable_barrier, raft_test_fixture) {
     BOOST_REQUIRE(success);
     leader_raft = gr.get_member(leader_id).consensus;
     result<model::offset> r(raft::errc::timeout);
-    retry_with_leader(gr, 5, 30s, [&r](raft_node& leader_node) {
-        return leader_node.consensus->linearizable_barrier().then(
-          [&r](result<model::offset> l_offset) {
-              r = l_offset;
-              return l_offset.has_value();
-          });
-    }).get();
-
+    auto leader_offsets = leader_raft->log()->offsets();
+    retry_with_leader(
+      gr,
+      5,
+      30s,
+      [&r, &leader_offsets](raft_node& leader_node) {
+          return leader_node.consensus->linearizable_barrier().then(
+            [&r, &leader_offsets, &leader_node](
+              result<model::offset> l_offset) {
+                r = l_offset;
+                leader_offsets = leader_node.log->offsets();
+                return l_offset.has_value();
+            });
+      })
+      .get();
+    // query leader again as leadership may changed
+    leader_id = wait_for_group_leader(gr);
+    leader_raft = gr.get_member(leader_id).consensus;
     // linerizable barrier must succeed with stable leader
     BOOST_REQUIRE(r.has_value());
-    BOOST_REQUIRE_EQUAL(r.value(), leader_raft->committed_offset());
-    BOOST_REQUIRE_EQUAL(r.value(), leader_raft->dirty_offset());
+    BOOST_REQUIRE_EQUAL(r.value(), leader_offsets.committed_offset);
+    BOOST_REQUIRE_EQUAL(r.value(), leader_offsets.dirty_offset);
 };
 
 FIXTURE_TEST(test_linarizable_barrier_single_node, raft_test_fixture) {