[tests] fix flakiness in TableLocationsCacheMultiMasterTest

The TableLocationsCacheMultiMasterTest.ResetCache scenario showed
signs of flakiness.  It turned out that the root case of the issue
was the absence of Raft leader leases.

This patch fixes the issue.  I ran the scenario using dist-test before
and after this patch (DEBUG builds):

  before: 4 out of 256 failed
    http://dist-test.cloudera.org/job?job_id=aserbin.1619919271.11533

  after : 0 out of 256 failed
    http://dist-test.cloudera.org/job?job_id=aserbin.1619919744.16115

Change-Id: Id53f0c537bdf1ec65da7d4bbf695864011f3e2ae
Reviewed-on: http://gerrit.cloudera.org:8080/17384
Tested-by: Alexey Serbin <[email protected]>
Reviewed-by: Grant Henke <[email protected]>

src/kudu/integration-tests/table_locations-itest.cc

@@ -40,9 +40,9 @@
 #include "kudu/common/common.pb.h"
 #include "kudu/common/partial_row.h"
 #include "kudu/common/row_operations.h"
+#include "kudu/common/row_operations.pb.h"
 #include "kudu/common/schema.h"
 #include "kudu/common/wire_protocol.h"
-#include "kudu/common/wire_protocol.pb.h"
 #include "kudu/gutil/ref_counted.h"
 #include "kudu/gutil/strings/substitute.h"
 #include "kudu/integration-tests/cluster_itest_util.h"
@@ -1265,6 +1265,7 @@ TEST_F(TableLocationsCacheMultiMasterTest, ResetCache) {
   int former_leader_master_idx;
   ASSERT_OK(cluster_->GetLeaderMasterIndex(&former_leader_master_idx));
 
+  int leader_master_idx = -1;
   ASSERT_EVENTUALLY([&] {
     // Induce a change in master leadership (maybe, even few of them, up to the
     // number of masters in the cluster).
@@ -1278,15 +1279,14 @@ TEST_F(TableLocationsCacheMultiMasterTest, ResetCache) {
           3 * kRaftHeartbeatIntervalMs));
       ASSERT_OK(cluster_->master(idx)->Resume());
     }
-    int leader_master_idx;
     ASSERT_OK(cluster_->GetLeaderMasterIndex(&leader_master_idx));
     ASSERT_NE(former_leader_master_idx, leader_master_idx);
   });
+  // An extra sanity check.
+  ASSERT_NE(-1, leader_master_idx);
 
   // Make sure all the cache's metrics are reset once master just has become
   // a leader.
-  int leader_master_idx;
-  ASSERT_OK(cluster_->GetLeaderMasterIndex(&leader_master_idx));
   NO_FATALS(CheckCacheMetricsReset(leader_master_idx));
 }