fix(rocksdb): fix the problem that the usage of block cache is not se…

…t for the metric (apache#2060)

apache#2061

The monitor of the block cache usage is server-level and created by
std::call_once in a replica-level object, running periodically to update
the block cache usage.

However, once the replica-level object is stopped, the server-level
monitor would be cancelled; as a result, the block cache usage would
never be updated.

src/server/pegasus_server_impl.cpp

@@ -1817,6 +1817,12 @@ dsn::error_code pegasus_server_impl::start(int argc, char **argv)
     std::call_once(flag, [&]() {
         // The timer task will always running even though there is no replicas
         CHECK_NE(kServerStatUpdateTimeSec.count(), 0);
+
+        // TODO(wangdan): _update_server_rdb_stat is server-level, thus it could not be simply
+        // cancelled in the destructor of pegasus_server_impl which is replica-level.
+        //
+        // We should refactor to make _update_server_rdb_stat exit gracefully by
+        // `_update_server_rdb_stat->cancel(true)`.
         _update_server_rdb_stat = dsn::tasking::enqueue_timer(
             LPC_REPLICATION_LONG_COMMON,
             nullptr, // TODO: the tracker is nullptr, we will fix it later
@@ -1869,10 +1875,7 @@ ::dsn::error_code pegasus_server_impl::stop(bool clear_state)
         _update_replica_rdb_stat->cancel(true);
         _update_replica_rdb_stat = nullptr;
     }
-    if (_update_server_rdb_stat != nullptr) {
-        _update_server_rdb_stat->cancel(true);
-        _update_server_rdb_stat = nullptr;
-    }
+
     _tracker.cancel_outstanding_tasks();
 
     _context_cache.clear();