view_builder: consumer: flush_fragments: close reader on error

Make sure to close the reader created by flush_fragments
if an exception occurs before it's moved to `populate_views`.

Note that it is also ok to close the reader _after_ it has been
moved, in case populate_views itself throws after closing the
reader that was moved it.  For conveience flat_mutation_reader::close
supports close-after-move.

Fixes #9479

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Message-Id: <20211024164138.1100304-1-bhalevy@scylladb.com>

db/view/view.cc

@@ -2013,12 +2013,14 @@ public:
             auto base_schema = _step.base->schema();
             auto views = with_base_info_snapshot(_views_to_build);
             auto reader = make_flat_mutation_reader_from_fragments(_step.reader.schema(), _builder._permit, std::move(_fragments));
+            auto close_reader = defer([&reader] { reader.close().get(); });
             reader.upgrade_schema(base_schema);
             _step.base->populate_views(
                     std::move(views),
                     _step.current_token(),
                     std::move(reader),
                     _now).get();
+            close_reader.cancel();
             _fragments.clear();
             _fragments_memory_usage = 0;
         }