sstables: Fix sstable reshaping for STCS

The heuristic of STCS reshape is correct, and it built the compaction
descriptor correctly, but forgot to return it to the caller, so no
reshape was ever done on behalf of STCS even when the strategy
needed it.

Fixes #7774.

Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <20201209175044.1609102-1-raphaelsc@scylladb.com>
(cherry picked from commit e4b55f40f3)

sstables/size_tiered_compaction_strategy.cc

@@ -256,6 +256,7 @@ size_tiered_compaction_strategy::get_reshaping_job(std::vector<shared_sstable> i
             bucket.resize(std::min(max_sstables, bucket.size()));
             compaction_descriptor desc(std::move(bucket), std::optional<sstables::sstable_set>(), iop);
             desc.options = compaction_options::make_reshape();
+            return desc;
         }
     }
 

test/boost/sstable_datafile_test.cc

@@ -6797,3 +6797,23 @@ SEASTAR_TEST_CASE(max_ongoing_compaction_test) {
         BOOST_REQUIRE(max_ongoing_compaction == 1);
     });
 }
+
+SEASTAR_TEST_CASE(stcs_reshape_test) {
+    return test_env::do_with_async([] (test_env& env) {
+        simple_schema ss;
+        auto s = ss.schema();
+        std::vector<shared_sstable> sstables;
+        sstables.reserve(s->max_compaction_threshold());
+        for (auto gen = 1; gen <= s->max_compaction_threshold(); gen++) {
+            auto sst = env.make_sstable(s, "", gen, la, big);
+            sstables::test(sst).set_data_file_size(1);
+            sstables.push_back(std::move(sst));
+        }
+
+        auto cs = sstables::make_compaction_strategy(sstables::compaction_strategy_type::size_tiered,
+                                                    s->compaction_strategy_options());
+
+        BOOST_REQUIRE(cs.get_reshaping_job(sstables, s, default_priority_class(), reshape_mode::strict).sstables.size());
+        BOOST_REQUIRE(cs.get_reshaping_job(sstables, s, default_priority_class(), reshape_mode::relaxed).sstables.size());
+    });
+}