Merge 'Harden table truncate' from Benny Halevy

This series fixes a few issue on the table truncate path:
- "memtable_list: safely futurize clear_and_add"
  - reinstates an async version of table::clear_and_add, just safe against #10421
- a unit test reproducing #10421 was added to make sure the new version is indeed safe.
- "table: clear: serialize with ongoing flush" fixes #10423
- a unit test reproducing #10423 was added

Fixes #10281
Fixes #10423

Test: unit(dev), database_test. test_truncate_without_snapshot_during_{writes,flushes} (debug)

Closes #10424

* github.com:scylladb/scylla:
  test: database_test: add test_truncate_without_snapshot_during_writes
  memtable_list: safely futurize clear_and_add
  table: clear: serialize with ongoing flush

test/boost/database_test.cc

@@ -93,6 +93,48 @@ SEASTAR_TEST_CASE(test_safety_after_truncate) {
     }, cfg);
 }
 
+// Reproducer for:
+//   https://github.com/scylladb/scylla/issues/10421
+//   https://github.com/scylladb/scylla/issues/10423
+SEASTAR_TEST_CASE(test_truncate_without_snapshot_during_writes) {
+    auto cfg = make_shared<db::config>();
+    cfg->auto_snapshot.set(false);
+    return do_with_cql_env_thread([] (cql_test_env& e) {
+        e.execute_cql("create table ks.cf (k text, v int, primary key (k));").get();
+        auto& db = e.local_db();
+        auto& ks = db.find_keyspace("ks");
+        auto& cf = db.find_column_family("ks", "cf");
+        auto s = cf.schema();
+        int count = 0;
+
+        auto insert_data = [&] (uint32_t begin, uint32_t end) {
+            return parallel_for_each(boost::irange(begin, end), [&] (auto i) -> future<> {
+                auto pkey = partition_key::from_single_value(*s, to_bytes(fmt::format("key{}", i)));
+                mutation m(s, pkey);
+                m.set_clustered_cell(clustering_key_prefix::make_empty(), "v", int32_t(42), {});
+                return do_with(freeze(m), [&] (const auto& fm) {
+                    return db.apply(s, fm, tracing::trace_state_ptr(), db::commitlog::force_sync::no, db::no_timeout).then([&] {
+                        return cf.flush();
+                    }).handle_exception([] (std::exception_ptr ex) {
+                        BOOST_FAIL(format("db.apply failed: {}", ex));
+                    });
+                }).then([&] {
+                    ++count;
+                });
+            });
+        };
+
+        uint32_t num_keys = 1000;
+
+        auto f0 = insert_data(0, num_keys);
+        auto f1 = do_until([&] { return count >= num_keys; }, [&] {
+            return db.truncate(ks, cf, [] { return make_ready_future<db_clock::time_point>(db_clock::now()); }, false /* with_snapshot */).then([] { return yield(); });
+        });
+        f0.get();
+        f1.get();
+    }, cfg);
+}
+
 SEASTAR_TEST_CASE(test_querying_with_limits) {
     return do_with_cql_env([](cql_test_env& e) {
         return seastar::async([&] {