table: snapshot_on_all_shards: Get directory from storage options

There are several things that are changed here

- The target directory for snapshot is evaluated using table directory
  taken from its storage options, not from config

- If the storage options are not "local", the snapshot_on_all_shards is
  failed early, it's impossible to snapshot sstables anyway

- If the storage is not configured for the obtained local options,
  snapshotting is skilled, because it's a virtual table that's probably
  not supposed to have snapshots

- The late failure to snapshot non-local sstables is converted into
  internal error, as this functionality cannot be executed as per
  previous change

- The target path is created using fs::path operator/ overload, not by
  concatenating strings (it's minor change)

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

replica/table.cc

@@ -2595,7 +2595,15 @@ future<> table::write_schema_as_cql(database& db, sstring dir) const {
 
 // Runs the orchestration code on an arbitrary shard to balance the load.
 future<> table::snapshot_on_all_shards(sharded<database>& sharded_db, const global_table_ptr& table_shards, sstring name) {
-    auto jsondir = table_shards->_config.datadir + "/snapshots/" + name;
+    auto* so = std::get_if<storage_options::local>(&table_shards->get_storage_options().value);
+    if (so == nullptr) {
+        throw std::runtime_error("Snapshotting non-local tables is not implemented");
+    }
+    if (so->dir.empty()) { // virtual tables don't have initialized local storage
+        co_return;
+    }
+
+    auto jsondir = (so->dir / sstables::snapshots_dir / name).native();
     auto orchestrator = std::hash<sstring>()(jsondir) % smp::count;
 
     co_await smp::submit_to(orchestrator, [&] () -> future<> {

sstables/storage.cc

@@ -643,7 +643,8 @@ future<> s3_storage::remove_by_registry_entry(entry_descriptor desc) {
 }
 
 future<> s3_storage::snapshot(const sstable& sst, sstring dir, absolute_path abs, std::optional<generation_type> gen) const {
-    co_await coroutine::return_exception(std::runtime_error("Snapshotting S3 objects not implemented"));
+    on_internal_error(sstlog, "Snapshotting S3 objects not implemented");
+    co_return;
 }
 
 std::unique_ptr<sstables::storage> make_storage(sstables_manager& manager, const data_dictionary::storage_options& s_opts, sstable_state state) {