lwt: introduce cql_statment_opt_metadata

cql_statement_opt_metadata is an interim node
in cql (prepared) statement hierarchy parenting
modification_statement and batch_statement. If there
is IF condition in such statements, they return a result set,
and thus have a result set metadata.

The metadata itself is filled in a subsequent patch.

cql3/cql_statement.hh

@@ -115,4 +115,21 @@ public:
     }
 };
 
+// Conditional modification statements and batches
+// return a result set and have metadata, while same
+// statements without conditions do not.
+class cql_statement_opt_metadata : public cql_statement {
+protected:
+    // Result set metadata, may be empty for simple updates and batches
+    shared_ptr<metadata> _metadata;
+public:
+    using cql_statement::cql_statement;
+    virtual shared_ptr<const metadata> get_result_metadata() const override {
+        if (_metadata) {
+            return _metadata;
+        }
+        return make_empty_metadata();
+    }
+};
+
 }

cql3/statements/batch_statement.cc

@@ -60,7 +60,7 @@ batch_statement::batch_statement(int bound_terms, type type_,
                                  std::vector<single_statement> statements,
                                  std::unique_ptr<attributes> attrs,
                                  cql_stats& stats)
-    : cql_statement_no_metadata(timeout_for_type(type_))
+    : cql_statement_opt_metadata(timeout_for_type(type_))
     , _bound_terms(bound_terms), _type(type_), _statements(std::move(statements))
     , _attrs(std::move(attrs))
     , _has_conditions(boost::algorithm::any_of(_statements, [] (auto&& s) { return s.statement->has_conditions(); }))

cql3/statements/batch_statement.hh

@@ -62,7 +62,7 @@ namespace statements {
  * A <code>BATCH</code> statement parsed from a CQL query.
  *
  */
-class batch_statement : public cql_statement_no_metadata {
+class batch_statement : public cql_statement_opt_metadata {
     static logging::logger _logger;
 public:
     using type = raw::batch_statement::type;

cql3/statements/modification_statement.cc

@@ -74,7 +74,7 @@ modification_statement_timeout(const schema& s) {
 }
 
 modification_statement::modification_statement(statement_type type_, uint32_t bound_terms, schema_ptr schema_, std::unique_ptr<attributes> attrs_, cql_stats& stats)
-    : cql_statement_no_metadata(modification_statement_timeout(*schema_))
+    : cql_statement_opt_metadata(modification_statement_timeout(*schema_))
     , type{type_}
     , _bound_terms{bound_terms}
     , s{schema_}

cql3/statements/modification_statement.hh

@@ -75,7 +75,7 @@ namespace raw { class modification_statement; }
 /*
  * Abstract parent class of individual modifications, i.e. INSERT, UPDATE and DELETE.
  */
-class modification_statement : public cql_statement_no_metadata {
+class modification_statement : public cql_statement_opt_metadata {
 private:
     static thread_local const ::shared_ptr<column_identifier> CAS_RESULT_COLUMN;