MDEV-23766: Make Json_writer assert when one tries to author invalid JSON

- Add unit test.

unittest/sql/my_json_writer-t.cc

@@ -0,0 +1,130 @@
+/*
+   Copyright (c) 2021, MariaDB Corporation.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA */
+
+#include <my_global.h>
+#include <my_pthread.h>
+#include <my_sys.h>
+#include <stdio.h>
+#include <tap.h>
+
+/*
+  Unit tests for class Json_writer. At the moment there are only tests for the
+  "Fail an assertion if one attempts to produce invalid JSON" feature.
+*/
+
+struct TABLE;
+struct JOIN_TAB;
+class Json_writer;
+
+
+/* Several fake objects */
+class Opt_trace 
+{
+public:
+  void enable_tracing_if_required() {}
+  void disable_tracing_if_required() {}
+  Json_writer *get_current_json() { return nullptr; }
+};
+
+class THD 
+{
+public:
+  Opt_trace opt_trace;
+};
+
+#define JSON_WRITER_UNIT_TEST
+#include "../sql/my_json_writer.h"
+#include "../sql/my_json_writer.cc"
+
+int main(int args, char **argv)
+{
+  plan(NO_PLAN);
+  diag("Testing Json_writer checks");
+
+  {
+    Json_writer w;
+    w.start_object();
+    w.add_member("foo"); 
+    w.end_object();
+    ok(w.invalid_json, "Started a name but didn't add a value");
+  }
+
+  {
+    Json_writer w;
+    w.start_object();
+    w.add_ull(123);
+    ok(w.invalid_json, "Unnamed value in an object");
+  }
+
+  {
+    Json_writer w;
+    w.start_array();
+    w.add_member("bebebe").add_ull(345);
+    ok(w.invalid_json, "Named member in array");
+  }
+
+  {
+    Json_writer w;
+    w.start_object();
+    w.start_array();
+    ok(w.invalid_json, "Unnamed array in an object");
+  }
+
+  {
+    Json_writer w;
+    w.start_object();
+    w.start_object();
+    ok(w.invalid_json, "Unnamed object in an object");
+  }
+
+  {
+    Json_writer w;
+    w.start_array();
+    w.add_member("zzz");
+    w.start_object();
+    ok(w.invalid_json, "Named object in an array");
+  }
+  {
+    Json_writer w;
+    w.start_array();
+    w.add_member("zzz");
+    w.start_array();
+    ok(w.invalid_json, "Named array in an array");
+  }
+
+  // BAD:
+  {
+    Json_writer w;
+    w.start_array();
+    w.end_object();
+    ok(!w.invalid_json, "BAD: not checked!");
+  }
+
+  // BAD:
+  {
+    Json_writer w;
+    w.start_object();
+    w.end_array();
+    ok(!w.invalid_json, "BAD: not checked!");
+  }
+
+
+
+  diag("Done");
+
+  return exit_status();
+}
+