Fix #754: improve compatibility with lua-cjson

test/app/json.result

 TAP version 13
 # json
-1..8
+1..9
     # unsigned
     1..56
     ok - encode/decode for 0
@@ -184,4 +184,10 @@ ok - table
     ok - udata  hook priority
     # ucdata: end
 ok - ucdata
+    # misc
+    1..2
+    ok - .NULL is cdata
+    ok - .NULL == nil
+    # misc: end
+ok - misc
 # json: end

test/app/json.test.lua

@@ -14,9 +14,15 @@ local function is_array(s)
     return string.sub(s, 1, 1) == "["
 end
 
+local function test_misc(test, s)
+    test:plan(2)
+    test:iscdata(s.NULL, 'void *', '.NULL is cdata')
+    test:ok(s.NULL == nil, '.NULL == nil')
+end
+
 tap.test("json", function(test)
     local serializer = require('json')
-    test:plan(8)
+    test:plan(9)
     test:test("unsigned", common.test_unsigned, serializer)
     test:test("signed", common.test_signed, serializer)
     test:test("double", common.test_double, serializer)
@@ -25,4 +31,5 @@ tap.test("json", function(test)
     test:test("nil", common.test_nil, serializer)
     test:test("table", common.test_table, serializer, is_array, is_map)
     test:test("ucdata", common.test_ucdata, serializer)
+    test:test("misc", test_misc, serializer)
 end)

third_party/lua-cjson/lua_cjson.c

@@ -1037,6 +1037,8 @@ luaopen_json(lua_State *L)
     json_create_tokens();
     luaL_json_default = luaL_newserializer(L, jsonlib);
     luaL_register_module(L, "json", NULL);
+    luaL_pushnull(L);
+    lua_setfield(L, -2, "null"); /* compatibility with cjson */
     return 1;
 }