feat: cjson.as_array metamethod to enforce empty array encoding

A proposed improved patch of openresty/lua-cjson#1 (a patch commonly
proposed to lua-cjson and its forks), taking into considerations
comments from the original PR.

- use a lightuserdata key to store the metatable in the Lua Registry
  (more efficient and avoiding conflicts)
- provide a lightuserdata resulting in empty arrays as well
- tests cases moved to t/agentzh.t, where cases for
  'encode_empty_table_as_object' are already written. It seems like a
  better place for tests specific to the OpenResty fork's additions.
- a more complex test case

2 files changed, 108 insertions, 5 deletions

diff --git a/lua_cjson.c b/lua_cjson.c
index 5ff3bf7..5f4faf2 100644
--- a/lua_cjson.c
+++ b/lua_cjson.c
@@ -75,6 +75,8 @@
 #define DEFAULT_DECODE_INVALID_NUMBERS 0
 #endif
 
+static const char * const *json_empty_array;
+
 typedef enum {
     T_OBJ_BEGIN,
     T_OBJ_END,
@@ -698,8 +700,21 @@ static void json_append_data(lua_State *l, json_config_t *cfg,
         len = lua_array_length(l, cfg, json);
         if (len > 0 || (len == 0 && !cfg->encode_empty_table_as_object))
             json_append_array(l, cfg, current_depth, json, len);
-        else
-            json_append_object(l, cfg, current_depth, json);
+        else {
+            int as_array = 0;
+            if (lua_getmetatable(l, -1)) {
+                lua_pushlightuserdata(l, &json_empty_array);
+                lua_rawget(l, LUA_REGISTRYINDEX);
+                as_array = lua_rawequal(l, -1, -2);
+                lua_pop(l, 2);
+            }
+
+            if (as_array) {
+                json_append_array(l, cfg, current_depth, json, 0);
+            } else {
+                json_append_object(l, cfg, current_depth, json);
+            }
+        }
         break;
     case LUA_TNIL:
         strbuf_append_mem(json, "null", 4);
@@ -707,8 +722,10 @@ static void json_append_data(lua_State *l, json_config_t *cfg,
     case LUA_TLIGHTUSERDATA:
         if (lua_touserdata(l, -1) == NULL) {
             strbuf_append_mem(json, "null", 4);
-            break;
+        } else if (lua_touserdata(l, -1) == &json_empty_array) {
+            json_append_array(l, cfg, current_depth, json, 0);
         }
+        break;
     default:
         /* Remaining types (LUA_TFUNCTION, LUA_TUSERDATA, LUA_TTHREAD,
          * and LUA_TLIGHTUSERDATA) cannot be serialised */
@@ -1378,6 +1395,11 @@ static int lua_cjson_new(lua_State *l)
     /* Initialise number conversions */
     fpconv_init();
 
+    /* Create empty array metatable */
+    lua_pushlightuserdata(l, &json_empty_array);
+    lua_newtable(l);
+    lua_rawset(l, LUA_REGISTRYINDEX);
+
     /* cjson module table */
     lua_newtable(l);
 
@@ -1389,6 +1411,15 @@ static int lua_cjson_new(lua_State *l)
     lua_pushlightuserdata(l, NULL);
     lua_setfield(l, -2, "null");
 
+    /* Set cjson.empty_array_mt */
+    lua_pushlightuserdata(l, &json_empty_array);
+    lua_rawget(l, LUA_REGISTRYINDEX);
+    lua_setfield(l, -2, "empty_array_mt");
+
+    /* Set cjson.empty_array */
+    lua_pushlightuserdata(l, &json_empty_array);
+    lua_setfield(l, -2, "empty_array");
+
     /* Set module name / version fields */
     lua_pushliteral(l, CJSON_MODNAME);
     lua_setfield(l, -2, "_NAME");
diff --git a/tests/agentzh.t b/tests/agentzh.t
index 0b546ff..e76f910 100644
--- a/tests/agentzh.t
+++ b/tests/agentzh.t
@@ -41,7 +41,79 @@ print(cjson.encode({dogs = {}}))
 
 
 
-=== TEST 4: & in JSON
+=== TEST 4: empty_array userdata
+--- lua
+local cjson = require "cjson"
+print(cjson.encode({arr = cjson.empty_array}))
+--- out
+{"arr":[]}
+
+
+
+=== TEST 5: empty_array_mt
+--- lua
+local cjson = require "cjson"
+local empty_arr = setmetatable({}, cjson.empty_array_mt)
+print(cjson.encode({arr = empty_arr}))
+--- out
+{"arr":[]}
+
+
+
+=== TEST 6: empty_array_mt and empty tables as objects (explicit)
+--- lua
+local cjson = require "cjson"
+local empty_arr = setmetatable({}, cjson.empty_array_mt)
+print(cjson.encode({obj = {}, arr = empty_arr}))
+--- out
+{"arr":[],"obj":{}}
+
+
+
+=== TEST 7: empty_array_mt and empty tables as objects (explicit)
+--- lua
+local cjson = require "cjson"
+cjson.encode_empty_table_as_object(true)
+local empty_arr = setmetatable({}, cjson.empty_array_mt)
+local data = {
+  arr = empty_arr,
+  foo = {
+    obj = {},
+    foobar = {
+      arr = cjson.empty_array,
+      obj = {}
+    }
+  }
+}
+print(cjson.encode(data))
+--- out
+{"foo":{"foobar":{"obj":{},"arr":[]},"obj":{}},"arr":[]}
+
+
+
+=== TEST 8: empty_array_mt on non-empty tables
+--- lua
+local cjson = require "cjson"
+cjson.encode_empty_table_as_object(true)
+local array = {"hello", "world", "lua"}
+setmetatable(array, cjson.empty_array_mt)
+local data = {
+  arr = array,
+  foo = {
+    obj = {},
+    foobar = {
+      arr = cjson.empty_array,
+      obj = {}
+    }
+  }
+}
+print(cjson.encode(data))
+--- out
+{"foo":{"foobar":{"obj":{},"arr":[]},"obj":{}},"arr":["hello","world","lua"]}
+
+
+
+=== TEST 9: & in JSON
 --- lua
 local cjson = require "cjson"
 local a="[\"a=1&b=2\"]"
@@ -52,7 +124,7 @@ print(cjson.encode(b))
 
 
 
-=== TEST 5: default and max precision
+=== TEST 10: default and max precision
 --- lua
 local math = require "math"
 local cjson = require "cjson"