bugfix: preserve 'empty_array_mt' behavior upon multiple loadings of the module.

Prior to this fix, when the module would be loaded several times
(by-passing `package.loaded`), the `lua_cjson_new` function would
override the `empty_array_mt` table in the registry with a new one.
Comparison for equality between those tables would then fail, and the
behavior would be broken.

This was discovered after loading `cjson` *and* `cjson.safe` in the same
application, resulting in two calls to `lua_cjson_new`.

Signed-off-by: Yichun Zhang (agentzh) <agentzh@gmail.com>

2 files changed, 40 insertions, 5 deletions

diff --git a/lua_cjson.c b/lua_cjson.c
index 3a77644..1695979 100644
--- a/lua_cjson.c
+++ b/lua_cjson.c
@@ -1401,10 +1401,21 @@ static int lua_cjson_new(lua_State *l)
     /* Initialise number conversions */
     fpconv_init();
 
-    /* Create empty array metatable */
+    /* Test if empty array metatable is in registry */
     lua_pushlightuserdata(l, &json_empty_array);
-    lua_newtable(l);
-    lua_rawset(l, LUA_REGISTRYINDEX);
+    lua_rawget(l, LUA_REGISTRYINDEX);
+    if (lua_isnil(l, -1)) {
+        /* Create empty array metatable.
+         *
+         * If multiple calls to lua_cjson_new() are made,
+         * this prevents overriding the table at the given
+         * registry's index with a new one.
+         */
+        lua_pop(l, 1);
+        lua_pushlightuserdata(l, &json_empty_array);
+        lua_newtable(l);
+        lua_rawset(l, LUA_REGISTRYINDEX);
+    }
 
     /* cjson module table */
     lua_newtable(l);
diff --git a/tests/agentzh.t b/tests/agentzh.t
index e76f910..3b0ecf1 100644
--- a/tests/agentzh.t
+++ b/tests/agentzh.t
@@ -113,7 +113,31 @@ print(cjson.encode(data))
 
 
 
-=== TEST 9: & in JSON
+=== TEST 9: multiple calls to lua_cjson_new (1/2)
+--- lua
+local cjson = require "cjson"
+package.loaded["cjson"] = nil
+require "cjson"
+local arr = setmetatable({}, cjson.empty_array_mt)
+print(cjson.encode(arr))
+--- out
+[]
+
+
+
+=== TEST 10: multiple calls to lua_cjson_new (2/2)
+--- lua
+local cjson = require "cjson.safe"
+-- load another cjson instance (not in package.loaded)
+require "cjson"
+local arr = setmetatable({}, cjson.empty_array_mt)
+print(cjson.encode(arr))
+--- out
+[]
+
+
+
+=== TEST 11: & in JSON
 --- lua
 local cjson = require "cjson"
 local a="[\"a=1&b=2\"]"
@@ -124,7 +148,7 @@ print(cjson.encode(b))
 
 
 
-=== TEST 10: default and max precision
+=== TEST 12: default and max precision
 --- lua
 local math = require "math"
 local cjson = require "cjson"