fix(json): allow objects with empty keys #25564

Problem:
Empty string is a valid JSON key, but json_decode() treats an object
with empty key as ":help msgpack-special-dict". #20757

    :echo json_decode('{"": "1"}')
    {'_TYPE': [], '_VAL': [['', '1']]}

Note: vim returns `{'': '1'}`.

Solution:
Allow empty string as an object key.

Note that we still (currently) disallow empty keys in object_to_vim() (since 7c01d5ff92):
f64e4b43e1/src/nvim/api/private/converter.c (L333-L334)

Fix #20757

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

runtime/doc/builtin.txt

@@ -3873,8 +3873,7 @@ json_decode({expr})                                              *json_decode()*
 		Vim value. In the following cases it will output
 		|msgpack-special-dict|:
 		1. Dictionary contains duplicate key.
-		2. Dictionary contains empty key.
-		3. String contains NUL byte.  Two special dictionaries: for
+		2. String contains NUL byte.  Two special dictionaries: for
 		   dictionary and for string will be emitted in case string
 		   with NUL byte was a dictionary key.
 
@@ -4954,7 +4953,6 @@ msgpackparse({data})                                            *msgpackparse()*
 			   are binary strings).
 			2. String with NUL byte inside.
 			3. Duplicate key.
-			4. Empty key.
 		ext	|List| with two values: first is a signed integer
 			representing extension type. Second is
 			|readfile()|-style list of strings.

runtime/lua/vim/_meta/vimfn.lua

@@ -4678,8 +4678,7 @@ function vim.fn.join(list, sep) end
 --- Vim value. In the following cases it will output
 --- |msgpack-special-dict|:
 --- 1. Dictionary contains duplicate key.
---- 2. Dictionary contains empty key.
---- 3. String contains NUL byte.  Two special dictionaries: for
+--- 2. String contains NUL byte.  Two special dictionaries: for
 ---    dictionary and for string will be emitted in case string
 ---    with NUL byte was a dictionary key.
 ---
@@ -5922,7 +5921,6 @@ function vim.fn.msgpackdump(list, type) end
 ---      are binary strings).
 ---   2. String with NUL byte inside.
 ---   3. Duplicate key.
----   4. Empty key.
 --- ext  |List| with two values: first is a signed integer
 ---   representing extension type. Second is
 ---   |readfile()|-style list of strings.

src/nvim/eval.lua

@@ -5735,8 +5735,7 @@ M.funcs = {
       Vim value. In the following cases it will output
       |msgpack-special-dict|:
       1. Dictionary contains duplicate key.
-      2. Dictionary contains empty key.
-      3. String contains NUL byte.  Two special dictionaries: for
+      2. String contains NUL byte.  Two special dictionaries: for
          dictionary and for string will be emitted in case string
          with NUL byte was a dictionary key.
 
@@ -7155,7 +7154,6 @@ M.funcs = {
       	   are binary strings).
       	2. String with NUL byte inside.
       	3. Duplicate key.
-      	4. Empty key.
       ext	|List| with two values: first is a signed integer
       	representing extension type. Second is
       	|readfile()|-style list of strings.

src/nvim/eval/decode.c

@@ -141,9 +141,7 @@ static inline int json_decoder_pop(ValuesStackItem obj, ValuesStack *const stack
     ValuesStackItem key = kv_pop(*stack);
     if (last_container.special_val == NULL) {
       // These cases should have already been handled.
-      assert(!(key.is_special_string
-               || key.val.vval.v_string == NULL
-               || *key.val.vval.v_string == NUL));
+      assert(!(key.is_special_string || key.val.vval.v_string == NULL));
       dictitem_T *const obj_di = tv_dict_item_alloc(key.val.vval.v_string);
       tv_clear(&key.val);
       if (tv_dict_add(last_container.container.vval.v_dict, obj_di)
@@ -170,11 +168,10 @@ static inline int json_decoder_pop(ValuesStackItem obj, ValuesStack *const stack
       tv_clear(&obj.val);
       return FAIL;
     }
-    // Handle empty key and key represented as special dictionary
+    // Handle special dictionaries
     if (last_container.special_val == NULL
         && (obj.is_special_string
             || obj.val.vval.v_string == NULL
-            || *obj.val.vval.v_string == NUL
             || tv_dict_find(last_container.container.vval.v_dict, obj.val.vval.v_string, -1))) {
       tv_clear(&obj.val);
 
@@ -404,13 +401,6 @@ static inline int parse_json_string(const char *const buf, const size_t buf_len,
     semsg(_("E474: Expected string end: %.*s"), (int)buf_len, buf);
     goto parse_json_string_fail;
   }
-  if (len == 0) {
-    POP(((typval_T) {
-      .v_type = VAR_STRING,
-      .vval = { .v_string = NULL },
-    }), false);
-    goto parse_json_string_ret;
-  }
   char *str = xmalloc(len + 1);
   int fst_in_pair = 0;
   char *str_end = str;

test/functional/lua/json_spec.lua

@@ -101,6 +101,8 @@ describe('vim.json.decode()', function()
     eq({['1']=2}, exec_lua([[return vim.json.decode('{"1": 2}')]]))
     eq({['1']=2, ['3']={{['4']={['5']={{}, 1}}}}},
        exec_lua([[return vim.json.decode('{"1": 2, "3": [{"4": {"5": [ [], 1]}}]}')]]))
+    -- Empty string is a valid key. #20757
+    eq({['']=42}, exec_lua([[return vim.json.decode('{"": 42}')]]))
   end)
 
   it('parses strings properly', function()
@@ -161,6 +163,8 @@ describe('vim.json.encode()', function()
   it('dumps dictionaries', function()
     eq('{}', exec_lua([[return vim.json.encode(vim.empty_dict())]]))
     eq('{"d":[]}', exec_lua([[return vim.json.encode({d={}})]]))
+    -- Empty string is a valid key. #20757
+    eq('{"":42}', exec_lua([[return vim.json.encode({['']=42})]]))
   end)
 
   it('dumps vim.NIL', function()

test/functional/vimscript/json_functions_spec.lua

@@ -467,19 +467,18 @@ describe('json_decode() function', function()
                  '[1, {"d": {"b": 3, "a": 1, "c": 4, "a": 2, "c": 4}}]')
     sp_decode_eq({1, {a={}, d={_TYPE='map', _VAL={{'b', 3}, {'a', 1}, {'c', 4}, {'a', 2}, {'c', 4}}}}},
                  '[1, {"a": [], "d": {"b": 3, "a": 1, "c": 4, "a": 2, "c": 4}}]')
-  end)
-
-  it('parses dictionaries with empty keys to special maps', function()
-    sp_decode_eq({_TYPE='map', _VAL={{'', 4}}},
-                 '{"": 4}')
-    sp_decode_eq({_TYPE='map', _VAL={{'b', 3}, {'a', 1}, {'c', 4}, {'d', 2}, {'', 4}}},
-                 '{"b": 3, "a": 1, "c": 4, "d": 2, "": 4}')
     sp_decode_eq({_TYPE='map', _VAL={{'', 3}, {'a', 1}, {'c', 4}, {'d', 2}, {'', 4}}},
                  '{"": 3, "a": 1, "c": 4, "d": 2, "": 4}')
     sp_decode_eq({{_TYPE='map', _VAL={{'', 3}, {'a', 1}, {'c', 4}, {'d', 2}, {'', 4}}}},
                  '[{"": 3, "a": 1, "c": 4, "d": 2, "": 4}]')
   end)
 
+  it('parses dictionaries with empty keys', function()
+    eq({[""] = 4}, funcs.json_decode('{"": 4}'))
+    eq({b = 3, a = 1, c = 4, d = 2, [""] = 4},
+       funcs.json_decode('{"b": 3, "a": 1, "c": 4, "d": 2, "": 4}'))
+  end)
+
   it('parses dictionaries with keys with NUL bytes to special maps', function()
     sp_decode_eq({_TYPE='map', _VAL={{{_TYPE='string', _VAL={'a\n', 'b'}}, 4}}},
                  '{"a\\u0000\\nb": 4}')
@@ -577,6 +576,8 @@ describe('json_encode() function', function()
     eq('{}', eval('json_encode({})'))
     eq('{"d": []}', funcs.json_encode({d={}}))
     eq('{"d": [], "e": []}', funcs.json_encode({d={}, e={}}))
+    -- Empty keys not allowed (yet?) in object_to_vim() (since 7c01d5ff9286). #25564
+    -- eq('{"": []}', funcs.json_encode({['']={}}))
   end)
 
   it('cannot dump generic mapping with generic mapping keys and values',