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 7c01d5ff9286d262097484c680e3a4eab49e2911):
https://github.com/neovim/neovim/blob/f64e4b43e1191ff30d902730f752875aa55682ce/src/nvim/api/private/converter.c#L333-L334

Fix #20757

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

1 files changed, 2 insertions, 12 deletions

diff --git a/src/nvim/eval/decode.c b/src/nvim/eval/decode.c
index a6407693d7..64b65b42a5 100644
--- a/src/nvim/eval/decode.c
+++ b/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;