MDEV-14402 JSON_VALUE doesn't escape quote.

        Result unescaping added.

mysql-test/r/func_json.result

@@ -28,6 +28,9 @@ NULL
 select json_value('{"key1": [1,2,3], "key1":123}', '$.key1');
 json_value('{"key1": [1,2,3], "key1":123}', '$.key1')
 123
+select JSON_VALUE('{ "x": [0,1], "y": "[0,1]", "z": "Mon\\\"t\\\"y" }','$.z');
+JSON_VALUE('{ "x": [0,1], "y": "[0,1]", "z": "Mon\\\"t\\\"y" }','$.z')
+Mon"t"y
 select json_query('{"key1":{"a":1, "b":[1,2]}}', '$.key2');
 json_query('{"key1":{"a":1, "b":[1,2]}}', '$.key2')
 NULL

mysql-test/t/func_json.test

@@ -9,6 +9,7 @@ select json_value('{"key1":123}', '$.key2');
 select json_value('{"key1":123}', '$.key1');
 select json_value('{"key1":[1,2,3]}', '$.key1');
 select json_value('{"key1": [1,2,3], "key1":123}', '$.key1');
+select JSON_VALUE('{ "x": [0,1], "y": "[0,1]", "z": "Mon\\\"t\\\"y" }','$.z');
 
 select json_query('{"key1":{"a":1, "b":[1,2]}}', '$.key2');
 select json_query('{"key1":{"a":1, "b":[1,2]}}', '$.key1');

sql/item_jsonfunc.cc

@@ -69,6 +69,7 @@ static inline bool append_simple(String *s, const uchar *a, size_t a_len)
 /*
   Appends JSON string to the String object taking charsets in
   consideration.
+*/
 static int st_append_json(String *s,
              CHARSET_INFO *json_cs, const uchar *js, uint js_len)
 {
@@ -82,9 +83,8 @@ static int st_append_json(String *s,
     return 0;
   }
 
-  return js_len;
+  return str_len;
 }
-*/
 
 
 /*
@@ -475,6 +475,9 @@ String *Item_func_json_value::val_str(String *str)
   json_scan_start(&je, js->charset(),(const uchar *) js->ptr(),
                   (const uchar *) js->ptr() + js->length());
 
+  str->length(0);
+  str->set_charset(&my_charset_utf8mb4_bin);
+
   path.cur_step= path.p.steps;
 continue_search:
   if (json_find_path(&je, &path.p, &path.cur_step, array_counters))
@@ -515,8 +518,7 @@ bool Item_func_json_value::check_and_get_value(json_engine_t *je, String *res,
     return true;
   }
 
-  res->set((const char *) je->value, je->value_len, je->s.cs);
-  return false;
+  return st_append_json(res, je->s.cs, je->value, je->value_len); 
 }