MDEV-13104 Json functions.

        An extra ',' added to the JSON_MERGE result making it invalid.

mysql-test/r/func_json.result

@@ -683,3 +683,6 @@ JSON_OBJECT("foo", "bar`bar")
 SELECT JSON_SET('{}', '$.age', 87);
 JSON_SET('{}', '$.age', 87)
 {"age": 87}
+SELECT JSON_MERGE('[]', '{"c":"d"}');
+JSON_MERGE('[]', '{"c":"d"}')
+[{"c": "d"}]

mysql-test/t/func_json.test

@@ -333,3 +333,8 @@ SELECT JSON_OBJECT("foo", "bar`bar");
 #
 SELECT JSON_SET('{}', '$.age', 87);
 
+#
+# MDEV-13104 Json functions.
+#
+SELECT JSON_MERGE('[]', '{"c":"d"}');
+

sql/item_jsonfunc.cc

@@ -1971,14 +1971,25 @@ static int do_merge(String *str, json_engine_t *je1, json_engine_t *je2)
   else
   {
     const uchar *end1, *beg1, *end2, *beg2;
+    int empty_array= 0;
 
     beg1= je1->value_begin;
 
     /* Merge as a single array. */
     if (je1->value_type == JSON_VALUE_ARRAY)
     {
-      if (json_skip_level(je1))
+      int cur_level= je1->stack_p;
+      empty_array= 1;
+      while (json_scan_next(je1) == 0)
+      {
+        if (je1->stack_p < cur_level)
+          break;
+        empty_array= 0;
+      }
+
+      if (je1->s.error)
         return 1;
+
       end1= je1->s.c_str - je1->sav_c_len;
     }
     else
@@ -1995,8 +2006,8 @@ static int do_merge(String *str, json_engine_t *je1, json_engine_t *je2)
         end1= je1->value_end;
     }
 
-    if (str->append((const char*) beg1, end1 - beg1),
-        str->append(", ", 2))
+    if (str->append((const char*) beg1, end1 - beg1) ||
+        (!empty_array && str->append(", ", 2)))
       return 3;
 
     if (json_value_scalar(je2))