MDEV-17121 JSON_ARRAY_APPEND.

Extra comma added to the result when an json array is empty.

include/json_lib.h

@@ -319,6 +319,12 @@ int json_skip_to_level(json_engine_t *j, int level);
   json_skip_to_level((json_engine), (json_engine)->stack_p)
 
 
+/*
+  works as json_skip_level() but also counts items on the current
+  level skipped.
+*/
+int json_skip_level_and_count(json_engine_t *j, int *n_items_skipped);
+
 #define json_skip_array_item json_skip_key
 
 /*

mysql-test/r/func_json.result

@@ -790,3 +790,9 @@ JSON_SET('{}', '$.a', _utf8 0xC3B6, '$.b', _utf8 0xC3B6)
 SELECT JSON_SET('{}', '$.a', _utf8 X'C3B6', '$.x', 1, '$.b', _utf8 X'C3B6');
 JSON_SET('{}', '$.a', _utf8 X'C3B6', '$.x', 1, '$.b', _utf8 X'C3B6')
 {"a": "", "x": 1, "b": ""}
+#
+# MDEV-17121 JSON_ARRAY_APPEND
+#
+select json_array_append('[             ]', '$', 'aue');
+json_array_append('[             ]', '$', 'aue')
+["aue"]

mysql-test/t/func_json.test

@@ -448,3 +448,10 @@ SELECT JSON_SET('{}', '$.a', _utf8 0xC3B6);
 SELECT JSON_SET('{}', '$.a', _utf8 0xC3B6, '$.b', _utf8 0xC3B6);
 SELECT JSON_SET('{}', '$.a', _utf8 X'C3B6', '$.x', 1, '$.b', _utf8 X'C3B6');
 
+
+--echo #
+--echo # MDEV-17121 JSON_ARRAY_APPEND
+--echo #
+
+select json_array_append('[             ]', '$', 'aue');
+

sql/item_jsonfunc.cc

@@ -1621,13 +1621,15 @@ String *Item_func_json_array_append::val_str(String *str)
 
     if (je.value_type == JSON_VALUE_ARRAY)
     {
-      if (json_skip_level(&je))
+      int n_items;
+      if (json_skip_level_and_count(&je, &n_items))
         goto js_error;
 
       ar_end= je.s.c_str - je.sav_c_len;
       str_rest_len= js->length() - (ar_end - (const uchar *) js->ptr());
       str->q_append(js->ptr(), ar_end-(const uchar *) js->ptr());
-      str->append(", ", 2);
+      if (n_items)
+        str->append(", ", 2);
       if (append_json_value(str, args[n_arg+1], &tmp_val))
         goto return_null; /* Out of memory. */
 

strings/json_lib.c

@@ -1197,6 +1197,31 @@ int json_skip_to_level(json_engine_t *j, int level)
 }
 
 
+#define json_skip_level(json_engine) \
+  json_skip_to_level((json_engine), (json_engine)->stack_p)
+
+
+/*
+  works as json_skip_level() but also counts items on the current
+  level skipped.
+*/
+int json_skip_level_and_count(json_engine_t *j, int *n_items_skipped)
+{
+  int level= j->stack_p;
+
+  *n_items_skipped= 0;
+  while (json_scan_next(j) == 0)
+  {
+    if (j->stack_p < level)
+      return 0;
+    if (j->stack_p == level && j->state == JST_VALUE)
+      (*n_items_skipped)++;
+  }
+
+  return 1;
+}
+
+
 int json_skip_key(json_engine_t *j)
 {
   if (json_read_value(j))