MDEV-25560 Creating table with certain generated column crashes server

Fix RPAD() handling without 3rd argument of padding, in which case default
padding is used.

mysql-test/main/func_str.result

@@ -5153,5 +5153,21 @@ c1
 42
 DROP TABLE t1, t2;
 #
+# MDEV-25560 Creating table with certain generated column crashes server
+#
+CREATE TABLE t1 (i int, b int AS (RPAD(123,1)) stored);
+# Original case from the reporter
+CREATE TABLE crash_test_2 (
+DATA_VALUE CHAR(10) NULL,
+HAS_DATA BIT NOT NULL,
+TEST_COLUMN CHAR(10) AS (RPAD(CASE WHEN HAS_DATA = 1
+THEN DATA_VALUE ELSE NULL END, 10)) STORED);
+Warnings:
+Warning	1901	Function or expression 'rpad(case when `HAS_DATA` = 1 then `DATA_VALUE` else NULL end,10)' cannot be used in the GENERATED ALWAYS AS clause of `TEST_COLUMN`
+Warning	1105	Expression depends on the @@sql_mode value PAD_CHAR_TO_FULL_LENGTH
+# Cleanup
+DROP TABLE t1;
+DROP TABLE crash_test_2;
+#
 # End of 10.3 tests
 #

mysql-test/main/func_str.test

@@ -2090,6 +2090,24 @@ SELECT c1 FROM t2;
 DROP TABLE t1, t2;
 
 
+--echo #
+--echo # MDEV-25560 Creating table with certain generated column crashes server
+--echo #
+
+CREATE TABLE t1 (i int, b int AS (RPAD(123,1)) stored);
+
+--echo # Original case from the reporter
+CREATE TABLE crash_test_2 (
+  DATA_VALUE CHAR(10) NULL,
+  HAS_DATA BIT NOT NULL,
+  TEST_COLUMN CHAR(10) AS (RPAD(CASE WHEN HAS_DATA = 1
+                                THEN DATA_VALUE ELSE NULL END, 10)) STORED);
+
+--echo # Cleanup
+DROP TABLE t1;
+DROP TABLE crash_test_2;
+
+
 --echo #
 --echo # End of 10.3 tests
 --echo #

sql/item_strfunc.cc

@@ -3192,6 +3192,14 @@ String *Item_func_binlog_gtid_pos::val_str(String *str)
 }
 
 
+static String *default_pad_str(String *pad_str, CHARSET_INFO *collation)
+{
+  pad_str->set_charset(collation);
+  pad_str->length(0);
+  pad_str->append(" ", 1);
+  return pad_str;
+}
+
 bool Item_func_pad::fix_length_and_dec()
 {
   if (arg_count == 3)
@@ -3207,9 +3215,7 @@ bool Item_func_pad::fix_length_and_dec()
   {
     if (agg_arg_charsets_for_string_result(collation, &args[0], 1, 1))
       return TRUE;
-    pad_str.set_charset(collation.collation);
-    pad_str.length(0);
-    pad_str.append(" ", 1);
+    default_pad_str(&pad_str, collation.collation);
   }
 
   DBUG_ASSERT(collation.collation->mbmaxlen > 0);
@@ -3232,17 +3238,18 @@ bool Item_func_pad::fix_length_and_dec()
 Sql_mode_dependency Item_func_rpad::value_depends_on_sql_mode() const
 {
   DBUG_ASSERT(fixed);
-  DBUG_ASSERT(arg_count == 3);
+  DBUG_ASSERT(arg_count >= 2);
   if (!args[1]->value_depends_on_sql_mode_const_item() ||
-      !args[2]->value_depends_on_sql_mode_const_item())
+          (arg_count == 3 && !args[2]->value_depends_on_sql_mode_const_item()))
     return Item_func::value_depends_on_sql_mode();
   Longlong_hybrid len= args[1]->to_longlong_hybrid();
   if (args[1]->null_value || len.neg())
     return Sql_mode_dependency();                  // will return NULL
   if (len.abs() > 0 && len.abs() < args[0]->max_char_length())
     return Item_func::value_depends_on_sql_mode();
   StringBuffer<64> padstrbuf;
-  String *padstr= args[2]->val_str(&padstrbuf);
+  String *padstr= arg_count == 3 ? args[2]->val_str(&padstrbuf) :
+                               default_pad_str(&padstrbuf, collation.collation);
   if (!padstr || !padstr->length())
     return Sql_mode_dependency();                  // will return NULL
   if (padstr->lengthsp() != 0)