MDEV-28651 quote(NULL) returns incorrect result in view ('NU' instead of 'NULL')

Item_func_quote did not calculate its max_length correctly for nullable
arguments.

Fix:

In case if the argument is nullable, reserve at least 4 characters
so the string "NULL" fits.

mysql-test/main/func_str.result

@@ -5286,3 +5286,30 @@ ERROR 42000: Incorrect parameter count in the call to native function 'DECODE'
 #
 # End of 10.4 tests
 #
+#
+# Start of 10.5 tests
+#
+#
+# MDEV-28651 quote(NULL) returns incorrect result in view ('NU' instead of 'NULL')
+#
+CREATE VIEW v1 AS SELECT quote(NULL);
+SELECT * FROM v1;
+quote(NULL)
+NULL
+DESCRIBE v1;
+Field	Type	Null	Key	Default	Extra
+quote(NULL)	varbinary(4)	YES		NULL	
+CREATE TABLE t1 AS SELECT * FROM v1;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `quote(NULL)` varbinary(4) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
+SELECT * FROM t1;
+quote(NULL)
+NULL
+DROP TABLE t1;
+DROP VIEW v1;
+#
+# End of 10.5 tests
+#

mysql-test/main/func_str.test

@@ -2329,3 +2329,25 @@ SELECT DECODE(NULL, NULL, NULL);
 --echo #
 --echo # End of 10.4 tests
 --echo #
+
+--echo #
+--echo # Start of 10.5 tests
+--echo #
+
+--echo #
+--echo # MDEV-28651 quote(NULL) returns incorrect result in view ('NU' instead of 'NULL')
+--echo #
+
+CREATE VIEW v1 AS SELECT quote(NULL);
+SELECT * FROM v1;
+DESCRIBE v1;
+CREATE TABLE t1 AS SELECT * FROM v1;
+SHOW CREATE TABLE t1;
+SELECT * FROM t1;
+DROP TABLE t1;
+DROP VIEW v1;
+
+
+--echo #
+--echo # End of 10.5 tests
+--echo #

sql/item_strfunc.h

@@ -1504,6 +1504,9 @@ class Item_func_quote :public Item_str_func
     collation.set(args[0]->collation);
     ulonglong max_result_length= (ulonglong) args[0]->max_length * 2 +
                                   2 * collation.collation->mbmaxlen;
+    // NULL argument is returned as a string "NULL" without quotes
+    if (args[0]->maybe_null)
+      set_if_bigger(max_result_length, 4 * collation.collation->mbmaxlen);
     max_length= (uint32) MY_MIN(max_result_length, MAX_BLOB_WIDTH);
     return FALSE;
   }