MDEV-28315 Fix ASAN stack-buffer-overflow in String::copy_aligned

Starting since this commit 36cdd5c
there is an ASAN stack-buffer-overflow error because we append a NULL
terminator beyond the length of memory allocated.

Reviewed by: Monty and Nayuta Yanagisawa

mysql-test/main/strings.result

@@ -18,3 +18,13 @@ LENGTH(CONCAT_WS(d, ' '))
 1
 1
 DROP TABLE t1;
+#
+# MDEV-28315 ASAN stack-buffer-overflow in String::copy_aligned
+#
+CREATE TABLE t1 (a VARBINARY(128)) CHARACTER SET utf32;
+INSERT INTO t1 VALUES ('South Carolina, Vermont, New Jersey, New Mexico, Wisconsin, Missouri, Delaware');
+CREATE TABLE t2 (b SET('South Carolina', 'Vermont', 'Texas', 'New Mexico', 'Wisconsin', 'Missouri', 'Delaware', 'Wyoming', 'New Jersey', 'Maryland', 'Illinois', 'New York')) CHARACTER SET utf32;
+INSERT INTO t2 SELECT * FROM t1;
+ERROR 01000: Data truncated for column 'b' at row 1
+DROP TABLE t1;
+DROP TABLE t2;

mysql-test/main/strings.test

@@ -24,3 +24,15 @@ CREATE TABLE t1 (d DATE);
 INSERT INTO t1 VALUES ('1920-03-02'),('2020-12-01');
 SELECT LENGTH(CONCAT_WS(d, ' ')) FROM t1;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-28315 ASAN stack-buffer-overflow in String::copy_aligned
+--echo #
+
+CREATE TABLE t1 (a VARBINARY(128)) CHARACTER SET utf32;
+INSERT INTO t1 VALUES ('South Carolina, Vermont, New Jersey, New Mexico, Wisconsin, Missouri, Delaware');
+CREATE TABLE t2 (b SET('South Carolina', 'Vermont', 'Texas', 'New Mexico', 'Wisconsin', 'Missouri', 'Delaware', 'Wyoming', 'New Jersey', 'Maryland', 'Illinois', 'New York')) CHARACTER SET utf32;
+--error WARN_DATA_TRUNCATED
+INSERT INTO t2 SELECT * FROM t1;
+DROP TABLE t1;
+DROP TABLE t2;

sql/sql_string.cc

@@ -398,7 +398,7 @@ bool String::copy_aligned(const char *str, size_t arg_length, size_t offset,
   DBUG_ASSERT(offset && offset != cs->mbminlen);
 
   size_t aligned_length= arg_length + offset;
-  if (alloc(aligned_length))
+  if (alloc(aligned_length+1))
     return TRUE;
 
   /*

sql/sql_string.h

@@ -690,7 +690,7 @@ class Binary_string: public Sql_alloc
 
       Note that if arg_length == Alloced_length then we don't allocate.
       This ensures we don't do any extra allocations in protocol and String:int,
-      but the string will not be atomically null terminated if c_ptr() is not
+      but the string will not be automatically null terminated if c_ptr() is not
       called.
     */
     if (arg_length <= Alloced_length && Alloced_length)