This is a backport from 5.1.32 into 5.1.31sp1.

The original fix was done by Gleb Shchepa on 2009-01-21
All comments are copied from the original fix:


Bug#42188: crash and/or memory corruption with user variables 
           in trigger

Interchangeable calls to the mysql_change_user client function 
and invocations of a trigger changing some user variable caused 
a memory corruption and a crash.

The mysql_change_user API call forces TDH::cleanup() on a server 
that frees user variable entries.
However it didn't reset Item_func_set_user_var::entry to NULL 
because Item_func_set_user_var::cleanup() was not overloaded.
So, Item_func_set_user_var::entry held a pointer to freed memory,
that caused a crash.

The Item_func_set_user_var::cleanup method has been overloaded
to cleanup the Item_func_set_user_var::entry field.

mysql-test/r/user_var.result:
  Added test case for bug #42188.
mysql-test/t/user_var.test:
  Added test case for bug #42188
  and
  After-push test file update (see bug 42321)
sql/item_func.cc:
  Bug#42188: crash and/or memory corruption with user variables
             in trigger
  
  The Item_func_set_user_var::cleanup method has been overloaded
  to cleanup the Item_func_set_user_var::entry field.
sql/item_func.h:
  Bug#42188: crash and/or memory corruption with user variables
             in trigger
  
  The Item_func_set_user_var::cleanup method has been overloaded
  to cleanup the Item_func_set_user_var::entry field.

mysql-test/r/user_var.result

@@ -399,4 +399,9 @@ select @lastid != id, @lastid, @lastid := id from t1;
 0	3	3
 1	3	4
 drop table t1;
+CREATE TABLE t1 (i INT);
+CREATE TRIGGER t_after_insert AFTER INSERT ON t1 FOR EACH ROW SET @bug42188 = 10;
+INSERT INTO t1 VALUES (1);
+INSERT INTO t1 VALUES (1);
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/user_var.test

@@ -285,4 +285,15 @@ set @lastid=-1;
 select @lastid != id, @lastid, @lastid := id from t1;
 drop table t1;
 
+#
+# Bug#42188: crash and/or memory corruption with user variables in trigger
+#
+
+CREATE TABLE t1 (i INT);
+CREATE TRIGGER t_after_insert AFTER INSERT ON t1 FOR EACH ROW SET @bug42188 = 10;
+INSERT INTO t1 VALUES (1);
+--change_user root,,test
+INSERT INTO t1 VALUES (1);
+DROP TABLE t1;
+
 --echo End of 5.1 tests

sql/item_func.cc

@@ -3808,6 +3808,13 @@ static user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
 }
 
 
+void Item_func_set_user_var::cleanup()
+{
+  Item_func::cleanup();
+  entry= NULL;
+}
+
+
 bool Item_func_set_user_var::set_entry(THD *thd, bool create_if_not_exists)
 {
   if (entry && thd->thread_id == entry_thread_id)

sql/item_func.h

@@ -1356,6 +1356,7 @@ class Item_func_set_user_var :public Item_func
   void save_org_in_field(Field *field) { (void)save_in_field(field, 1, 0); }
   bool register_field_in_read_map(uchar *arg);
   bool set_entry(THD *thd, bool create_if_not_exists);
+  void cleanup();
 };