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Backport from mysql-5.5 to mysql-5.1 of: Bug19770858: MYSQLD CAN BE DRIVEN TO OOM WITH TWO SIMPLE SESSION VARS The problem was that the maximum value of the transaction_prealloc_size session system variable was ULONG_MAX which meant that it was possible to cause the server to allocate excessive amounts of memory. This patch fixes the problem by reducing the maxmimum value of transaction_prealloc_size and transaction_alloc_block_size down to 128K. Note that transactions will still be able to allocate more than 128K if needed, this patch just reduces the amount that can be preallocated - as well as the maximum size of the incremental allocation blocks. (cherry picked from commit 540c9f7ebb428bbf9ec028feabe1f7f919fdefd9) Conflicts: mysql-test/suite/sys_vars/r/transaction_alloc_block_size_basic.result mysql-test/suite/sys_vars/r/transaction_alloc_block_size_basic_64.result mysql-test/suite/sys_vars/t/disabled.def mysql-test/suite/sys_vars/t/transaction_alloc_block_size_basic.test sql/sys_vars.cc