> revision-id: gshchepa@mysql.com-20100801181236-uyuq6ewaq43rw780
 > parent: alexey.kopytov@sun.com-20100723115254-jjwmhq97b9wl932l
 > committer: Gleb Shchepa <gshchepa@mysql.com>
 > branch nick: mysql-5.1-security
 > timestamp: Sun 2010-08-01 22:12:36 +0400
 > Bug #54461: crash with longblob and union or update with subquery
 >
 > Queries may crash, if
 >   1) the GREATEST or the LEAST function has a mixed list of
 >      numeric and LONGBLOB arguments and
 >   2) the result of such a function goes through an intermediate
 >      temporary table.
 >
 > An Item that references a LONGBLOB field has max_length of
 > UINT_MAX32 == (2^32 - 1).
 >
 > The current implementation of GREATEST/LEAST returns REAL
 > result for a mixed list of numeric and string arguments (that
 > contradicts with the current documentation, this contradiction
 > was discussed and it was decided to update the documentation).
 >
 > The max_length of such a function call was calculated as a
 > maximum of argument max_length values (i.e. UINT_MAX32).
 >
 > That max_length value of UINT_MAX32 was used as a length for
 > the intermediate temporary table Field_double to hold
 > GREATEST/LEAST function result.
 >
 > The Field_double::val_str() method call on that field
 > allocates a String value.
 >
 > Since an allocation of String reserves an additional byte
 > for a zero-termination, the size of String buffer was
 > set to (UINT_MAX32 + 1), that caused an integer overflow:
 > actually, an empty buffer of size 0 was allocated.
 >
 > An initialization of the "first" byte of that zero-size
 > buffer with '\0' caused a crash.
 >
 > The Item_func_min_max::fix_length_and_dec() has been
 > modified to calculate max_length for the REAL result like
 > we do it for arithmetical operators.