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.


******
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.



mysql-test/r/func_misc.result:
  Test case for bug #54461.
  
  ******
  Test case for bug #54461.
mysql-test/t/func_misc.test:
  Test case for bug #54461.
  
  ******
  Test case for bug #54461.
sql/item_func.cc:
  Bug #54461: crash with longblob and union or update with subquery
  
  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.
  
  ******
  Bug #54461: crash with longblob and union or update with subquery
  
  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.