Make max_user_connections signed, with min allowed value being -1.

So removed.

the depth of subquery nestedness to less than 31
(sizeof(ulong)-1).

This bug in the function Loose_scan_opt::check_ref_access_part1 could lead
to choosing an invalid execution plan employing a loose scan access to a
semi-join table even in the cases when such access could not be used at all.
This could result in wrong answers for some queries with IN subqueries.

Analysis:
The optimizer distinguishes two kinds of 'constant' conditions:
expensive ones, and non-expensive ones. The non-expensive conditions
are evaluated inside make_join_select(), and if false, already the
optimizer detects empty query results.

In order to avoid arbitrarily expensive optimization, the evaluation of
expensive constant conditions is delayed until execution. These conditions
are attached to JOIN::exec_const_cond and evaluated in the beginning of
JOIN::exec. The relevant execution logic is:

JOIN::exec()
{
  if (! join->exec_const_cond->val_int())
  {
    produce an empty result;
    stop execution
  }
  continue execution
  execute the original WHERE clause (that contains exec_const_cond)
 ...
}

As a result, when an expensive constant condition is
TRUE, it is evaluated twice - once through
JOIN::exec_const_cond, and once through JOIN::cond.
When the expensive constant condition is a subquery,
predicate, the subquery is evaluated twice. If we have
many levels of subqueries, this logic results in a chain
of recursive subquery executions that walk a perfect
binary tree. The result is that for subquries with depth N,
JOIN::exec is executed O(2^N) times.

Solution:
Notice that the second execution of the constant conditions
happens inside do_select(), in the branch:
if (join->table_count == join->const_tables) { ... }
In this case exec_const_cond is equivalent to the whole WHERE
clause, therefore the WHERE clause has already been checked in
the beginnig of JOIN::exec, and has been found to be true.
The bug is addressed by not evaluating the WHERE clause if there
was exec_const_conds, and it was TRUE.

set to 'on' by default.

A non-first execution of a prepared statement missed a call of the
TABLE_LIST::process_index_hints() method in the code of the function
setup_tables().
At some scenarios this could lead to the choice of a quite inefficient
execution plan for the base query of the prepared statement.

  Checks for thd->killed state added to the long loops in geometry calculations.

per-file comments:
  sql/gcalc_slicescan.cc
Fix for bug #809849 spatial operations must be KILL-able.
        checks for TERMINATED_STATE added.
  sql/gcalc_slicescan.h
Fix for bug #809849 spatial operations must be KILL-able.
        defines added to include checks for termination in the
        library.
  sql/gcalc_tools.cc
Fix for bug #809849 spatial operations must be KILL-able.
        checks for TERMINATED_STATE added.
  sql/gcalc_tools.h
Fix for bug #809849 spatial operations must be KILL-able.
        TERMINATED_STATE pointers added.
  sql/item_geofunc.cc
Fix for bug #809849 spatial operations must be KILL-able.
  sql/item_geofunc.h
Fix for bug #809849 spatial operations must be KILL-able.

This bug in the function setup_semijoin_dups_elimination() could 
lead to invalid choice of the sequence of tables for which semi-join
duplicate elimination was applied.

Due to this bug the function SEL_IMERGE::or_sel_tree_with_checks()
could build an inconsistent merge tree if one of the SEL_TREEs in the
resulting index merge happened to contain a full key range.
This could trigger an assertion failure.

That's not a good option for an onrdinary user.

The function key_and() erroneously called SEL_ARG::increment_use_count()
when SEL_ARG::incr_refs() should had been called. This could lead to
wrong values of use_count for some SEL_ARG trees.

Some optimizer switches were missing in the help lists of mysqld.

Apart from the fix, the patch also adds few more unrelated test
cases for partial matching, and fixes few typos.

Analysis:
This bug uncovered that partial matching via rowid intersection
didn't handle the case when:
- the left IN argument has some NULLs,
- there are no non-null value matches, and there is no non-null
  column,
- the subquery columns that are not covered with the NULLs in
  the left IN argument contain at least one row, such that it
  has NULL values in all columns where the left IN operand has
  no NULLs.
In this case there is a partial match.

In addition the analysis of the related code uncovered incorrect
handling of few other related cases.

Solution:
The solution for the bug is to check if there exists a row with
NULLs in all columns other than the ones having NULL in the
let IN operand.

The check is implemented via checking whether the bitmaps that
store NULL information in class Ordered_key have a non-empty
intersection for the relevant columns.

The intersection itself is implemented via the function
bitmap_exists_intersection() in my_bitmap.c.

The function setup_semijoin_dups_elimination erroneously assumed
that if join_cache_level is set to 3 or 4 then the type of the
access to a table cannot be JT_REF or JT_EQ_REF. This could lead
to wrong query result sets.

If the optimizer switch 'semijoin_with_cache' is set to 'off' then 
join cache cannot be used to join inner tables of a semijoin.

Also fixed a bug in the function check_join_cache_usage() that led
to wrong output of the EXPLAIN commands for some test cases.

MariaDB 5.5 merges changes from MySQL 5.5 where all constant
expressions are wrapped into an Item_cache. As a result, constant
single-row subqueries were also wrapped in an Item_cache.
When analyzing the where clause for constant expressions that
can be evaluated during optimization, subqueries wrapped into
an Item_cache did not appear as expensive, and were therefore
evaluated during optimization. Such evaluation is against the
current architecture of MariaDB 5.3 where subquries are executed
during the execute phase.

The patch adds the is_expensive() predicate to Item_cache.
This makes Item_cache consistent with other wrapping Item
classes that need to look at the properties of the wrapped
object.

don't assume that the http reply packet will arrive in all in one piece

Fix for walk() method of subqueries: always call the method on the subquery.