The bug was caused by a misplaced construct opt_with_clause
for one of the variants of CREATE ... SELECT.

The code for st_select_lex::find_table_def_in_with_clauses()
did not take into account the fact that the specs for mergeable
CTEs were cloned and were not processed by the function
With_element::check_dependencies_in_spec().

There was no implementation of the virtual method print()
for the Item_window_func class. As a result for a view
containing window function an invalid view definition could
be written in the frm file. When a query that refers to
this view was executed a syntax error was reported.

MDEV-10908: innodb_zip.bug56680 fails in buildbot with InnoDB: Failing assertion: bpage->id.space() == page_id.space()

Incorrect merge introduced too strict assertion.

MDEV-10918 Innodb/Linux - Fallback to simulated aio if io_setup() fails, e.g due to insufficient resources

The bug was in the code of the recursive method
With_element::check_unrestricted_recursive. For recursive
calls of this method sel->get_with_element()->owner != owner.

If a materialized derived table / view is specified by a unit
with SELECTs containing ORDER BY ... LIMIT then condition pushdown
cannot be done for these SELECTs.
If a materialized derived table / view is specified by a unit
containing global ORDER BY ... LIMIT then condition pushdown
cannot be done for this unit.

The server missed to call check_dependencies_in_with_clauses()
when processing PREPARE ... FROM CREATE ... SELECT / INSERT ... SELECT
with WITH clause before SELECT.

When a prepared statement uses a CTE definition with a column list
renaming of columns of the CTE expression  must be performed
for every execution of the prepared statement.

Fixes file descriptor leak after unsuccessful attempt to open
unix socket connection.

(Continued after a95e38) Update more test results.

Make sure we select one window function if we have an empty over clause.

The number is irrelevant and may differ by architecture. We are only
interested if it happened or not

Add support for having multiple IO_CACHEs with type=READ_CACHE to share
the file they are reading from.
Each IO_CACHE keeps its own in-memory buffer. When doing a read or seek
operation on the file, it notifies other IO_CACHEs that the file position
has been changed.

Make Rowid_seq_cursor use cloned IO_CACHE when reading filesort result.

Implement nth_value correctly and add a test case for it.

Refactour out (into a copy for now) the logic of Item_sum_hybrid, to
allow for multiple arguments. It does not contain the comparator
members. The result is the class Item_sum_hybrid_simple.

LEAD and LAG make use of this Item to store previous rows in a chache.
It also helps in specifying the field type. Currently LEAD/LAG do not
support default values.

NTH_VALUE behaves identical to LEAD and LAG, except that the starting
position cursor is placed on the top of the frame instead of the current
row.

Also remove some whitespace

Make window functions work with an empty over clause by forcing
a sort on the first column of the current join_tab. This is a temporary
fix until we get window functions to work with big tables.

The positional cursor now fetches rows based on the positional
cursor and an offset (if present). It will fetch rows, based on the
offset, only if the required position is not out of bounds.

Add a test case to check this behavior.

With clever use of partition bounds, we only need to add one row to the
items at a time. This way we remove the need to "reset" the item and run
through the full partition again.

Currently the implementation doesn't support removal, thus the
computation is performed by running over the window frame again.

The test case also checks correct functionality of Frame_scan_cursor, as
currently STD does not implement the remove() call.

We can set values in the record buffer first and only perform one table
write call at the end. No need to write to file every time one column is
updated.

Also, remove unused method from Table_read_cursor.