buf_flush_try_neighbors(): Before invoking buf_page_t::ready_for_flush(),
check that the freshly looked up buf_pool.page_hash entry actually is
a buffer page and not a buf_pool.watch[] sentinel for purge buffering.

This race condition was introduced in MDEV-15053
(commit b1ab211d).
It is rather hard to hit this bug, because
buf_flush_check_neighbors() already checked the condition.
The problem exists if buf_pool.watch_set() was invoked for
a page in the range after the check in buf_flush_check_neighbor()
had been finished.

Thanks to Varun Gupta for suggesting this. This seems to
make main.innodb_ext_key,off more stable.

Invoking memcpy() on a NULL pointer is undefined behaviour
(even if the length is 0) and gives the compiler permission to
assume that the pointer is nonnull. Recent versions of GCC
(starting with version 8) are more aggressively optimizing away
checks for NULL pointers. This undefined behaviour would cause
a SIGSEGV in the test main.func_encrypt on an optimized debug build
on GCC 10.2.0.

This regression was introduced in
commit afc9d00c.
This is a partial backport of
commit 199863d7 from 10.4.

 MDEV-24033: SIGSEGV in __memcmp_avx2_movbe from queue_insert | SIGSEGV in __memcmp_avx2_movbe from native_compare

The issue here was the system variable max_sort_length was being applied
to decimals and it was truncating the value for decimals to the number
of bytes set by max_sort_length.
This was leading to a buffer overflow as the values were written
to the buffer without truncation and then we moved the offset to
the number of bytes(set by max_sort_length), that are needed for comparison.

The fix is to not apply max_sort_length for fixed size types like INT,
DECIMALS and only apply max_sort_length for CHAR, VARCHARS, TEXT and
BLOBS.

Also, revert the work-around for the test that was attempted in
commit 85613a32.

This issue was caught by MemorySanitizer as well as on the
Microsoft Windows debug builds, thanks to /MD being used
starting with 10.4.

The code fix will also be applied to 10.2 because the regression
was introduced in commit afc9d00c.

Test itself is not deterministic.

Disable galera_var_replicate_myisam until fixed on 10.4

Corrects: 7803601d

Use 9 byte (min length packet)

This bug was already fixed in a previous commit.
Added test case from the MDEV to prove it's fixed.

The crash happens because a double free in the case CREATE TABLE fails
because there is a conflicting tables on disk.

Fixed by ensuring that the double free can't happen.

On my system, OpenSuse, I got a compilation error that some arguments
to getgrouplist() where not initialized

For some reason, in the test main,innodb_ext_key,off
we frequently get unexpected EXPLAIN output, in particular
on Microsoft Windows debug builders. Let us comment out that
EXPLAIN statement for now.

The problem was that opt_sum_query() was, as part of MIN/MAX optimization,
doing read operations on constant tables that where already closed

Fixed by ensuring we don't try to read from tables that are closed.

Starting with commit ef3f71fa
MemorySanitizer would complain that we are writing uninitialized
data via the doublewrite buffer.

buf_dblwr_t::add_to_batch(): Zero out any unused part of the
doublewrite buffer, for PAGE_COMPRESSED and ROW_FORMAT=COMPRESSED
tables.

Reviewed by: Eugene Kosov

This prevents the clash between NOT_FIXED_DEC differing between server
and plugins if MYSQL_SERVER is not defined during plugin compilation.

It's a virtual method and it can't be inlined anyway. This allows type
plugins (mysql_json in particular) to use Type_handler_blob and / or
subclass it, without needing to explicitly expose the
vers_type_timestamp object.

Cleanup other linker errors

Additional case for PS protocol: UPDATE is converted to multi-update
in mysql_multi_update_prepare().

and restore the test modified in the same commit
(the non-replication related deadlock will be reported separately)

Implement a different fix for
"MDEV-19232: Floating point precision / value comparison problem"

Instead of truncating decimal values after every division,
truncate them for comparison purposes.

This reverts commit 62d73df6 but keeps the test.

followup for 3e807d25 and eae10a87