MDEV-37042 innodb_undo_log_truncate=ON leads to out-of-bounds write

[Thirunarayanan]

Reason:

During undo tablespace truncation, pages with LSNs older than the tablespace creation LSN may still exist in the buffer pool and get submitted to the doublewrite buffer. When mtr_t::commit_shrink() is invoked shortly after doublewrite batch submission, this can lead to out-of-bounds write errors.

Fix:

buf_dblwr_t::flush_buffered_writes_completed() : skip doublewrite processing for pages where the page LSN is older
than the tablespace creation LSN. Such pages belong to the old tablespace before truncation and should not be written through the doublewrite buffer.

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