buf_pool_t::shrink(): When relocating a buffer page, invalidate
the page identifier of the original page so that buf_pool_t::page_guess()
will not accidentally match it.
Before commit b6923420f3 (MDEV-29445)
introduced buf_pool_t::page_guess(), the validity of block descriptor
pointers was checked by buf_pool_t::is_uncompressed(const buf_block_t*).
Therefore, any block descriptors that used to be part of a larger buffer
pool would not be accessed at all.
This race condition is very hard to reproduce. To reproduce it,
an optimistic btr_pcur_t::restore_position() or similar will have to
be invoked on a block that has been relocated by buf_pool_t::shrink()
and that had not meanwhile been replaced with another page with a
different identifier.
Reviewed by: Vladislav Lesin
21bb6a3e34