Race condition between buf_pool.page_hash and buffer pool resizing

The replacement of buf_pool.page_hash with a different type of hash table in MDEV-22871 introduced a race condition with buffer pool resizing.

We have an execution trace where buf_pool.page_hash.array is changed to point to something else while page_hash_latch::read_lock() is executing. The same should also affect page_hash_latch::write_lock().

The wait loop currently fails to notice that buffer pool resizing is in progress. A part of the problem is that we are waiting too deep in the code:

    template<bool exclusive> page_hash_latch *lock(ulint fold)

    {

      for (;;)

      {

        auto n= n_cells;

        page_hash_latch *latch= lock_get(fold, n);

        latch->acquire<exclusive>();

        /* Our latch prevents n_cells from changing. */

        if (UNIV_LIKELY(n == n_cells))

          return latch;

        /* Retry, because buf_pool_t::resize_hash() affected us. */

        latch->release<exclusive>();

      }

    }

We are actually waiting inside page_hash_latch::read_lock() or page_hash_latch::write_lock() on memory that may no longer belong to the buf_pool.page_hash.array. We would need some notion of timeout or temporary failure when the buffer pool is being resized.