Assertion fold == id.fold() failed in buf_flush_check_neighbor()

The setting innodb_flush_neighbors is broken on 64-bit platforms ever since the code was refactored in MDEV-23399 to avoid recomputing page_id_t::fold() in buf_flush_check_neighbors(). The following fixes the problem that was repeatable when attempting to flush pages in the temporary tablespace around page number 0x200002:

diff --git a/storage/innobase/include/buf0types.h b/storage/innobase/include/buf0types.h

--- a/storage/innobase/include/buf0types.h

+++ b/storage/innobase/include/buf0types.h

@@ -162,7 +162,10 @@ class page_id_t

   /** Retrieve the fold value.

   @return fold value */

-  ulint fold() const { return (space() << 20) + space() + page_no(); }

+  ulint fold() const

+  {

+    return sizeof(ulint) == 8 ? m_id : (space() << 20) + space() + page_no();

+  }

   /** Reset the page number only.

   @param[in]	page_no	page number */

The problem is that the return type of page_id_t::space() is uint32_t, and therefore on 64-bit systems the fold value would not be the nonzero value (((ulint)0xfffffffe)<<20)+0xfffffffe+0x200002 (as it could be assumed on a quick read of the code), but 0, because that is what (((uint32_t)0xfffffffe)<<20)+0xfffffffe+0x200002 evaluates to.

Furthermore, there is some indication that despite MDEV-17380, neighbour flushing was attempted on a NVMe device (major number: 259, minor number 0).

The performance of the above patch will have to be tested.