MDEV-37294 segv in flst::remove_complete(buf_block_t*, unsigned short, unsigned char*, mtr_t*)

[Thirunarayanan]

Problem:

During system tablespace defragmentation, extent movement occurs in two phases: prepare and complete.

1. prepare phase validates involved pages and acquires necessary resources.
2. complete phase performs the actual data copy.

Prepare phase fails to check whether allocating a page will make the extent FULL. When an extent has exactly (extent_size - 1) pages used, the prepare phase returns early without latching the prev/next extent descriptors needed for list manipulation.

Complete phase then allocates the final page, making the extent full, and attempts to move it from
FSEG_NOT_FULL/FSP_FREE_FRAG to FSEG_FULL/FSP_FULL_FRAG list. This fails with an assertion because the required blocks were never latched, causing a crash in flst::remove_complete().

Solution:

alloc_from_fseg_prepare(), alloc_from_free_frag_prepare(): Check if the extent will become full after allocation. This makes the prepare phase to acquire the necessary pages for FSP list manipulation

find_new_extents(): Print more revised information about moving of extent data and destination extent also.

defragment_level(): Move get_child_pages(new_block) before committing changes to enable proper rollback on failure. Well, this failure is theoretically impossible (new block is exact copy of validated old block).

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[dr-m]

The return DB_CORRUPTION is avoided if n_used < m_extent_size. Therefore, before this fix, it looks like the code after the return DB_SUCCESS must have been unreachable; the branch must always have been taken.

Can we put the most distinctive condition first? Something like the following:

if (n_used + 1 != m_extent_size)
  return n_used < 1 || n_used >= m_extent_size ? DB_CORRUPTION : DB_SUCCESS;