The block_fits() predicate was borked. It would check that a block
fits within the bounds of the whole heap. But that's not enough:
because of alignment changes between levels the sub-blocks may be
adjusted forward. It needs to fit inside the PARENT block that it was
split from.
What could happen at runtime is that the last subblocks of a
misaligned parent block would overlap memory from subsequent blocks,
or even run off the end of the heap. That's bad.
Change the API of block_fits() a little so it can extract the parent
region and do this properly.
Fixes#15279. Passes test introduced in #16728 to demonstrate what
seems like the same issue.
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
d0490fe9f9