This has existed for some time in the form of the lfs_traverse
function, however lfs_traverse is relatively unconventional and
has proven to not have been the most intuitive for users.
As pointed out by davidefer, the lookahead pointer modular arithmetic
does not work around integer overflow when the pointer size is not a
multiple of the block count.
To avoid overflow problems, the easy solution is to stop trying to
work around integer overflows and keep the lookahead offset inside the
block device. To make this work, the ack was modified into a resetable
counter that is decremented every block allocation.
As a plus, quite a bit of the allocation logic ended up simplified.
One of the big simplifications in littlefs's implementation is the
complete lack of tracking free blocks, allowing operations to simply
drop blocks that are no longer in use.
However, this means the lookahead buffer can easily contain outdated
blocks that were previously deleted. This is usually fine, as littlefs
will rescan the storage if it can't find a free block in the lookahead
buffer, but after changes that caused littlefs to more conservatively
respect the alloc acks (e611cf5), any scanned blocks after an ack would
be incorrectly trusted.
The fix is to eagerly scan ahead in the lookahead when we allocate so
that alloc acks are better able to discredit old lookahead blocks. Since
usually alloc acks are tightly coupled to allocations of one or two blocks,
this allows littlefs to properly rescan every set of allocations.
This may still be a concern if there is a long series of worn out
blocks, but in the worst case littlefs will conservatively avoid using
blocks it's not sure about.
Found by davidefer
