lfs: correct alignment restriction on lookahead buffer

[pabigot]

The buffer need only be 32-bit aligned.

Signed-off-by: Peter A. Bigot pab@pabigot.com

[geky]

Hi @pabigot, thanks for creating a pr!

This was discussed over here: #134, though I realize it's not easy to find and should be documented in lfs.h.

One of the possible solutions for #75 involves storing pairs of pointers (address+size pairs). The lookahead buffer would then need to be a multiple of 64-bits.

Well, it doesn't need to be, but any unaligned size would be wasted. It's cheaper and more transparent to the user to present this as a strict requirement to the user.

If we don't adopt this solution for #75, we can just remove the 64-bit requirement. But adopting it early makes it possible to introduce without breaking compatibility.

It's not needed yet but the alignment is reserved as it is likely it will be needed to solve #75. If it isn't we can remove the assert.

[pabigot]

@geky I'm confused again. I thought with #239 you agreed: yes, the length of the lookahead array needs to be a multiple of 8 bytes, but even with the proposed solution of pairs of address and size the alignment of the array is to 4-byte values.

So it is documented in lfs.h. I hadn't realized the code enforced the too-strong alignment until I finally ran into the situation.

[geky]

Ah! You're right, I'm getting confused again.

After some more coffee this looks great 👍