net: buf: Make a net_buf_move(&buf) API

[jori-nordic]

We sometimes do an implicit move of netbufs.
In that case, the best we do is leave a comment, and maybe NULL out the original variable, like this

	/* Move `buf` into `temp` */
	temp = buf;
	buf = NULL;

It would be nice to have a proper API to do this in net/buf.h, something like

	temp = net_buf_move(&buf);
	__ASSERT_NO_MSG(buf == NULL);

It could be defined like so

struct net_buf *net_buf_move(struct net_buf **original)
{
	struct net_buf *new = *original;

	__ASSERT_NO_MSG(new->ref == 1);
	*original = NULL;

	return new;
}

[jhedberg]

@jori-nordic thanks for creating this issue. Just a few quick thoughts:

• I don't think we want to constrain this API to ref == 1. Moving a reference doesn't need to imply that this is the only reference there is - that's a separate use-case specific constraint IMO.
• Since we have atomic_ptr_clear() I'd just use it. Most places may not need it right now, but as we move towards supporting preemptible threads then being able to use this API without locking will be nice.

[jukkar]

To me this new function gives impression of moving the data from/to the net_buf, like what memmove() does. Not sure what it could be called, but some examples could be net_buf_replace(), net_buf_swap() or similar.

[jhedberg]

To me this new function gives impression of moving the data from/to the net_buf, like what memmove() does. Not sure what it could be called, but some examples could be net_buf_replace(), net_buf_swap() or similar.

@jukkar point taken. That said, we do tend to use net_buf_*_mem() in the net_buf API for memory operations. Also, "move" is verb that's used for analogous things in other languages, like Rust. Would it help to append _ref() to the API, i.e. net_buf_move_ref()? (not sure I prefer that more, but that could just be due to my dislike for unnecessarily long symbol names :)

[jukkar]

Yes, net_buf_move_ref() would indeed be better here.