aya: rework links

[alessandrod]

Remove LinkRef and remove the Rc<RefCell<_>> that was used to store type-erased link values in ProgramData. Among other things, this allows Bpf to be Send, which makes it easier to use it with async runtimes.

Change the link API to:

let link_id = prog.attach(...)?;
...
prog.detach(link_id)?;

Link ids are strongly typed, so it's impossible to eg:

let link_id = uprobe.attach(...)?;
xdp.detach(link_id);

As it would result in a compile time error.

Links are still stored inside ProgramData, and unless detached explicitly, they are automatically detached when the parent program gets dropped.

[alessandrod]

This is another take on similar work that happened in #123

[alessandrod]

I guess to explain the gist of the PR:

• links are stored in ProgramData::links
• ProgramData::links is a hash map, where the keys are the link ids, and the values are the links
• attach(...) inserts into ProgramData::links and returns the corresponding link_id (map key)
• detach(link_id) deletes from ProgramData::links using link_id as the key

As a follow up to this PR, I'm going to add:

let link_id = program.attach(...)?;

// this leaves the link attached, transferring ownership to the caller (the name forget_link is provisional)
let owned_link = program.forget_link(link_id);

// this will make it so that the link stays attached when owned_link is dropped
owned_link.forget(); 

[vadorovsky]

I tested it with 2 different projects and it works perfectly. No remarks to the code, I really like the new API!

[vadorovsky]

I actually have one remark now nah, it was a brainfart

[vadorovsky]

Or hmm... actually using remove on HashMap always solves the problem. But anyway, there might be people wanting to use a reference for some other reason.

[alessandrod]

Nope taking self by value is very much intended: after detach a link becomes unusable (can't detach twice), and moving self ensures that trying to detach twice would fail at compile time.

[vadorovsky]

OK, good point, then seems like remove is the only way to go with HashMaps on the userspace side.

[vadorovsky]

Or hmm... actually using remove on HashMap always solves the problem. But anyway, there might be people wanting to use a reference for some other reason.

[dave-tucker]

LGTM.
@alessandrod just to be 100% clear. Per the comment in #123, we'll need #51 implemented... and to add to the release notes for 0.11.0 that there is a change for LircMode2, that links will need to be link_id.forget() - or whatever the API will be - to restore the pre 0.11.0 behaviour where Lirc programs would remain attached even after drop. Correct?