Add hashing for activity ids

[uOJackDu]

For issue #6341.

Add hashing for activity ids so that the ids of the activity objects do not change on every request.

[uOJackDu]

This is the only place where I am passing in the object_id. I think it makes the ids for Announce and Create stable. Not sure where else I could use it.

[Nutomic]

Also needs to be in crates/apub/activities/src/create_or_update/post.rs so both code paths generate the same id.

[Nutomic]

Use sha2 which we are already using as dependency.

[Nutomic]

Suggested change

	CreateOrUpdatePage::new(post.into(), &person, &community, kind, None, &context).await?;
	CreateOrUpdatePage::new(post.into(), &person, &community, kind, Some(post.ap_id), &context).await?;

[Nutomic]

Actually this can be a problem when editing the post, then each Update activity will have the same id. So you also need to hash the timestamp (published_at or updated_at).

[Nutomic]

Suggested change

	let id = generate_activity_id(kind.clone(), None, &context)?;
	let id = generate_activity_id(kind.clone(), Some(comment.ap_id), &context)?;

[Nutomic]

Suggested change

	object_id: Option<&str>,
	object_id: Option<&Url>,

[Nutomic]

To avoid passing None in so many places, you could do:

fn generate_activity_id_with_object_id(kind, context) {
  generate_activity_id(kind, None, context)
}

fn generate_activity_id(kind, object_id, context) {
  generate_activity_id(kind, context)
}

[dessalines]

What is the minimum amount of info needed to create uniqueness for these generated ap_ids?

Seems like kind/action, numeric_object_id should be enough right?

[Nutomic]

That is not enough, because if you edit a post, it is federated as Update<Page> activity each time. So the kind and object id will be identical every time, but the generated ap_id must be different (otherwise it will be ignored as duplicate on the receiving side). So the object created_at/updated_at timestamp also needs to go into the hash.