feat: let apps define the actor of an activity via the activity manager

[bakiburakogun]

Problem

CurrentUser::getUID() reads the user straight from the session:

public function getUID(): ?string {
	$user = $this->userSession->getUser();
	if ($user instanceof IUser) {
		return $user->getUID();
	}
	return null;
}

FilesHooks asks CurrentUser for the actor of a file activity, so anything an app does on a user's behalf outside of their session — from a background job, from a webhook — cannot be attributed to them. The activity stream renders it as "remote account" created ….

IManager::setCurrentUserId() exists for exactly this case, and OC\Activity\Manager::getCurrentUserId() honours it, but the activity app never consults it. There is currently no way for an app to name the actor.

Change

Ask the activity manager first, and fall back to the session when it has nothing to offer.

Without an override nothing changes:

• when a user is logged in, Manager::getCurrentUserId() returns that session user, which is what this method returned before;
• when there is neither a session nor a valid feed token it throws \UnexpectedValueException, which is caught here so the previous null is still returned.

The one behavioural difference is a request authenticated with an activity feed token: getCurrentUserId() resolves it to the owning user, where this method previously returned null and getUserIdentifier() fell through to the cloud id or the nickname header. Attributing those to the token owner looks more correct to me, but say the word if you would rather keep them going through the fallback chain — restricting the new lookup to the case where the session has no user would do it.

Motivation

Raised in nextcloud/spreed#19078, where call recordings are stored by a background job and end up attributed to nobody. The current workaround there is to swap the session user around the file write, which @nickvergessen rightly pushed back on:

I'm not sure it's the best idea to overwrite the session user like this. Also it's not going to work on the new chunked-uploading of recordings we implemented recently. I think instead we need work in the activity app to allow defining different actors […] I think CurrentUser should first check IManager::getCurrentUserId() that would allow Talk to overwrite the user for the action

With this in place Talk can call setCurrentUserId() around storing the recording, and the follow-up on the Talk side drops the session juggling entirely.

Testing

Two unit tests added: one asserting the override is used and the session is not consulted, one asserting the fallback to the session when the manager throws. The existing CurrentUser tests pass unchanged — an unstubbed getCurrentUserId() returns '' from the mock, which falls through to the session lookup.

[nickvergessen]

From my perspective this is what Talk needs to be able to overwrite the "remote user" with the current user.
Not sure if possible, but if Talk would be able to influence this part even more so it could refer to the recording backend explicitely that'd be even better, but not sure how we could do that. So leaving the approval to @miaulalala and @artonge