mastodon/app/services/activitypub/synchronize_followers_service.rb
ThibG ca56527140
Add follower synchronization mechanism (#14510)
* Add support for followers synchronization on the receiving end

Check the `collectionSynchronization` attribute on `Create` and `Announce`
activities and synchronize followers from provided collection if possible.

* Add tests for followers synchronization on the receiving end

* Add support for follower synchronization on the sender's end

* Add tests for the sending end

* Switch from AS attributes to HTTP header

Replace the custom `collectionSynchronization` ActivityStreams attribute by
an HTTP header (`X-AS-Collection-Synchronization`) with the same syntax as
the `Signature` header and the following fields:
- `collectionId` to specify which collection to synchronize
- `digest` for the SHA256 hex-digest of the list of followers known on the
   receiving instance (where “receiving instance” is determined by accounts
   sharing the same host name for their ActivityPub actor `id`)
- `url` of a collection that should be fetched by the instance actor

Internally, move away from the webfinger-based `domain` attribute and use
account `uri` prefix to group accounts.

* Add environment variable to disable followers synchronization

Since the whole mechanism relies on some new preconditions that, in some
extremely rare cases, might not be met, add an environment variable
(DISABLE_FOLLOWERS_SYNCHRONIZATION) to disable the mechanism altogether and
avoid followers being incorrectly removed.

The current conditions are:
1. all managed accounts' actor `id` and inbox URL have the same URI scheme and
   netloc.
2. all accounts whose actor `id` or inbox URL share the same URI scheme and
   netloc as a managed account must be managed by the same Mastodon instance
   as well.

As far as Mastodon is concerned, breaking those preconditions require extensive
configuration changes in the reverse proxy and might also cause other issues.

Therefore, this environment variable provides a way out for people with highly
unusual configurations, and can be safely ignored for the overwhelming majority
of Mastodon administrators.

* Only set follower synchronization header on non-public statuses

This is to avoid unnecessary computations and allow Follow-related
activities to be handled by the usual codepath instead of going through
the synchronization mechanism (otherwise, any Follow/Undo/Accept activity
would trigger the synchronization mechanism even if processing the activity
itself would be enough to re-introduce synchronization)

* Change how ActivityPub::SynchronizeFollowersService handles follow requests

If the remote lists a local follower which we only know has sent a follow
request, consider the follow request as accepted instead of sending an Undo.

* Integrate review feeback

- rename X-AS-Collection-Synchronization to Collection-Synchronization
- various minor refactoring and code style changes

* Only select required fields when computing followers_hash

* Use actor URI rather than webfinger domain in synchronization endpoint

* Change hash computation to be a XOR of individual hashes

Makes it much easier to be memory-efficient, and avoid sorting discrepancy issues.

* Marginally improve followers_hash computation speed

* Further improve hash computation performances by using pluck_each
2020-10-21 18:04:09 +02:00

75 lines
2.7 KiB
Ruby

# frozen_string_literal: true
class ActivityPub::SynchronizeFollowersService < BaseService
include JsonLdHelper
include Payloadable
def call(account, partial_collection_url)
@account = account
items = collection_items(partial_collection_url)
return if items.nil?
# There could be unresolved accounts (hence the call to .compact) but this
# should never happen in practice, since in almost all cases we keep an
# Account record, and should we not do that, we should have sent a Delete.
# In any case there is not much we can do if that occurs.
@expected_followers = items.map { |uri| ActivityPub::TagManager.instance.uri_to_resource(uri, Account) }.compact
remove_unexpected_local_followers!
handle_unexpected_outgoing_follows!
end
private
def remove_unexpected_local_followers!
@account.followers.local.where.not(id: @expected_followers.map(&:id)).each do |unexpected_follower|
UnfollowService.new.call(unexpected_follower, @account)
end
end
def handle_unexpected_outgoing_follows!
@expected_followers.each do |expected_follower|
next if expected_follower.following?(@account)
if expected_follower.requested?(@account)
# For some reason the follow request went through but we missed it
expected_follower.follow_requests.find_by(target_account: @account)&.authorize!
else
# Since we were not aware of the follow from our side, we do not have an
# ID for it that we can include in the Undo activity. For this reason,
# the Undo may not work with software that relies exclusively on
# matching activity IDs and not the actor and target
follow = Follow.new(account: expected_follower, target_account: @account)
ActivityPub::DeliveryWorker.perform_async(build_undo_follow_json(follow), follow.account_id, follow.target_account.inbox_url)
end
end
end
def build_undo_follow_json(follow)
Oj.dump(serialize_payload(follow, ActivityPub::UndoFollowSerializer))
end
def collection_items(collection_or_uri)
collection = fetch_collection(collection_or_uri)
return unless collection.is_a?(Hash)
collection = fetch_collection(collection['first']) if collection['first'].present?
return unless collection.is_a?(Hash)
case collection['type']
when 'Collection', 'CollectionPage'
collection['items']
when 'OrderedCollection', 'OrderedCollectionPage'
collection['orderedItems']
end
end
def fetch_collection(collection_or_uri)
return collection_or_uri if collection_or_uri.is_a?(Hash)
return if invalid_origin?(collection_or_uri)
fetch_resource_without_id_validation(collection_or_uri, nil, true)
end
end