Reject existing Follow in addition to sending a Block (mastodon#9811)

Mastodon expects remote servers to remove follow relationships upon receiving
a Block. However, the spec only evokes Block activities in a C2S context, never
in a S2S context.

This PR, in addition to federating the Block, explicitly sends a Reject for any
affected follow relationship, which makes a bit more sense with regards to the
spec.

app/services/unfollow_service.rb

@@ -20,6 +20,7 @@ def unfollow!
 
     follow.destroy!
     create_notification(follow) unless @target_account.local?
+    create_reject_notification(follow) if @target_account.local? && !@source_account.local?
     UnmergeWorker.perform_async(@target_account.id, @source_account.id)
     follow
   end
@@ -42,6 +43,12 @@ def create_notification(follow)
     end
   end
 
+  def create_reject_notification(follow)
+    # Rejecting an already-existing follow request
+    return unless follow.account.activitypub?
+    ActivityPub::DeliveryWorker.perform_async(build_reject_json(follow), follow.target_account_id, follow.account.inbox_url)
+  end
+
   def build_json(follow)
     ActiveModelSerializers::SerializableResource.new(
       follow,
@@ -50,6 +57,14 @@ def build_json(follow)
     ).to_json
   end
 
+  def build_reject_json(follow)
+    ActiveModelSerializers::SerializableResource.new(
+      follow,
+      serializer: ActivityPub::RejectFollowSerializer,
+      adapter: ActivityPub::Adapter
+    ).to_json
+  end
+
   def build_xml(follow)
     OStatus::AtomSerializer.render(OStatus::AtomSerializer.new.unfollow_salmon(follow))
   end

spec/services/unfollow_service_spec.rb

@@ -56,4 +56,22 @@
       expect(a_request(:post, 'http://example.com/inbox')).to have_been_made.once
     end
   end
+
+  describe 'remote ActivityPub (reverse)' do
+    let(:bob) { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob', protocol: :activitypub, domain: 'example.com', inbox_url: 'http://example.com/inbox')).account }
+
+    before do
+      bob.follow!(sender)
+      stub_request(:post, 'http://example.com/inbox').to_return(status: 200)
+      subject.call(bob, sender)
+    end
+
+    it 'destroys the following relation' do
+      expect(bob.following?(sender)).to be false
+    end
+
+    it 'sends a reject activity' do
+      expect(a_request(:post, 'http://example.com/inbox')).to have_been_made.once
+    end
+  end
 end