Improved remote thread fetching (#10106)

* Fetch up to 5 replies when discovering a new remote status

This is used for resolving threads downwards. The originating
server must add a “replies” attributes with such replies for it to
be useful.

* Add some tests for ActivityPub::FetchRepliesWorker

* Add specs for ActivityPub::FetchRepliesService

* Serialize up to 5 public self-replies for ActivityPub notes

* Add specs for ActivityPub::NoteSerializer

* Move exponential backoff logic to a worker concern

* Fetch first page of paginated collections when fetching thread replies

* Add specs for paginated collections in replies

* Move Note replies serialization to a first CollectionPage

The collection isn't actually paginable yet as it has no id nor
a `next` field. This may come in another PR.

* Use pluck(:uri) instead of map(&:uri) to improve performances

* Fix fetching replies when they are in a CollectionPage

spec/workers/activitypub/fetch_replies_worker_spec.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe ActivityPub::FetchRepliesWorker do
+  subject { described_class.new }
+
+  let(:account) { Fabricate(:account, uri: 'https://example.com/user/1') }
+  let(:status)  { Fabricate(:status, account: account) }
+
+  let(:payload) do
+    {
+      '@context': 'https://www.w3.org/ns/activitystreams',
+      id: 'https://example.com/statuses_replies/1',
+      type: 'Collection',
+      items: [],
+    }
+  end
+
+  let(:json) { Oj.dump(payload) }
+
+  describe 'perform' do
+    it 'performs a request if the collection URI is from the same host' do
+      stub_request(:get, 'https://example.com/statuses_replies/1').to_return(status: 200, body: json)
+      subject.perform(status.id, 'https://example.com/statuses_replies/1')
+      expect(a_request(:get, 'https://example.com/statuses_replies/1')).to have_been_made.once
+    end
+
+    it 'does not perform a request if the collection URI is from a different host' do
+      stub_request(:get, 'https://other.com/statuses_replies/1').to_return(status: 200)
+      subject.perform(status.id, 'https://other.com/statuses_replies/1')
+      expect(a_request(:get, 'https://other.com/statuses_replies/1')).to_not have_been_made
+    end
+
+    it 'raises when request fails' do
+      stub_request(:get, 'https://example.com/statuses_replies/1').to_return(status: 500)
+      expect { subject.perform(status.id, 'https://example.com/statuses_replies/1') }.to raise_error Mastodon::UnexpectedResponseError
+    end
+  end
+end