the most basic test you could imagine

app/services/activitypub/fetch_all_replies_service.rb

@@ -24,9 +24,15 @@ def call(collection_or_uri, allow_synchronous_requests: true, request_id: nil)
   def filtered_replies
     return if @items.nil?
 
-    # find all statuses that we *shouldn't* update the replies for, and use that as a filter
-    # Typically we assume this is smaller than the replies we *should* fetch,
-    # so we minimize the number of uris we should load here.
+    # Find all statuses that we *shouldn't* update the replies for, and use that as a filter.
+    # We don't assume that we have the statuses before they're created,
+    # hence the negative filter -
+    # "keep all these uris except the ones we already have"
+    # instead of
+    # "keep all these uris that match some conditions on existing Status objects"
+    #
+    # Typically we assume the number of replies we *shouldn't* fetch is smaller than the
+    # replies we *should* fetch, so we also minimize the number of uris we should load here.
     uris = @items.map { |item| value_or_id(item) }
     dont_update = Status.where(uri: uris).shouldnt_fetch_replies.pluck(:uri)
 

spec/services/activitypub/fetch_all_replies_service_spec.rb

@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe ActivityPub::FetchAllRepliesService do
+  subject { described_class.new }
+
+  let(:actor)          { Fabricate(:account, domain: 'example.com', uri: 'http://example.com/account') }
+  let(:status)         { Fabricate(:status, account: actor) }
+  let(:collection_uri) { 'http://example.com/replies/1' }
+
+  let(:items) do
+    [
+      'http://example.com/self-reply-1',
+      'http://example.com/self-reply-2',
+      'http://example.com/self-reply-3',
+      'http://other.com/other-reply-1',
+      'http://other.com/other-reply-2',
+      'http://other.com/other-reply-3',
+      'http://example.com/self-reply-4',
+      'http://example.com/self-reply-5',
+      'http://example.com/self-reply-6',
+    ]
+  end
+
+  let(:payload) do
+    {
+      '@context': 'https://www.w3.org/ns/activitystreams',
+      type: 'Collection',
+      id: collection_uri,
+      items: items,
+    }.with_indifferent_access
+  end
+
+  describe '#call' do
+    it 'fetches more than the default maximum and from multiple domains' do
+      allow(FetchReplyWorker).to receive(:push_bulk)
+
+      subject.call(payload)
+
+      expect(FetchReplyWorker).to have_received(:push_bulk).with(%w(http://example.com/self-reply-1 http://example.com/self-reply-2 http://example.com/self-reply-3 http://other.com/other-reply-1 http://other.com/other-reply-2 http://other.com/other-reply-3 http://example.com/self-reply-4
+                                                                    http://example.com/self-reply-5 http://example.com/self-reply-6))
+    end
+
+    context 'with a recent status' do
+      before do
+        Fabricate(:status, uri: 'http://example.com/self-reply-2', fetched_replies_at: 1.second.ago, local: false)
+      end
+
+      it 'skips statuses that have been updated recently' do
+        allow(FetchReplyWorker).to receive(:push_bulk)
+
+        subject.call(payload)
+
+        expect(FetchReplyWorker).to have_received(:push_bulk).with(%w(http://example.com/self-reply-1 http://example.com/self-reply-3 http://other.com/other-reply-1 http://other.com/other-reply-2 http://other.com/other-reply-3 http://example.com/self-reply-4 http://example.com/self-reply-5 http://example.com/self-reply-6))
+      end
+    end
+
+    context 'with an old status' do
+      before do
+        Fabricate(:status, uri: 'http://other.com/other-reply-1', fetched_replies_at: 1.year.ago, created_at: 1.year.ago, account: actor)
+      end
+
+      it 'updates the time that fetched statuses were last fetched' do
+        allow(FetchReplyWorker).to receive(:push_bulk)
+
+        subject.call(payload)
+
+        expect(Status.find_by(uri: 'http://other.com/other-reply-1').fetched_replies_at).to be >= 1.minute.ago
+      end
+    end
+  end
+end