1 files changed, 60 insertions, 0 deletions
diff --git a/app/services/activitypub/fetch_collection_items_service.rb b/app/services/activitypub/fetch_collection_items_service.rb
new file mode 100644
index 000000000..2d990dbf5
--- /dev/null
+++ b/app/services/activitypub/fetch_collection_items_service.rb
@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+class ActivityPub::FetchCollectionItemsService < BaseService
+  include JsonLdHelper
+
+  def call(collection_or_uri, account, page_limit: 10, item_limit: 100, allow_synchronous_requests: true)
+    @account = account
+    @allow_synchronous_requests = allow_synchronous_requests
+
+    collection_items(collection_or_uri, page_limit, item_limit)
+  end
+
+  private
+
+  def collection_items(collection_or_uri, page_limit, item_limit)
+    collection = fetch_collection(collection_or_uri)
+    return [] unless collection.is_a?(Hash)
+
+    collection = fetch_collection(collection['first']) if collection['first'].present?
+    page_count = 0
+    item_count = 0
+    items = []
+
+    while collection.present? && collection.is_a?(Hash) && collection['type'].present?
+      batch = case collection['type']
+              when 'Collection', 'CollectionPage'
+                collection['items'].each
+              when 'OrderedCollection', 'OrderedCollectionPage'
+                collection['orderedItems']
+              end
+
+      batch_size = [batch.count, item_limit - item_count].min
+      items.push(
+        batch.take(batch_size)
+             .map { |item| value_or_id(item) }
+             .reject { |uri| unsupported_uri_scheme?(uri) || ActivityPub::TagManager.instance.local_uri?(uri) }
+      )
+
+      item_count += batch_size
+      page_count += 1
+
+      break unless item_count < item_limit && page_count < page_limit && collection['next'].present?
+
+      collection = fetch_collection(collection['next'])
+    end
+
+    items.uniq
+  end
+
+  def fetch_collection(collection_or_uri)
+    return collection_or_uri if collection_or_uri.is_a?(Hash)
+    return unless @allow_synchronous_requests
+    return if invalid_origin?(collection_or_uri)
+
+    on_behalf_of = @account.present? ? @account.followers.local.first : nil
+    fetch_resource_without_id_validation(collection_or_uri, on_behalf_of, true)
+  rescue Mastodon::UnexpectedResponseError
+    nil
+  end
+end