1 files changed, 88 insertions, 0 deletions
diff --git a/app/services/projects/branches_by_mode_service.rb b/app/services/projects/branches_by_mode_service.rb
new file mode 100644
index 00000000000..fb66bfa073b
--- /dev/null
+++ b/app/services/projects/branches_by_mode_service.rb
@@ -0,0 +1,88 @@
+# frozen_string_literal: true
+
+# Projects::BranchesByModeService uses Gitaly page-token pagination
+# in order to optimally fetch branches.
+# The drawback of the page-token pagination is that it doesn't provide
+# an option of going to the previous page of the collection.
+# That's why we need to fall back to offset pagination when previous page
+# is requested.
+class Projects::BranchesByModeService
+  include Gitlab::Routing
+
+  attr_reader :project, :params
+
+  def initialize(project, params = {})
+    @project = project
+    @params = params
+  end
+
+  def execute
+    return fetch_branches_via_gitaly_pagination if use_gitaly_pagination?
+
+    fetch_branches_via_offset_pagination
+  end
+
+  private
+
+  def mode
+    params[:mode]
+  end
+
+  def by_mode(branches)
+    return branches unless %w[active stale].include?(mode)
+
+    branches.select { |b| b.state.to_s == mode }
+  end
+
+  def use_gitaly_pagination?
+    return false if params[:page].present? || params[:search].present?
+
+    Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: true)
+  end
+
+  def fetch_branches_via_offset_pagination
+    branches = BranchesFinder.new(project.repository, params).execute
+    branches = Kaminari.paginate_array(by_mode(branches)).page(params[:page])
+
+    branches_with_links(branches, last_page: branches.last_page?)
+  end
+
+  def fetch_branches_via_gitaly_pagination
+    per_page = Kaminari.config.default_per_page
+    options = params.merge(per_page: per_page + 1, page_token: params[:page_token])
+
+    branches = BranchesFinder.new(project.repository, options).execute(gitaly_pagination: true)
+
+    # Branch is stale if it hasn't been updated for 3 months
+    # This logic is specified in Gitlab Rails and isn't specified in Gitaly
+    # To display stale branches we fetch branches sorted as most-stale-at-the-top
+    # If the result contains active branches we filter them out and define that no more stale branches left
+    # Same logic applies to fetching active branches
+    branches = by_mode(branches)
+    last_page = branches.size <= per_page
+
+    branches = branches.take(per_page) # rubocop:disable CodeReuse/ActiveRecord
+
+    branches_with_links(branches, last_page: last_page)
+  end
+
+  def branches_with_links(branches, last_page:)
+    # To fall back to offset pagination we need to track current page via offset param
+    # And increase it whenever we go to the next page
+    previous_offset = params[:offset].to_i
+
+    previous_path, next_path = nil, nil
+
+    return [branches, previous_path, next_path] if branches.blank?
+
+    unless last_page
+      next_path = project_branches_filtered_path(project, state: mode, page_token: branches.last.name, sort: params[:sort], offset: previous_offset + 1)
+    end
+
+    if previous_offset > 0
+      previous_path = project_branches_filtered_path(project, state: mode, sort: params[:sort], page: previous_offset, offset: previous_offset - 1)
+    end
+
+    [branches, previous_path, next_path]
+  end
+end