1 files changed, 67 insertions, 0 deletions
diff --git a/lib/gitlab/danger/roulette.rb b/lib/gitlab/danger/roulette.rb
index dbf42912882..9f7980dc20a 100644
--- a/lib/gitlab/danger/roulette.rb
+++ b/lib/gitlab/danger/roulette.rb
@@ -6,6 +6,46 @@ module Gitlab
   module Danger
     module Roulette
       ROULETTE_DATA_URL = 'https://about.gitlab.com/roulette.json'
+      OPTIONAL_CATEGORIES = [:qa, :test].freeze
+
+      Spin = Struct.new(:category, :reviewer, :maintainer, :optional_role)
+
+      # Assigns GitLab team members to be reviewer and maintainer
+      # for each change category that a Merge Request contains.
+      #
+      # @return [Array<Spin>]
+      def spin(project, categories, branch_name)
+        team =
+          begin
+            project_team(project)
+          rescue => err
+            warn("Reviewer roulette failed to load team data: #{err.message}")
+            []
+          end
+
+        canonical_branch_name = canonical_branch_name(branch_name)
+
+        spin_per_category = categories.each_with_object({}) do |category, memo|
+          memo[category] = spin_for_category(team, project, category, canonical_branch_name)
+        end
+
+        spin_per_category.map do |category, spin|
+          case category
+          when :test
+            if spin.reviewer.nil?
+              # Fetch an already picked backend reviewer, or pick one otherwise
+              spin.reviewer = spin_per_category[:backend]&.reviewer || spin_for_category(team, project, :backend, canonical_branch_name).reviewer
+            end
+          when :engineering_productivity
+            if spin.maintainer.nil?
+              # Fetch an already picked backend maintainer, or pick one otherwise
+              spin.maintainer = spin_per_category[:backend]&.maintainer || spin_for_category(team, project, :backend, canonical_branch_name).maintainer
+            end
+          end
+
+          spin
+        end
+      end
 
       # Looks up the current list of GitLab team members and parses it into a
       # useful form
@@ -58,6 +98,33 @@ module Gitlab
       def mr_author?(person)
         person.username == gitlab.mr_author
       end
+
+      def spin_role_for_category(team, role, project, category)
+        team.select do |member|
+          member.public_send("#{role}?", project, category, gitlab.mr_labels) # rubocop:disable GitlabSecurity/PublicSend
+        end
+      end
+
+      def spin_for_category(team, project, category, branch_name)
+        reviewers, traintainers, maintainers =
+          %i[reviewer traintainer maintainer].map do |role|
+            spin_role_for_category(team, role, project, category)
+          end
+
+        # TODO: take CODEOWNERS into account?
+        # https://gitlab.com/gitlab-org/gitlab/issues/26723
+
+        # Make traintainers have triple the chance to be picked as a reviewer
+        random = new_random(branch_name)
+        reviewer = spin_for_person(reviewers + traintainers + traintainers, random: random)
+        maintainer = spin_for_person(maintainers, random: random)
+
+        Spin.new(category, reviewer, maintainer).tap do |spin|
+          if OPTIONAL_CATEGORIES.include?(category)
+            spin.optional_role = :maintainer
+          end
+        end
+      end
     end
   end
 end