Fallback to backend reviewer/mtainer for test/eng-prod changes

Signed-off-by: Rémy Coutable <remy@rymai.me>

danger/roulette/Dangerfile

@@ -38,24 +38,32 @@ MARKDOWN
 NO_REVIEWER = 'No reviewer available'.freeze
 NO_MAINTAINER = 'No maintainer available'.freeze
 
-def spin_for_category(team, project, category, branch_name)
-  random = roulette.new_random(branch_name)
-  labels = gitlab.mr_labels
+Spin = Struct.new(:reviewer, :maintainer)
+
+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 |kind|
-      team.select do |member|
-        member.public_send(kind, project, category, labels) # rubocop:disable GitlabSecurity/PublicSend
-      end
+    %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 = roulette.new_random(branch_name)
   reviewer = roulette.spin_for_person(reviewers + traintainers + traintainers, random: random)
   maintainer = roulette.spin_for_person(maintainers, random: random)
 
+  Spin.new(reviewer, maintainer)
+end
+
+def markdown_row_for_category(category, reviewer, maintainer)
   "| #{helper.label_for_category(category)} | #{reviewer&.markdown_name || NO_REVIEWER} | #{maintainer&.markdown_name || NO_MAINTAINER} |"
 end
 
@@ -85,8 +93,29 @@ if changes.any? && !gitlab.mr_labels.include?('CSS cleanup')
 
   project = helper.project_name
   unknown = changes.fetch(:unknown, [])
+  spin_per_category = categories.each_with_object({}) do |category, memo|
+    memo[category] = spin_for_category(team, project, category, canonical_branch_name)
+  end
+
+  rows = spin_per_category.map do |category, spin|
+    reviewer = spin.reviewer
+    maintainer = spin.maintainer
+
+    case category
+    when :test
+      if reviewer.nil?
+        # Fetch an already picked backend reviewer, or pick one otherwise
+        reviewer = spin_per_category[:backend]&.reviewer || spin_for_category(team, project, :backend, canonical_branch_name).reviewer
+      end
+    when :engineering_productivity
+      if maintainer.nil?
+        # Fetch an already picked backend maintainer, or pick one otherwise
+        maintainer = spin_per_category[:backend]&.maintainer || spin_for_category(team, project, :backend, canonical_branch_name).maintainer
+      end
+    end
 
-  rows = categories.map { |category| spin_for_category(team, project, category, canonical_branch_name) }
+    markdown_row_for_category(category, reviewer, maintainer)
+  end
 
   markdown(MESSAGE)
   markdown(CATEGORY_TABLE_HEADER + rows.join("\n")) unless rows.empty?

lib/gitlab/danger/helper.rb

@@ -100,6 +100,7 @@ module Gitlab
         test: "~test ~Quality for `spec/features/*`",
         engineering_productivity: '~"Engineering Productivity" for CI, Danger'
       }.freeze
+      # First-match win, so be sure to put more specific regex at the top...
       CATEGORIES = {
         %r{\Adoc/} => :none, # To reinstate roulette for documentation, set to `:docs`.
         %r{\A(CONTRIBUTING|LICENSE|MAINTENANCE|PHILOSOPHY|PROCESS|README)(\.md)?\z} => :none, # To reinstate roulette for documentation, set to `:docs`.
@@ -145,9 +146,8 @@ module Gitlab
         %r{\A(ee/)?app/(?!assets|views)[^/]+} => :backend,
         %r{\A(ee/)?(bin|config|generator_templates|lib|rubocop)/} => :backend,
         %r{\A(ee/)?spec/features/} => :test,
-        %r{\A(ee/)?spec/(?!javascripts|frontend)[^/]+} => :backend,
-        %r{\A(ee/)?vendor/(?!assets)[^/]+} => :backend,
-        %r{\A(ee/)?vendor/(languages\.yml|licenses\.csv)\z} => :backend,
+        %r{\A(ee/)?spec/} => :backend,
+        %r{\A(ee/)?vendor/} => :backend,
         %r{\A(Gemfile|Gemfile.lock|Rakefile)\z} => :backend,
         %r{\A[A-Z_]+_VERSION\z} => :backend,
         %r{\A\.rubocop(_todo)?\.yml\z} => :backend,