Assign reviews to hungry reviewers first

This causes Reviewer Roulette to suggest reviewers with a "hungry"
status instead of other reviewers.

lib/gitlab/danger/roulette.rb

@@ -146,13 +146,18 @@ module Gitlab
           %i[reviewer traintainer maintainer].map do |role|
             spin_role_for_category(team, role, project, category)
           end
+        hungry_reviewers = reviewers.select { |member| member.hungry }
 
         # 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(mr_source_branch)
-        reviewer = spin_for_person(reviewers + traintainers + traintainers, random: random, timezone_experiment: timezone_experiment)
+
+        # Make hungry traintainers have 4x the chance to be picked as a reviewer
+        # Make traintainers have 3x the chance to be picked as a reviewer
+        # Make hungry reviewers have 2x the chance to be picked as a reviewer
+        weighted_reviewers = reviewers + hungry_reviewers + traintainers + traintainers
+        reviewer = spin_for_person(weighted_reviewers, random: random, timezone_experiment: timezone_experiment)
         maintainer = spin_for_person(maintainers, random: random, timezone_experiment: timezone_experiment)
 
         Spin.new(category, reviewer, maintainer, false, timezone_experiment)

lib/gitlab/danger/teammate.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module Danger
     class Teammate
-      attr_reader :options, :username, :name, :role, :projects, :available, :tz_offset_hours
+      attr_reader :options, :username, :name, :role, :projects, :available, :hungry, :tz_offset_hours
 
       # The options data are produced by https://gitlab.com/gitlab-org/gitlab-roulette/-/blob/master/lib/team_member.rb
       def initialize(options = {})
@@ -14,6 +14,7 @@ module Gitlab
         @role = options['role']
         @projects = options['projects']
         @available = options['available']
+        @hungry = options['hungry']
         @tz_offset_hours = options['tz_offset_hours']
       end
 

spec/lib/gitlab/danger/roulette_spec.rb

@@ -67,14 +67,18 @@ RSpec.describe Gitlab::Danger::Roulette do
     )
   end
 
-  let(:teammate_json) do
+  let(:teammates) do
     [
       backend_maintainer.to_h,
       frontend_maintainer.to_h,
       frontend_reviewer.to_h,
       software_engineer_in_test.to_h,
       engineering_productivity_reviewer.to_h
-    ].to_json
+    ]
+  end
+
+  let(:teammate_json) do
+    teammates.to_json
   end
 
   subject(:roulette) { Object.new.extend(described_class) }
@@ -210,6 +214,69 @@ RSpec.describe Gitlab::Danger::Roulette do
         end
       end
     end
+
+    describe 'reviewer suggestion probability' do
+      let(:reviewer) { teammate_with_capability('reviewer', 'reviewer backend') }
+      let(:hungry_reviewer) { teammate_with_capability('hungry_reviewer', 'reviewer backend', hungry: true) }
+      let(:traintainer) { teammate_with_capability('traintainer', 'trainee_maintainer backend') }
+      let(:hungry_traintainer) { teammate_with_capability('hungry_traintainer', 'trainee_maintainer backend', hungry: true) }
+      let(:teammates) do
+        [
+          reviewer.to_h,
+          hungry_reviewer.to_h,
+          traintainer.to_h,
+          hungry_traintainer.to_h
+        ]
+      end
+
+      let(:categories) { [:backend] }
+
+      # This test is testing probability with inherent randomness.
+      # The variance is inversely related to sample size
+      # Given large enough sample size, the variance would be smaller,
+      # but the test would take longer.
+      # Given smaller sample size, the variance would be larger,
+      # but the test would take less time.
+      let!(:sample_size) { 500 }
+      let!(:variance) { 0.1 }
+
+      before do
+        # This test needs actual randomness to simulate probabilities
+        allow(subject).to receive(:new_random).and_return(Random.new)
+        WebMock
+          .stub_request(:get, described_class::ROULETTE_DATA_URL)
+          .to_return(body: teammate_json)
+      end
+
+      it 'has 1:2:3:4 probability of picking reviewer, hungry_reviewer, traintainer, hungry_traintainer' do
+        picks = Array.new(sample_size).map do
+          spins = subject.spin(project, categories, timezone_experiment: timezone_experiment)
+          spins.first.reviewer.name
+        end
+
+        expect(probability(picks, 'reviewer')).to be_within(variance).of(0.1)
+        expect(probability(picks, 'hungry_reviewer')).to be_within(variance).of(0.2)
+        expect(probability(picks, 'traintainer')).to be_within(variance).of(0.3)
+        expect(probability(picks, 'hungry_traintainer')).to be_within(variance).of(0.4)
+      end
+
+      def probability(picks, role)
+        picks.count(role).to_f / picks.length
+      end
+
+      def teammate_with_capability(name, capability, hungry: false)
+        Gitlab::Danger::Teammate.new(
+          {
+            'name' => name,
+            'projects' => {
+              'gitlab' => capability
+            },
+            'available' => true,
+            'hungry' => hungry
+          }
+        )
+      end
+    end
   end
 
   RSpec::Matchers.define :match_teammates do |expected|