Add build policy for pipeline expressions

lib/gitlab/ci/build/policy/variables.rb

+module Gitlab
+  module Ci
+    module Build
+      module Policy
+        class Variables < Policy::Specification
+          def initialize(expressions)
+            @expressions = Array(expressions)
+          end
+
+          def satisfied_by?(pipeline)
+            statements = @expressions.map do |statement|
+              ::Gitlab::Ci::Pipeline::Expression::Statement
+                .new(statement, pipeline)
+            end
+
+            statements.any? { |statement| statement.truthful? }
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/build/policy/variables_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Build::Policy::Variables do
+  let(:pipeline) { build(:ci_pipeline, ref: 'master') }
+
+  before do
+    pipeline.variables.build(key: 'CI_PROJECT_NAME', value: '')
+  end
+
+  describe '#satisfied_by?' do
+    it 'is satisfied by a defined and existing variable' do
+      policy = described_class.new(['$CI_PROJECT_ID', '$UNDEFINED'])
+
+      expect(policy).to be_satisfied_by(pipeline)
+    end
+
+    it 'is not satisfied by an overriden empty variable' do
+      policy = described_class.new(['$CI_PROJECT_NAME'])
+
+      expect(policy).not_to be_satisfied_by(pipeline)
+    end
+
+    it 'is satisfied by a truthy pipeline expression' do
+      policy = described_class.new([%($CI_PIPELINE_SOURCE == "#{pipeline.source}")])
+
+      expect(policy).to be_satisfied_by(pipeline)
+    end
+
+    it 'is not satisfied by a falsy pipeline expression' do
+      policy = described_class.new([%($CI_PIPELINE_SOURCE == "invalid source")])
+
+      expect(policy).not_to be_satisfied_by(pipeline)
+    end
+
+    it 'is satisfied by a truthy expression using undefined variable' do
+      policy = described_class.new(['$UNDEFINED', '$UNDEFINED == null'])
+
+      expect(policy).to be_satisfied_by(pipeline)
+    end
+  end
+end