Add only/except pipeline build policy for `changes`

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

+module Gitlab
+  module Ci
+    module Build
+      module Policy
+        class Changes < Policy::Specification
+          def initialize(globs)
+            @globs = Array(globs)
+          end
+
+          def satisfied_by?(pipeline, seed)
+            return true unless pipeline.branch_updated?
+
+            pipeline.modified_paths.any? do |path|
+              @globs.any? { |glob| File.fnmatch?(glob, path, File::FNM_PATHNAME) }
+            end
+          end
+        end
+      end
+    end
+  end
+end

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

+require 'spec_helper'
+
+describe Gitlab::Ci::Build::Policy::Changes do
+  set(:project) { create(:project) }
+
+  let(:pipeline) do
+    build(:ci_empty_pipeline, project: project,
+                              ref: 'master',
+                              source: :push,
+                              sha: '1234abcd',
+                              before_sha: '0123aabb')
+  end
+
+  let(:ci_build) do
+    build(:ci_build, pipeline: pipeline, project: project, ref: 'master')
+  end
+
+  let(:seed) { double('build seed', to_resource: ci_build) }
+
+  before do
+    allow(pipeline).to receive(:modified_paths) do
+      %w[some/modified/ruby/file.rb some/other_file.txt]
+    end
+  end
+
+  describe '#satisfied_by?' do
+    it 'is satisfied by matching literal path' do
+      policy = described_class.new(%w[some/other_file.txt])
+
+      expect(policy).to be_satisfied_by(pipeline, seed)
+    end
+
+    it 'is satisfied by matching simple pattern' do
+      policy = described_class.new(%w[some/*.txt])
+
+      expect(policy).to be_satisfied_by(pipeline, seed)
+    end
+
+    it 'is satisfied by matching recusive pattern' do
+      policy = described_class.new(%w[some/**/*.rb])
+
+      expect(policy).to be_satisfied_by(pipeline, seed)
+    end
+
+    it 'is not satisfied when pattern does not match path' do
+      policy = described_class.new(%w[some/*.rb])
+
+      expect(policy).not_to be_satisfied_by(pipeline, seed)
+    end
+
+    it 'is not satisfied when pattern does not match' do
+      policy = described_class.new(%w[invalid/*.md])
+
+      expect(policy).not_to be_satisfied_by(pipeline, seed)
+    end
+
+    context 'when pipelines does not run for a branch update' do
+      before do
+        pipeline.before_sha = Gitlab::Git::BLANK_SHA
+      end
+
+      it 'is always satisfied' do
+        policy = described_class.new(%w[invalid/*])
+
+        expect(policy).to be_satisfied_by(pipeline, seed)
+      end
+    end
+  end
+end