Make it possible to access static builds variables

This makes it possible to access static build variables even when an
object is not persisted yet. This will allow us to evaluate build
variables using only/except policies before deciding whether we actually
want to persist the build in the database, or not.

app/models/ci/build.rb

@@ -547,16 +547,16 @@ module Ci
         variables.append(key: 'CI_SERVER_NAME', value: 'GitLab')
         variables.append(key: 'CI_SERVER_VERSION', value: Gitlab::VERSION)
         variables.append(key: 'CI_SERVER_REVISION', value: Gitlab::REVISION)
-        variables.append(key: 'CI_JOB_ID', value: id.to_s)
+        variables.append(key: 'CI_JOB_ID', value: id.to_s) if persisted?
         variables.append(key: 'CI_JOB_NAME', value: name)
         variables.append(key: 'CI_JOB_STAGE', value: stage)
-        variables.append(key: 'CI_JOB_TOKEN', value: token, public: false)
+        variables.append(key: 'CI_JOB_TOKEN', value: token, public: false) if persisted?
         variables.append(key: 'CI_COMMIT_SHA', value: sha)
         variables.append(key: 'CI_COMMIT_REF_NAME', value: ref)
         variables.append(key: 'CI_COMMIT_REF_SLUG', value: ref_slug)
-        variables.append(key: 'CI_REGISTRY_USER', value: CI_REGISTRY_USER)
-        variables.append(key: 'CI_REGISTRY_PASSWORD', value: token, public: false)
-        variables.append(key: 'CI_REPOSITORY_URL', value: repo_url, public: false)
+        variables.append(key: 'CI_REGISTRY_USER', value: CI_REGISTRY_USER) if persisted?
+        variables.append(key: 'CI_REGISTRY_PASSWORD', value: token, public: false) if persisted?
+        variables.append(key: 'CI_REPOSITORY_URL', value: repo_url, public: false) if persisted?
         variables.append(key: "CI_COMMIT_TAG", value: ref) if tag?
         variables.append(key: "CI_PIPELINE_TRIGGERED", value: 'true') if trigger_request
         variables.append(key: "CI_JOB_MANUAL", value: 'true') if action?
@@ -579,8 +579,11 @@ module Ci
 
     def legacy_variables
       Gitlab::Ci::Variables::Collection.new.tap do |variables|
+        if persisted?
           variables.append(key: 'CI_BUILD_ID', value: id.to_s)
           variables.append(key: 'CI_BUILD_TOKEN', value: token, public: false)
+        end
+
         variables.append(key: 'CI_BUILD_REF', value: sha)
         variables.append(key: 'CI_BUILD_BEFORE_SHA', value: before_sha)
         variables.append(key: 'CI_BUILD_REF_NAME', value: ref)

spec/models/ci/build_spec.rb

@@ -1977,6 +1977,59 @@ describe Ci::Build do
         end
       end
     end
+
+    context 'when build has not been persisted yet' do
+      let(:build) do
+        described_class.new(
+          name: 'rspec',
+          stage: 'test',
+          ref: 'feature',
+          project: project,
+          pipeline: pipeline
+        )
+      end
+
+      it 'does not persist the build' do
+        expect(build).to be_valid
+        expect(build).not_to be_persisted
+
+        variables = build.variables
+
+        expect(variables.size).to be >= 28
+        expect(build).not_to be_persisted
+      end
+
+      it 'returns static predefined variables' do
+        keys = %w[CI_JOB_NAME
+                  CI_COMMIT_SHA
+                  CI_COMMIT_REF_NAME
+                  CI_COMMIT_REF_SLUG
+                  CI_JOB_STAGE]
+
+        build.variables.map { |var| var.fetch(:key) }.tap do |names|
+          expect(names).to include(*keys)
+        end
+
+        expect(build.variables).to include(key: 'CI_COMMIT_REF_NAME',
+                                           value: 'feature',
+                                           public: true)
+      end
+
+      it 'does not return prohibited variables' do
+        keys = %w[CI_JOB_ID
+                  CI_JOB_TOKEN
+                  CI_BUILD_ID
+                  CI_BUILD_TOKEN
+                  CI_REGISTRY_USER
+                  CI_REGISTRY_PASSWORD
+                  CI_REPOSITORY_URL
+                  CI_ENVIRONMENT_URL]
+
+        build.variables.map { |var| var.fetch(:key) }.tap do |names|
+          expect(names).not_to include(*keys)
+        end
+      end
+    end
   end
 
   describe 'state transition: any => [:pending]' do