Refactor parallelization implementation

* Move the variables to ::Ci::Build#predefined_variables
* Tweak pipeline build seed implementation

app/models/ci/build.rb

@@ -801,10 +801,16 @@ module Ci
         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?
+        variables.append(key: "CI_NODE_INDEX", value: node_index.to_s) if self.options[:parallel]
+        variables.append(key: "CI_NODE_TOTAL", value: self.options.fetch(:parallel, 1).to_s)
         variables.concat(legacy_variables)
       end
     end
 
+    def node_index
+      name.match(%r{(\d+)/\d+$}).captures[0]
+    end
+
     def gitlab_version_info
       @gitlab_version_info ||= Gitlab::VersionInfo.parse(Gitlab::VERSION)
     end

lib/gitlab/ci/pipeline/seed/build.rb

@@ -24,22 +24,14 @@ module Gitlab
             end
           end
 
-          def parallelized?
-            @attributes[:options].include?(:parallel)
+          def parallel?
+            !!@attributes.dig(:options, :parallel)
           end
 
           def parallelize_build
-            builds = []
-
             total = @attributes[:options][:parallel]
-
-            total.times do |i|
-              build = ::Ci::Build.new(attributes.merge(options: { variables: { CI_NODE_INDEX: i + 1, CI_NODE_TOTAL: total } }))
-              build.name = build.name + "_#{i + 1}/#{total}"
-              builds << build
-            end
-
-            builds
+            Array.new(total) { ::Ci::Build.new(attributes) }
+              .each_with_index { |build, idx| build.name = "#{build.name} #{idx + 1}/#{total}" }
           end
 
           def attributes
@@ -56,11 +48,7 @@ module Gitlab
 
           def to_resource
             strong_memoize(:resource) do
-              if parallelized?
-                parallelize_build
-              else
-                ::Ci::Build.new(attributes)
-              end
+              parallel? ? parallelize_build : ::Ci::Build.new(attributes)
             end
           end
         end