1 files changed, 127 insertions, 0 deletions
diff --git a/lib/gitlab/ci/config/yaml/interpolator.rb b/lib/gitlab/ci/config/yaml/interpolator.rb
new file mode 100644
index 00000000000..4ae191dfedf
--- /dev/null
+++ b/lib/gitlab/ci/config/yaml/interpolator.rb
@@ -0,0 +1,127 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Ci
+    class Config
+      module Yaml
+        ##
+        # Config::Yaml::Interpolation performs includable file interpolation, and surfaces all possible interpolation
+        # errors. It is designed to provide an external file's validation context too.
+        #
+        class Interpolator
+          include ::Gitlab::Utils::StrongMemoize
+
+          attr_reader :config, :args, :ctx, :errors
+
+          def initialize(config, args, ctx = nil)
+            @config = config
+            @args = args.to_h
+            @ctx = ctx
+            @errors = []
+
+            validate!
+          end
+
+          def valid?
+            @errors.none?
+          end
+
+          def ready?
+            ##
+            # Interpolation is ready when it has been either interrupted by an error or finished with a result.
+            #
+            @result || @errors.any?
+          end
+
+          def interpolate?
+            enabled? && has_header? && valid?
+          end
+
+          def has_header?
+            config.has_header? && config.header.present?
+          end
+
+          def to_hash
+            @result.to_h
+          end
+
+          def error_message
+            # Interpolator can have multiple error messages, like: ["interpolation interrupted by errors", "unknown
+            # interpolation key: `abc`"] ?
+            #
+            # We are joining them together into a single one, because only one error can be surfaced when an external
+            # file gets included and is invalid. The limit to three error messages combined is more than required.
+            #
+            @errors.first(3).join(', ')
+          end
+
+          ##
+          # TODO Add `instrument.logger` instrumentation blocks:
+          #   https://gitlab.com/gitlab-org/gitlab/-/issues/396722
+          #
+          def interpolate!
+            return {} unless valid?
+            return @result ||= content.to_h unless interpolate?
+
+            return @errors.concat(header.errors) unless header.valid?
+            return @errors.concat(inputs.errors) unless inputs.valid?
+            return @errors.concat(context.errors) unless context.valid?
+            return @errors.concat(template.errors) unless template.valid?
+
+            if ctx&.user
+              ::Gitlab::UsageDataCounters::HLLRedisCounter.track_event('ci_interpolation_users', values: ctx.user.id)
+            end
+
+            @result ||= template.interpolated.to_h.deep_symbolize_keys
+          end
+          strong_memoize_attr :interpolate!
+
+          private
+
+          def validate!
+            return errors.push('content does not have a valid YAML syntax') unless config.valid?
+
+            return unless has_header? && !enabled?
+
+            errors.push('can not evaluate included file because interpolation is disabled')
+          end
+
+          def enabled?
+            return false if ctx.nil?
+
+            ::Feature.enabled?(:ci_includable_files_interpolation, ctx.project)
+          end
+
+          def header
+            @entry ||= Ci::Config::Header::Root.new(config.header).tap do |header|
+              header.key = 'header'
+
+              header.compose!
+            end
+          end
+
+          def content
+            @content ||= config.content
+          end
+
+          def spec
+            @spec ||= header.inputs_value
+          end
+
+          def inputs
+            @inputs ||= Ci::Input::Inputs.new(spec, args)
+          end
+
+          def context
+            @context ||= Ci::Interpolation::Context.new({ inputs: inputs.to_hash })
+          end
+
+          def template
+            @template ||= ::Gitlab::Ci::Interpolation::Template
+              .new(content, context)
+          end
+        end
+      end
+    end
+  end
+end