Merge branch 'refactor/ci-config-add-facade' into 'master'

Add new GitLab CI configuration facade class

## What does this MR do?

This MR is a first iteration of major CI configuration refactoring. See #17139 and #15060 for more details.

## What are the relevant issue numbers?

Closes #17139

See merge request !4462

3 files changed, 44 insertions, 7 deletions

diff --git a/lib/ci/gitlab_ci_yaml_processor.rb b/lib/ci/gitlab_ci_yaml_processor.rb
index 026a5ac97ca..130f5b0892e 100644
--- a/lib/ci/gitlab_ci_yaml_processor.rb
+++ b/lib/ci/gitlab_ci_yaml_processor.rb
@@ -12,18 +12,14 @@ module Ci
     attr_reader :before_script, :after_script, :image, :services, :path, :cache
 
     def initialize(config, path = nil)
-      @config = YAML.safe_load(config, [Symbol], [], true)
+      @config = Gitlab::Ci::Config.new(config).to_hash
       @path = path
 
-      unless @config.is_a? Hash
-        raise ValidationError, "YAML should be a hash"
-      end
-
-      @config = @config.deep_symbolize_keys
-
       initial_parsing
 
       validate!
+    rescue Gitlab::Ci::Config::Loader::FormatError => e
+      raise ValidationError, e.message
     end
 
     def builds_for_stage_and_ref(stage, ref, tag = false, trigger_request = nil)
diff --git a/lib/gitlab/ci/config.rb b/lib/gitlab/ci/config.rb
new file mode 100644
index 00000000000..ffe633d4b63
--- /dev/null
+++ b/lib/gitlab/ci/config.rb
@@ -0,0 +1,16 @@
+module Gitlab
+  module Ci
+    class Config
+      class LoaderError < StandardError; end
+
+      def initialize(config)
+        loader = Loader.new(config)
+        @config = loader.load!
+      end
+
+      def to_hash
+        @config
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/ci/config/loader.rb b/lib/gitlab/ci/config/loader.rb
new file mode 100644
index 00000000000..dbf6eb0edbe
--- /dev/null
+++ b/lib/gitlab/ci/config/loader.rb
@@ -0,0 +1,25 @@
+module Gitlab
+  module Ci
+    class Config
+      class Loader
+        class FormatError < StandardError; end
+
+        def initialize(config)
+          @config = YAML.safe_load(config, [Symbol], [], true)
+        end
+
+        def valid?
+          @config.is_a?(Hash)
+        end
+
+        def load!
+          unless valid?
+            raise FormatError, 'Invalid configuration format'
+          end
+
+          @config.deep_symbolize_keys
+        end
+      end
+    end
+  end
+end