fix(gitlab-ci-config): allow strings in the 'include' keyword

This fix is a followup to !24098 which introduced a validation of the
`include:` keyword of a gitlab-ci configuration file when triggered
from /ci/lint API calls.

However, there was a test case missing: the case of a single string as
value. I have added a test case for that which shows that the code was
not validating it correctly.

This commit fixes that to allow all `include:` valid inputs.

changelogs/unreleased/fix-include-ci-yaml.yml

+---
+title: Fix single string values for the 'include' keyword validation of gitlab-ci.yml.
+merge_request: 26998
+author: Paul Bonaud (@paulrbr)
+type: fixed

lib/gitlab/ci/config/entry/includes.rb

@@ -11,7 +11,7 @@ module Gitlab
           include ::Gitlab::Config::Entry::Validatable
 
           validations do
-            validates :config, type: Array
+            validates :config, array_or_string: true
           end
 
           def self.aspects

lib/gitlab/config/entry/validators.rb

@@ -54,6 +54,14 @@ module Gitlab
           end
         end
 
+        class ArrayOrStringValidator < ActiveModel::EachValidator
+          def validate_each(record, attribute, value)
+            unless value.is_a?(Array) || value.is_a?(String)
+              record.errors.add(attribute, 'should be an array or a string')
+            end
+          end
+        end
+
         class BooleanValidator < ActiveModel::EachValidator
           include LegacyValidationHelpers
 

spec/lib/gitlab/ci/yaml_processor_spec.rb

@@ -615,6 +615,14 @@ module Gitlab
         subject { Gitlab::Ci::YamlProcessor.new(YAML.dump(config), opts) }
 
         context "when validating a ci config file with no project context" do
+          context "when a single string is provided" do
+            let(:include_content) { "/local.gitlab-ci.yml" }
+
+            it "does not return any error" do
+              expect { subject }.not_to raise_error
+            end
+          end
+
           context "when an array is provided" do
             let(:include_content) { ["/local.gitlab-ci.yml"] }