Introduce DurationValidator, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9219#note_24032923

app/models/application_setting.rb

@@ -80,8 +80,7 @@ class ApplicationSetting < ActiveRecord::Base
             presence: true,
             numericality: { only_integer: true, greater_than: 0 }
 
-  validates :default_artifacts_expire_in, presence: true
-  validate  :check_default_artifacts_expire_in
+  validates :default_artifacts_expire_in, presence: true, duration: true
 
   validates :container_registry_token_expire_delay,
             presence: true,
@@ -305,10 +304,4 @@ class ApplicationSetting < ActiveRecord::Base
     errors.add(:repository_storages, "can't include: #{invalid.join(", ")}") unless
       invalid.empty?
   end
-
-  def check_default_artifacts_expire_in
-    ChronicDuration.parse(default_artifacts_expire_in)
-  rescue ChronicDuration::DurationParseError
-    errors.add(:default_artifacts_expire_in, "is not a correct duration")
-  end
 end

app/validators/duration_validator.rb

@@ -0,0 +1,17 @@
+# DurationValidator
+#
+# Validate the format conforms with ChronicDuration
+#
+# Example:
+#
+#   class ApplicationSetting < ActiveRecord::Base
+#     validates :default_artifacts_expire_in, presence: true, duration: true
+#   end
+#
+class DurationValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    ChronicDuration.parse(value)
+  rescue ChronicDuration::DurationParseError
+    record.errors.add(attribute, "is not a correct duration")
+  end
+end