Add support for PlantUML diagrams in Asciidoc.

This MR enables rendering of PlantUML diagrams in Asciidoc documents. To add a
PlantUML diagram all we need is to include a plantuml block like:

```
[plantuml, id="myDiagram", width="100px", height="100px"]
--
bob -> alice : ping
alice -> bob : pong
--
```

The plantuml block is substituted by an HTML img element with *src* pointing to
an external PlantUML server.

This MR also add a PlantUML integration section to the Administrator -> Settings
page to configure the PlantUML rendering service and to enable/disable it.

Closes: #17603

4 files changed, 20 insertions, 1 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index d2fadf6a3d0..885ce7d44bc 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -565,6 +565,8 @@ module API
       expose :repository_storages
       expose :koding_enabled
       expose :koding_url
+      expose :plantuml_enabled
+      expose :plantuml_url
     end
 
     class Release < Grape::Entity
diff --git a/lib/api/settings.rb b/lib/api/settings.rb
index 9eb9a105bde..c5eff16a5de 100644
--- a/lib/api/settings.rb
+++ b/lib/api/settings.rb
@@ -93,6 +93,10 @@ module API
       given koding_enabled: ->(val) { val } do
         requires :koding_url, type: String, desc: 'The Koding team URL'
       end
+      optional :plantuml_enabled, type: Boolean, desc: 'Enable PlantUML'
+      given plantuml_enabled: ->(val) { val } do
+        requires :plantuml_url, type: String, desc: 'The PlantUML server URL'
+      end
       optional :version_check_enabled, type: Boolean, desc: 'Let GitLab inform you when an update is available.'
       optional :email_author_in_body, type: Boolean, desc: 'Some email servers do not support overriding the email sender name. Enable this option to include the name of the author of the issue, merge request or comment in the email body instead.'
       optional :html_emails_enabled, type: Boolean, desc: 'By default GitLab sends emails in HTML and plain text formats so mail clients can choose what format to use. Disable this option if you only want to send emails in plain text format.'
@@ -114,7 +118,7 @@ module API
                       :shared_runners_enabled, :max_artifacts_size, :container_registry_token_expire_delay,
                       :metrics_enabled, :sidekiq_throttling_enabled, :recaptcha_enabled,
                       :akismet_enabled, :admin_notification_email, :sentry_enabled,
-                      :repository_storage, :repository_checks_enabled, :koding_enabled,
+                      :repository_storage, :repository_checks_enabled, :koding_enabled, :plantuml_enabled,
                       :version_check_enabled, :email_author_in_body, :html_emails_enabled,
                       :housekeeping_enabled
     end
diff --git a/lib/gitlab/asciidoc.rb b/lib/gitlab/asciidoc.rb
index fa234284361..0618107e2c3 100644
--- a/lib/gitlab/asciidoc.rb
+++ b/lib/gitlab/asciidoc.rb
@@ -1,5 +1,6 @@
 require 'asciidoctor'
 require 'asciidoctor/converter/html5'
+require "asciidoctor-plantuml"
 
 module Gitlab
   # Parser/renderer for the AsciiDoc format that uses Asciidoctor and filters
@@ -29,6 +30,8 @@ module Gitlab
       )
       asciidoc_opts[:attributes].unshift(*DEFAULT_ADOC_ATTRS)
 
+      plantuml_setup
+
       html = ::Asciidoctor.convert(input, asciidoc_opts)
 
       html = Banzai.post_process(html, context)
@@ -36,6 +39,15 @@ module Gitlab
       html.html_safe
     end
 
+    def self.plantuml_setup
+      Asciidoctor::PlantUml.configure do |conf|
+        conf.url = ApplicationSetting.current.plantuml_url
+        conf.svg_enable = ApplicationSetting.current.plantuml_enabled
+        conf.png_enable = ApplicationSetting.current.plantuml_enabled
+        conf.txt_enable = false
+      end
+    end
+
     class Html5Converter < Asciidoctor::Converter::Html5Converter
       extend Asciidoctor::Converter::Config
 
diff --git a/lib/gitlab/current_settings.rb b/lib/gitlab/current_settings.rb
index 9d142f1b82e..2ff27e46d64 100644
--- a/lib/gitlab/current_settings.rb
+++ b/lib/gitlab/current_settings.rb
@@ -35,6 +35,7 @@ module Gitlab
         signin_enabled: Settings.gitlab['signin_enabled'],
         gravatar_enabled: Settings.gravatar['enabled'],
         koding_enabled: false,
+        plantuml_enabled: false,
         sign_in_text: nil,
         after_sign_up_text: nil,
         help_page_text: nil,