PlantUML support for Markdown

Allow rendering of PlantUML diagrams in Markdown documents using fenced blocks:

    ```plantuml
    Bob -> Sara : Hello
    Sara -> Bob : Go away
    ```

Closes: #4048

3 files changed, 61 insertions, 0 deletions

diff --git a/lib/banzai/filter/plantuml_filter.rb b/lib/banzai/filter/plantuml_filter.rb
new file mode 100644
index 00000000000..e194cf59275
--- /dev/null
+++ b/lib/banzai/filter/plantuml_filter.rb
@@ -0,0 +1,39 @@
+require "nokogiri"
+require "asciidoctor-plantuml/plantuml"
+
+module Banzai
+  module Filter
+    # HTML that replaces all `code plantuml` tags with PlantUML img tags.
+    #
+    class PlantumlFilter < HTML::Pipeline::Filter
+      def call
+        return doc unless doc.at('pre.plantuml') and settings.plantuml_enabled
+
+        plantuml_setup
+
+        doc.css('pre.plantuml').each do |el|
+          img_tag = Nokogiri::HTML::DocumentFragment.parse(
+            Asciidoctor::PlantUml::Processor.plantuml_content(el.content, {}))
+          el.replace img_tag
+        end
+
+        doc
+      end
+
+      private
+
+      def settings
+        ApplicationSetting.current || ApplicationSetting.create_from_defaults
+      end
+
+      def plantuml_setup
+        Asciidoctor::PlantUml.configure do |conf|
+          conf.url = settings.plantuml_url
+          conf.png_enable = settings.plantuml_enabled
+          conf.svg_enable = false
+          conf.txt_enable = false
+        end
+      end
+    end
+  end
+end
diff --git a/lib/banzai/pipeline/gfm_pipeline.rb b/lib/banzai/pipeline/gfm_pipeline.rb
index ac95a79009b..b25d6f18d59 100644
--- a/lib/banzai/pipeline/gfm_pipeline.rb
+++ b/lib/banzai/pipeline/gfm_pipeline.rb
@@ -10,6 +10,7 @@ module Banzai
       def self.filters
         @filters ||= FilterArray[
           Filter::SyntaxHighlightFilter,
+          Filter::PlantumlFilter,
           Filter::SanitizationFilter,
 
           Filter::MathFilter,
diff --git a/lib/rouge/lexers/plantuml.rb b/lib/rouge/lexers/plantuml.rb
new file mode 100644
index 00000000000..7d5700b7f6d
--- /dev/null
+++ b/lib/rouge/lexers/plantuml.rb
@@ -0,0 +1,21 @@
+module Rouge
+  module Lexers
+    class Plantuml < Lexer
+      title "A passthrough lexer used for PlantUML input"
+      desc "A boring lexer that doesn't highlight anything"
+
+      tag 'plantuml'
+      mimetypes 'text/plain'
+
+      default_options token: 'Text'
+
+      def token
+        @token ||= Token[option :token]
+      end
+
+      def stream_tokens(string, &b)
+        yield self.token, string
+      end
+    end
+  end
+end