Add support for rendering diagrams with mermaid

[mermaid][Mermaid] generates diagrams and flowcharts from text in a
similar manner as Markdown.

This adds support for rendering diagrams from any markdown code blocks
with the language tag `mermaid`.

For example the code block:

```mermaid
sequenceDiagram
	Alice->>+John: Hello John, how are you?
	John-->>-Alice: Great!
```

Will now render an inline SVG diagram of the sequence instead of the raw
`<code>` block.

Keeping diagrams as code in this way makes it significantly easier to
keep diagrams up to date with the documentation, and can make reviewing
changes to them easier.

The implementation takes advantage of the existing dependecy on node.js to
install and execute the mermaid cli tool that translates the various
diagram code into SVG. A timeout is added to execution to workaround an
issue where the cli tool [fails to terminate on error][fail-exit].

The deafult mermaid themes for diagrams don't quite fit the GOV.UK
"brand" colors, and creating the CSS to get the colors pretty is out of
scope, so this just sets it to use the "neutral" (grayscale) theme,
which at least doesn't clash with anything.

[mermaid]: https://mermaid-js.github.io/mermaid/#/
[fail-exit]: mermaidjs/mermaid.cli#77

example/source/code.html.md

@@ -39,3 +39,14 @@ An example of a code block with a short length
 RSpec.describe ContentItem do
 end
 ```
+
+An example of a [mermaid](https://mermaid-js.github.io/mermaid) diagram
+
+```mermaid
+graph TD;
+	A-->B;
+	A-->C;
+	B-->D;
+	C-->D;
+```
+

lib/govuk_tech_docs/tech_docs_html_renderer.rb

@@ -1,4 +1,6 @@
 require "middleman-core/renderers/redcarpet"
+require "tmpdir"
+require "timeout"
 
 module GovukTechDocs
   class TechDocsHTMLRenderer < Middleman::Renderers::MiddlemanRedcarpetHTML
@@ -80,7 +82,9 @@ def table_row(body)
     end
 
     def block_code(text, lang)
-      if defined?(super)
+      if lang == "mermaid"
+        block_diagram(text)
+      elsif defined?(super)
         # Post-processing the block_code HTML to implement tabbable code blocks.
         #
         # Middleman monkey patches the Middleman::Renderers::MiddlemanRedcarpetHTML
@@ -108,5 +112,33 @@ def block_code(text, lang)
         pre.to_html
       end
     end
+
+    def block_diagram(code)
+      mmdc = "#{__dir__}/../../node_modules/.bin/mmdc"
+      Dir.mktmpdir do |tmp|
+        input_path = "#{tmp}/input"
+        output_path = "#{tmp}/output.svg"
+        File.open(input_path, "w") { |f| f.write(code) }
+        ok = exec_with_timeout("#{mmdc} -i #{input_path} -o #{output_path} --theme neutral", 10)
+        if ok && File.exist?(output_path)
+          File.read(output_path)
+        else
+          "<pre>#{code}</pre>"
+        end
+      end
+    end
+
+    def exec_with_timeout(cmd, timeout)
+      pid = Process.spawn(cmd, { %i[err out] => :close, :pgroup => true })
+      begin
+        Timeout.timeout(timeout) do
+          Process.waitpid(pid, 0)
+          $?.exitstatus.zero?
+        end
+      rescue Timeout::Error
+        Process.kill(15, -Process.getpgid(pid))
+        false
+      end
+    end
   end
 end

package.json

@@ -6,7 +6,8 @@
     "lint": "standard"
   },
   "dependencies": {
-    "govuk-frontend": "~5.7.1"
+    "govuk-frontend": "~5.7.1",
+    "@mermaid-js/mermaid-cli": "^8.4.8"
   },
   "devDependencies": {
     "standard": "^14.3.4"

spec/features/diagrams_spec.rb

@@ -0,0 +1,26 @@
+require "rack/file"
+require "capybara/rspec"
+
+Capybara.app = Rack::File.new("example/build")
+
+RSpec.describe "Diagrams in code blocks" do
+  include Capybara::DSL
+
+  it "generates static SVG from mermaid code blocks" do
+    when_the_site_is_created
+    and_i_visit_the_code_page
+    then_there_is_an_svg_diagram
+  end
+
+  def when_the_site_is_created
+    rebuild_site!
+  end
+
+  def and_i_visit_the_code_page
+    visit "/code.html"
+  end
+
+  def then_there_is_an_svg_diagram
+    expect(page.body).to match '<svg id="mermaid-'
+  end
+end