Weeknotes 79

With no more “Worship the Sun” to play, I bought “Ori and the Blind Forest” for the Nintendo Switch to meet my need for platforming games that border on the cruel.

I implemented a feature for a Ruby on Rails web application to watermark PDFs as they are downloaded. I used Prawn to generate a single A4 PDF with the user’s name and email address printed diagonally across the page, e.g.

Prawn::Document.new(page_size: 'A4', margin: 0) do
  font_size 36
  fill_color '000000'

  rotate 35.26, origin: [297.64, 420.945] do
    transparent 0.1 do
      text 'Generated for Alice Bloggs <alice@example.com>', align: :center, valign: :top
      text 'Generated for Alice Bloggs <alice@example.com>', align: :center, valign: :center
      text 'Generated for Alice Bloggs <alice@example.com>', align: :center, valign: :bottom
    end
  end
end

I then overlay this onto whatever PDF document they are downloading from Active Storage using QPDF.

As QPDF is a command-line tool, I needed a way to shell out to it in a safe way that could be used on-demand. Thankfully, MiniMagick does something very similar shelling out to ImageMagick’s various commands, e.g. convert, mogrify, etc.

Having seen ImageMagick commands cause an application to hang forever (bonus trivia: that’s my first ever pull request on GitHub) and being subject to Heroku’s 30 second timeout, I wanted to ensure the watermarking process had a strict time limit. Despite it being Ruby’s most dangerous API, I took a leaf from MiniMagick’s book and used Timeout.timeout like so:

def generate_pdf(infile, overlayfile, outfile)
  Open3.popen3(
    'qpdf', infile.path, '--overlay', overlayfile.path, '--from=', '--repeat=1', '--', outfile.path
  ) do |_stdin, _stdout, stderr, wait_thr|
    break if Timeout.timeout(timeout) { wait_thr.value }.success?

    raise WatermarkingError, stderr.read
  rescue Timeout::Error
    Process.kill('KILL', wait_thr.pid)
    raise WatermarkingError, 'qpdf timed out'
  end
end

At the end of this process, I have a Tempfile with the watermarked PDF which I need to send to the user. Rails’ send_file seems a good choice as it’ll handle setting the appropriate headers for the file type and avoid issues with cache headers in old versions of Internet Explorer. However, if you are trying to clean up your temporary files on every request rather than letting the Ruby Garbage Collector handle it, you can’t unlink the file in your controller. If you do, by the time your file is actually read and served, it will have been deleted and your users will end up with an empty response.

Thankfully, Rack comes with a Tempfile Reaper middleware which allows you to clean Tempfiles created during a request by adding them to request.env[Rack::RACK_TEMPFILES]. This is part of Rails’ default middleware stack so we can use it to remove Tempfiles only when they have been read.

As people aged 40 and over can now get the COVID-19 vaccine, I attempted to book my sister’s first dose. She hadn’t yet been contacted by her GP or the NHS so I looked for the next available appointment at the nearest vaccination centre and saw that there were slots within the next few days. However, demand is obviously high and the appointments disappeared from the site before I could book one.

On subsequent visits to the same site, I was then presented with the following message:

You were unable to go to/missed your 1st appointment to get the coronavirus vaccination.

This means you need to book both of your appointments again.

As we had never made an appointment, I assume this is due to some overly broad condition in the website, e.g.

if !users_first_time_using_site && user_has_not_had_first_dose
  display_scary_message
end

Thankfully, she is now booked in for her first dose next weekend.

I attended last night’s Boston Computation Club to watch Chris Patuzzo’s talk “Assembly Programming for the M1”. It was great to see Chris present his own work and to see him tease “Worship the Sun” at the end.

I learned that the club was inspired by London Computation Club and they found us due to our write-ups over the years particularly those regarding Benjamin C. Pierce’s “Types and Programming Languages”.