Add pngquant rake tasks

This commit adds two rake tasks: pngquant:compress and pngquant:lint.

These tasks automate the compression of our documentation images using
pngquant.

Gemfile

@@ -386,6 +386,10 @@ group :development, :test do
   gem 'simple_po_parser', '~> 1.1.2', require: false
 
   gem 'timecop', '~> 0.8.0'
+
+  gem 'png_quantizator', '~> 0.2.1', require: false
+
+  gem 'parallel', '~> 1.17.0', require: false
 end
 
 # Gems required in omnibus-gitlab pipeline

Gemfile.lock

@@ -734,6 +734,7 @@ GEM
     peek (1.1.0)
       railties (>= 4.0.0)
     pg (1.1.4)
+    png_quantizator (0.2.1)
     po_to_json (1.0.1)
       json (>= 1.6.0)
     premailer (1.11.1)
@@ -1286,8 +1287,10 @@ DEPENDENCIES
   omniauth_crowd (~> 2.2.0)
   omniauth_openid_connect (~> 0.3.3)
   org-ruby (~> 0.9.12)
+  parallel (~> 1.17.0)
   peek (~> 1.1)
   pg (~> 1.1)
+  png_quantizator (~> 0.2.1)
   premailer-rails (~> 1.10.3)
   prometheus-client-mmap (~> 0.9.10)
   pry-byebug (~> 3.5.1)

lib/tasks/pngquant.rake

+return if Rails.env.production?
+
+require 'png_quantizator'
+require 'parallel'
+
+# The amount of variance (in bytes) allowed in
+# file size when testing for compression size
+TOLERANCE = 10
+
+namespace :pngquant do
+  # Returns an array of all images eligible for compression
+  def doc_images
+    Dir.glob('doc/**/*.png', File::FNM_CASEFOLD)
+  end
+
+  # Runs pngquant on an image and optionally
+  # writes the result to disk
+  def compress_image(file, overwrite_original)
+    compressed_file = "#{file}.compressed"
+    FileUtils.copy(file, compressed_file)
+
+    pngquant_file = PngQuantizator::Image.new(compressed_file)
+
+    # Run the image repeatedly through pngquant until
+    # the change in file size is within TOLERANCE
+    loop do
+      before = File.size(compressed_file)
+      pngquant_file.quantize!
+      after = File.size(compressed_file)
+      break if before - after <= TOLERANCE
+    end
+
+    savings = File.size(file) - File.size(compressed_file)
+    is_uncompressed = savings > TOLERANCE
+
+    if is_uncompressed && overwrite_original
+      FileUtils.copy(compressed_file, file)
+    end
+
+    FileUtils.remove(compressed_file)
+
+    [is_uncompressed, savings]
+  end
+
+  # Ensures pngquant is available and prints an error if not
+  def check_executable
+    unless system('pngquant --version', out: File::NULL)
+      warn(
+        'Error: pngquant executable was not detected in the system.'.color(:red),
+        'Download pngquant at https://pngquant.org/ and place the executable in /usr/local/bin'.color(:green)
+      )
+      abort
+    end
+  end
+
+  desc 'GitLab | pngquant | Compress all documentation PNG images using pngquant'
+  task :compress do
+    check_executable
+
+    files = doc_images
+    puts "Compressing #{files.size} PNG files in doc/**"
+
+    Parallel.each(files) do |file|
+      was_uncompressed, savings = compress_image(file, true)
+
+      if was_uncompressed
+        puts "#{file} was reduced by #{savings} bytes"
+      end
+    end
+  end
+
+  desc 'GitLab | pngquant | Checks that all documentation PNG images have been compressed with pngquant'
+  task :lint do
+    check_executable
+
+    files = doc_images
+    puts "Checking #{files.size} PNG files in doc/**"
+
+    uncompressed_files = Parallel.map(files) do |file|
+      is_uncompressed, _ = compress_image(file, false)
+      if is_uncompressed
+        puts "Uncompressed file detected: ".color(:red) + file
+        file
+      end
+    end.compact
+
+    if uncompressed_files.empty?
+      puts "All documentation images are optimally compressed!".color(:green)
+    else
+      warn(
+        "The #{uncompressed_files.size} image(s) above have not been optimally compressed using pngquant.".color(:red),
+        'Please run "bin/rake pngquant:compress" and commit the result.'
+      )
+      abort
+    end
+  end
+end