Implement a global threadpool for faster builds.

Fixes rbuchberger#225

This implements a global Concurrent::ThreadPoolExecutor from
concurrent-ruby to avoid memory blowup, and de-duplicates files before
generation so we no longer need to rely on filesystem consistency to
avoid double-generating images.

This is an alternate to rbuchberger#282 with a litle bit more complexity, but with
the added benefits that all image generation can happen in a single
ThreadPool.

jekyll_picture_tag.gemspec

@@ -29,6 +29,8 @@ Gem::Specification.new do |spec|
 
   # addressable is used to url-encode image filenames.
   spec.add_runtime_dependency 'addressable', '~> 2.6'
+  # Needed for parallel excution of builds.
+  spec.add_runtime_dependency 'concurrent-ruby', '~> 1.1'
   # Jekyll versions older than 4.0 are not supported.
   spec.add_runtime_dependency 'jekyll', '~> 4.0'
   # MIME types are needed for <source> tags' type= attributes.

lib/jekyll_picture_tag.rb

@@ -6,6 +6,7 @@
 require_relative 'jekyll_picture_tag/instructions'
 require_relative 'jekyll_picture_tag/output_formats'
 require_relative 'jekyll_picture_tag/parsers'
+require_relative 'jekyll_picture_tag/pool'
 require_relative 'jekyll_picture_tag/router'
 require_relative 'jekyll_picture_tag/srcsets'
 require_relative 'jekyll_picture_tag/utils'
@@ -68,7 +69,10 @@ def render(context)
 
         ''
       else
-        PictureTag.output_class.new.to_s
+        PictureTag::Pool.start_pool
+        result = PictureTag.output_class.new.to_s
+        PictureTag::Pool.stop_pool
+        result
       end
     end
 

lib/jekyll_picture_tag/output_formats/basic.rb

@@ -79,7 +79,7 @@ def build_fallback_image
           width: checked_fallback_width
         )
 
-        image.generate
+        PictureTag::Pool.generate(image)
 
         image
       end

lib/jekyll_picture_tag/pool.rb

@@ -0,0 +1,49 @@
+require 'concurrent-ruby'
+require 'set'
+
+module PictureTag
+  # This is a global concurrent-ruby pool for executing tasks in parallel.
+  # Pool itself should only be used in a single threaded context, and its
+  # start method must be called prior to submitting to Pool.pool, and stop
+  # must be called after.
+  class Pool
+    # We use a class variable to store pool to emulate a global singleton.
+    # rubocop:disable Style/ClassVars
+    @@_pool = nil
+    # Because we're processing in parallel, we can't use existince in the
+    # filestystem to prevent duplicate handling of files without a race
+    # condition, so we manually track generated files here so we don't attempt
+    # to generate the same image twice.
+    @@_seen_files = Set[]
+
+    def self.start_pool
+      @@_pool = Concurrent::ThreadPoolExecutor.new(
+        min_threads: 1, max_threads: Concurrent.processor_count,
+        max_queue: 2 * Concurrent.processor_count
+      )
+    end
+
+    def self.start_test_pool
+      # Stub in an pool which executes immediately to avoid race conditions
+      # during unit tests.
+      @@_pool = Concurrent::ImmediateExecutor.new
+      @@_seen_files = Set[]
+    end
+
+    def self.generate(generated_image)
+      return if @@_seen_files.include?(generated_image.name)
+
+      @@_seen_files.add(generated_image.name)
+      @@_pool.post do
+        generated_image.generate
+      end
+    end
+
+    def self.stop_pool
+      @@_pool.shutdown
+      @@_pool.wait_for_termination
+      @@_seen_files = Set[]
+    end
+  end
+  # rubocop:enable Style/ClassVars
+end

lib/jekyll_picture_tag/srcsets/basic.rb

@@ -1,4 +1,5 @@
 require 'mime-types'
+
 module PictureTag
   # Handles srcset generation, which also handles file generation.
   module Srcsets
@@ -68,8 +69,7 @@ def build_files
 
         # This triggers GeneratedImage to actually build an image file.
         files = checked_targets
-        files.each(&:generate)
-
+        files.each { |file| PictureTag::Pool.generate(file) }
         files
       end
 

test/unit/images/test_generated_image_missing.rb

@@ -11,6 +11,7 @@ class GeneratedImageMissingTest < Minitest::Test
   # Lifecycle
   def setup
     PictureTag.stubs(config)
+    PictureTag::Pool.start_test_pool
     File.stubs(:exist?).with(destfile).returns(false)
   end
 

test/unit/output_formats/output_format_test_helper.rb

@@ -1,4 +1,5 @@
 require 'test_helper'
+require 'jekyll_picture_tag'
 
 # The fact that these stubs are so complicated is probably one of those code
 # smell things I keep hearing about.
@@ -7,6 +8,7 @@ module OutputFormatTestHelper
   include TestHelper
 
   def base_stubs
+    PictureTag::Pool.start_test_pool
     PictureTag.stubs(config)
     stub_srcsets
     stub_generated_image

test/unit/srcsets/srcsets_test_helper.rb

@@ -1,4 +1,5 @@
 require 'test_helper'
+require 'jekyll_picture_tag'
 
 module SrcsetTestHelper
   include TestHelper
@@ -7,6 +8,7 @@ module SrcsetTestHelper
   def setup
     [100, 150, 200, 300].each { |i| stub_generated(i, gstub(i)) }
 
+    PictureTag::Pool.start_test_pool
     PictureTag.stubs(config)
   end