Fix boot failure without mini_magick for apps on the Vips transformer

CHANGELOG.md

@@ -50,6 +50,12 @@ Some actions that application developers should consider taking when upgrading f
 
 * New gem `yabeda-hotcell`. It publishes Yabeda metrics for the application side. Call `Yabeda::HotCell.install!` one time at boot. The gem counts each call by cell, operation, code and cause, and measures the time the cell spent. On each scrape, it reads the counters of each registered cell. See the README's "Metrics collection".
 
+### ActiveStorage::HotCell::Client
+
+#### Fixed
+
+* An application that uses only the Vips transformer no longer needs the `mini_magick` gem. Previously, `require "active_storage/hot_cell/client"` raised `LoadError` when `mini_magick` was not installed. The gem now loads `Transformers::Image::Magick` when the application first names it.
+
 ### Tooling
 
 #### Changed

activestorage-hotcell-client/lib/active_storage/hot_cell/client.rb

@@ -6,7 +6,10 @@
 require "active_storage/hot_cell/client/version"
 require "active_storage/hot_cell/client/operations"
 require "active_storage/hot_cell/client/transformers/image/vips"
-require "active_storage/hot_cell/client/transformers/image/magick"
+# Autoloaded because it subclasses Rails' ImageMagick transformer, which requires mini_magick, and an application
+# on vips need not bundle that gem.
+ActiveStorage::HotCell::Client::Transformers::Image.autoload :Magick,
+                                                             "active_storage/hot_cell/client/transformers/image/magick"
 require "active_storage/hot_cell/client/analyzers/image/vips"
 require "active_storage/hot_cell/client/analyzers/image/magick"
 require "active_storage/hot_cell/client/analyzers/video/ffprobe"

activestorage-hotcell-client/test/transformers/image_magick_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "test_helper"
+require "open3"
 
 class TransformersImageMagickTest < ActiveStorageHotCellClientTest
   def test_it_yields_an_open_rewound_tempfile_of_the_converted_image
@@ -52,7 +53,40 @@ def test_a_cell_that_is_not_there_raises_the_transient_class
     end
   end
 
+  # image_processing 2.x no longer depends on mini_magick, so an application on vips may not have it. Only a
+  # fresh process can show what requiring the gem loads, and a stand-in first on the load path makes
+  # `require "mini_magick"` fail the way it does when the gem is not installed. Rails is loaded first, as
+  # Bundler.require does in an application, so the railtie is loaded too.
+  def test_the_gem_loads_without_mini_magick
+    output, status = ruby_without_mini_magick <<~RUBY
+      require "rails"
+      require "activestorage-hotcell-client"
+    RUBY
+
+    assert_predicate status, :success?, output
+  end
+
+  def test_naming_the_transformer_without_mini_magick_raises_load_error
+    output, status = ruby_without_mini_magick <<~RUBY
+      require "active_storage/hot_cell/client"
+      puts "loaded"
+      ActiveStorage::HotCell::Client::Transformers::Image::Magick
+    RUBY
+
+    refute_predicate status, :success?
+    assert_match(/^loaded$/, output)
+    assert_match "requires the mini_magick gem", output
+  end
+
   private
+    def ruby_without_mini_magick(script)
+      Dir.mktmpdir "hotcell-no-mini-magick" do |dir|
+        File.write File.join(dir, "mini_magick.rb"), 'raise LoadError, "cannot load such file -- mini_magick"'
+
+        Open3.capture2e RbConfig.ruby, "-I", dir, "-I", File.expand_path("../../lib", __dir__), "-e", script
+      end
+    end
+
     def transformer(transformations)
       ActiveStorage::HotCell::Client::Transformers::Image::Magick.new transformations
     end