Use overlayfs (when possible) to speed up specs

.rubocop.yml

@@ -10,6 +10,9 @@ Style/BlockComments:
 Style/Documentation:
   Enabled: false
 
+Style/AndOr:
+  Enabled: false
+
 Style/RescueModifier:
   Exclude:
     - spec/spec_helper.rb
@@ -18,6 +21,13 @@ Style/NestedParenthesizedCalls:
   Exclude:
     - spec/**/*
 
+Style/MultilineIfModifier:
+  Exclude:
+    - spec/support/dummy_app_helpers.rb
+Style/WhileUntilModifier:
+  Exclude:
+    - spec/support/dummy_app_helpers.rb
+
 Metrics/BlockLength:
   Exclude:
     - spec/**/*

README.md

@@ -133,5 +133,11 @@ sync('drivers/*').forEach((driverPath) => {
 module.exports = environment
 ```
 
+## Developing / Testing
+
+This gem doesn't actually load the dummy app environment like most Rails plugins. For each spec, it copies the entire dummy app into a new folder and runs rake tasks, generators, and `rails run` inside that new folder. This makes it easier for us to test file-structure related behavior. We can freely create and move files without worrying about cleaning up.
+
+Copying the entire dummy app on every spec is super slow. If you have [fuse-overlayfs](https://github.com/containers/fuse-overlayfs) installed, the test cases will use it to avoid the full copy on every test. Set `NO_OVERLAYFS=true` if you have it installed but don't want it to be used. If used, it speeds up tests a little bit, but running `rails run` is still pretty slow.
+
 ## License
 The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).

spec/support/dummy_app_helpers.rb

@@ -65,9 +65,7 @@ def find_js_pack(html, pack_name)
   def create_file(file_name, contents)
     full_path = File.expand_path(File.join(dummy_app, file_name))
 
-    dir = full_path.split('/')
-    dir.pop
-    FileUtils.mkdir_p(dir.join('/'))
+    FileUtils.mkdir_p(File.dirname(full_path))
 
     File.open(full_path, 'w') { |f| f.write contents }
   end
@@ -84,22 +82,46 @@ def dummy_app
     @dummy_app
   end
 
+  def dummy_app_workdir
+    "#{@dummy_app}.workdir"
+  end
+
+  def dummy_app_upper
+    "#{@dummy_app}.upper"
+  end
+
+  def all_dummy_app_dirs
+    [dummy_app, dummy_app_workdir, dummy_app_upper]
+  end
+
   def dummy_app_template
     File.expand_path File.join(__dir__, '../dummy')
   end
 
   def setup_dummy_app
     random_string = SecureRandom.hex.chars.first(4).join
     @dummy_app = File.expand_path File.join(__dir__, "../dummy-#{random_string}")
-    FileUtils.rm_r @dummy_app if File.exist?(@dummy_app)
-    FileUtils.cp_r dummy_app_template, @dummy_app
+
+    all_dummy_app_dirs.each do |dir|
+      FileUtils.rm_rf dir
+      FileUtils.mkdir dir
+    end
+
+    try_overlayfs or copy_dummy_app
   end
 
   def teardown_dummy_app
     return if @dummy_app.nil?
 
-    FileUtils.rm_r @dummy_app
+    until system "fusermount -u #{@dummy_app}"
+      sleep 1
+    end if @using_fuse
+    all_dummy_app_dirs.each do |dir|
+      FileUtils.rm_r dir
+    end
+
     @dummy_app = nil
+    @using_fuse = nil
   end
 
   #
@@ -119,4 +141,19 @@ def truncate_lines(stream, limit: 200)
 
     lines
   end
+
+  def try_overlayfs
+    return false if ENV['NO_OVERLAYFS']
+    low  = dummy_app_template
+    up   = dummy_app_upper
+    work = dummy_app_workdir
+
+    cmd = "fuse-overlayfs -o 'lowerdir=#{low},upperdir=#{up},workdir=#{work}' #{dummy_app}"
+    @using_fuse = system cmd
+  end
+
+  def copy_dummy_app
+    FileUtils.rmdir dummy_app
+    FileUtils.cp_r dummy_app_template, dummy_app
+  end
 end