Fix #3198.

actionpack/lib/sprockets/assets.rake

-namespace :assets do
-  desc "Compile all the assets named in config.assets.precompile"
-  task :precompile do
-    # We need to do this dance because RAILS_GROUPS is used
-    # too early in the boot process and changing here is already too late.
-    if ENV["RAILS_GROUPS"].to_s.empty? || ENV["RAILS_ENV"].to_s.empty?
-      ENV["RAILS_GROUPS"] ||= "assets"
-      ENV["RAILS_ENV"]    ||= "production"
-      ruby $0, *ARGV
-      exit
-    else
-      require "fileutils"
-      Rake::Task["tmp:cache:clear"].invoke
-      Rake::Task["assets:environment"].invoke
+require "fileutils"
 
-      unless Rails.application.config.assets.enabled
-        raise "Cannot precompile assets if sprockets is disabled. Please set config.assets.enabled to true"
-      end
+namespace :assets do
+  def invoke_precompile
+    args = [$0, "assets:internal_precompile"]
+    args << "--trace" if Rake.application.options.trace
+    ruby *args
+  end
 
-      # Ensure that action view is loaded and the appropriate sprockets hooks get executed
-      _ = ActionView::Base
+  desc "Compile all the assets named in config.assets.precompile"
+  task :precompile => "assets:clean" do
+    ENV["RAILS_GROUPS"] ||= "assets"
+    ENV["RAILS_ENV"]    ||= "production"
+    invoke_precompile
+  end
 
-      config = Rails.application.config
-      config.assets.compile = true
-      config.assets.digest = false if ENV["RAILS_ASSETS_NONDIGEST"]
+  task :internal_precompile => "assets:environment" do
+    unless Rails.application.config.assets.enabled
+      raise "Cannot precompile assets if sprockets is disabled. Please set config.assets.enabled to true"
+    end
 
-      env    = Rails.application.assets
+    # Ensure that action view is loaded and the appropriate sprockets hooks get executed
+    _ = ActionView::Base
 
-      # Always compile files and avoid use of existing precompiled assets
-      config.assets.compile = true
-      config.assets.digests = {}
+    config = Rails.application.config
+    config.assets.compile = true
+    config.assets.digest  = false if ENV["RAILS_ASSETS_NONDIGEST"]
+    config.assets.digests = {}
 
-      target = File.join(Rails.public_path, config.assets.prefix)
-      static_compiler = Sprockets::StaticCompiler.new(env, target, :digest => config.assets.digest)
+    env    = Rails.application.assets
+    target = File.join(Rails.public_path, config.assets.prefix)
+    static_compiler = Sprockets::StaticCompiler.new(env, target, :digest => config.assets.digest)
 
-      manifest = static_compiler.precompile(config.assets.precompile)
-      manifest_path = config.assets.manifest || target
-      FileUtils.mkdir_p(manifest_path)
+    manifest = static_compiler.precompile(config.assets.precompile)
+    manifest_path = config.assets.manifest || target
+    FileUtils.mkdir_p(manifest_path)
 
-      unless ENV["RAILS_ASSETS_NONDIGEST"]
-        File.open("#{manifest_path}/manifest.yml", 'wb') do |f|
-          YAML.dump(manifest, f)
-        end
-        ENV["RAILS_ASSETS_NONDIGEST"] = "true"
-        ruby $0, *ARGV
-        exit
+    unless ENV["RAILS_ASSETS_NONDIGEST"]
+      File.open("#{manifest_path}/manifest.yml", 'wb') do |f|
+        YAML.dump(manifest, f)
       end
+      ENV["RAILS_ASSETS_NONDIGEST"] = "true"
+      invoke_precompile
     end
   end
 
   desc "Remove compiled assets"
-  task :clean => ['assets:environment', 'tmp:cache:clear'] do
+  task :clean => "tmp:cache:clear" do
     config = Rails.application.config
     public_asset_path = File.join(Rails.public_path, config.assets.prefix)
     rm_rf public_asset_path, :secure => true