Fix #3198.

actionpack/lib/sprockets/assets.rake

+require "fileutils"
+
 namespace :assets do
+  def invoke_precompile
+    args = [$0, "assets:internal_precompile"]
+    args << "--trace" if Rake.application.options.trace
+    ruby *args
+  end
+
   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?
+  task :precompile => "assets:clean" do
     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
+    invoke_precompile
+  end
 
+  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
@@ -23,13 +25,9 @@ namespace :assets do
     config = Rails.application.config
     config.assets.compile = true
     config.assets.digest  = false if ENV["RAILS_ASSETS_NONDIGEST"]
-
-      env    = Rails.application.assets
-
-      # Always compile files and avoid use of existing precompiled assets
-      config.assets.compile = true
     config.assets.digests = {}
 
+    env    = Rails.application.assets
     target = File.join(Rails.public_path, config.assets.prefix)
     static_compiler = Sprockets::StaticCompiler.new(env, target, :digest => config.assets.digest)
 
@@ -42,14 +40,12 @@ namespace :assets do
         YAML.dump(manifest, f)
       end
       ENV["RAILS_ASSETS_NONDIGEST"] = "true"
-        ruby $0, *ARGV
-        exit
-      end
+      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