Don't redirect $stdout in :ruby filter.

Redirecting $stdout isn't thread safe.

Provide `haml_io`, an IO object that writes to the buffer.

CHANGELOG.md

@@ -5,8 +5,9 @@
 * The Haml exectutable now accepts an `--autoclose` option. You can now
   specify a list of tags that should be autoclosed
 
-* The `:ruby` filter now runs the generated code with a exclusive lock, to
-  prevent issues with sharing `$stdout` across threads.
+* The `:ruby` filter no longer redirects $stdout to the Haml document, as this
+  is not thread safe. Instead it provides a `haml_io` local variable, which is
+  an IO object that writes to the document.
 
 * HTML5 is now the default output format rather than XHTML. This was already
   the default on Rails 3+, so many users will notice no difference.

lib/haml/filters.rb

@@ -258,9 +258,13 @@ RUBY
       end
     end
 
-    # Parses the filtered text with the normal Ruby interpreter. All output sent
-    # to `$stdout`, such as with `puts`, is output into the Haml document. Not
-    # available if the {file:REFERENCE.md#suppress_eval-option `:suppress_eval`}
+    # Parses the filtered text with the normal Ruby interpreter. Creates an IO
+    # object named `haml_io`, anything written to it is output into the Haml
+    # document. In previous version this filter redirected any output to `$stdout`
+    # to the Haml document, this was not threadsafe and has been removed, you
+    # should use `haml_io` instead.
+    #
+    # Not available if the {file:REFERENCE.md#suppress_eval-option `:suppress_eval`}
     # option is set to true. The Ruby code is evaluated in the same context as
     # the Haml template.
     module Ruby
@@ -272,11 +276,13 @@ RUBY
         return if compiler.options[:suppress_eval]
         compiler.instance_eval do
           push_silent <<-FIRST.gsub("\n", ';') + text + <<-LAST.gsub("\n", ';')
-            _haml_old_stdout = $stdout
-            $stdout = StringIO.new(_hamlout.buffer, 'a')
+            begin
+              haml_io = StringIO.new(_hamlout.buffer, 'a')
           FIRST
-            _haml_old_stdout, $stdout = $stdout, _haml_old_stdout
-            _haml_old_stdout.close
+            ensure
+              haml_io.close
+              haml_io = nil
+            end
           LAST
         end
       end

test/filters_test.rb

@@ -235,4 +235,24 @@ class EscapedFilterTest < MiniTest::Unit::TestCase
     haml = ":escaped\n  &"
     assert_equal(html, render(haml))
   end
+end
+
+class RubyFilterTest < MiniTest::Unit::TestCase
+  test "can write to haml_io" do
+    haml = ":ruby\n  haml_io.puts 'hello'\n"
+    html = "hello\n"
+    assert_equal(html, render(haml))
+  end
+
+  test "haml_io appends to output" do
+    haml = "hello\n:ruby\n  haml_io.puts 'hello'\n"
+    html = "hello\nhello\n"
+    assert_equal(html, render(haml))
+  end
+
+  test "can create local variables" do
+    haml = ":ruby\n  a = 7\n=a"
+    html = "7\n"
+    assert_equal(html, render(haml))
+  end
 end