Include a Mutex mashal hack for Ruby 1.9.

lib/pdf/core_ext/mutex.rb

@@ -0,0 +1,11 @@
+
+unless RUBY_VERSION < '1.9'
+  class ::Mutex
+    def marshal_dump
+    end unless method_defined?(:marshal_dump)
+    def marshal_load(data)
+    end unless method_defined?(:marshal_load)
+  end
+end
+
+

lib/pdf/writer.rb

@@ -12,6 +12,7 @@
 require 'open-uri'
 
 require 'transaction/simple'
+require 'pdf/core_ext/mutex'
 require 'color'
 
   # A class to provide the core functionality to create a PDF document

pdf-writer.gemspec

@@ -23,6 +23,8 @@ Gem::Specification.new do |s|
     "lib/pdf/charts",
     "lib/pdf/charts/stddev.rb",
     "lib/pdf/charts.rb",
+    "lib/pdf/core_ext",
+    "lib/pdf/core_ext/mutex.rb",
     "lib/pdf/math.rb",
     "lib/pdf/quickref.rb",
     "lib/pdf/simpletable.rb",