Create a mutex instance variable in URI and synchronize when freezing.

Fixes #388.

Author: gkellogg

lib/rdf/model/uri.rb

@@ -223,6 +223,7 @@ def self.normalize_path(path)
     #   @param [Boolean] canonicalize (false)
     def initialize(*args, validate: false, canonicalize: false, **options)
       @value = @object = @hash = nil
+      @mutex = Mutex.new
       uri = args.first
       if uri
         @value = uri.to_s
@@ -665,12 +666,14 @@ def dup
     # @private
     def freeze
       unless frozen?
-        # Create derived components
-        authority; userinfo; user; password; host; port
-        @value  = value.freeze
-        @object = object.freeze
-        @hash = hash.freeze
-        super
+        @mutex.synchronize do
+          # Create derived components
+          authority; userinfo; user; password; host; port
+          @value  = value.freeze
+          @object = object.freeze
+          @hash = hash.freeze
+          super
+        end
       end
       self
     end