Use sets to check if methods are invokable without symbolizing.

lib/liquid/context.rb

@@ -39,6 +39,7 @@ module Liquid
 
       filters.each do |f|
         raise ArgumentError, "Expected module but got: #{f.class}" unless f.is_a?(Module)
+        Strainer.add_known_filter(f)
         strainer.extend(f)
       end
     end

lib/liquid/drop.rb

@@ -1,3 +1,5 @@
+require 'set'
+
 module Liquid
 
   # A drop in liquid is a class which allows you to export DOM like things to liquid.
@@ -31,8 +33,8 @@ module Liquid
 
     # called by liquid to invoke a drop
     def invoke_drop(method_or_key)
-      if method_or_key && method_or_key != EMPTY_STRING && drop_method_defined?(method_or_key.to_s)
+      if method_or_key && method_or_key != EMPTY_STRING && self.class.invokable?(method_or_key)
-        send(method_or_key.to_s)
+        send(method_or_key)
       else
         before_method(method_or_key)
       end
@@ -51,8 +53,9 @@ module Liquid
     private
 
     # Check for method existence without invoking respond_to?, which creates symbols
-    def drop_method_defined?(method_name)
+    def self.invokable?(method_name)
-      self.class.public_instance_methods.any? {|method| method.to_s == method_name }
+      @invokable_methods ||= Set.new((public_instance_methods - Liquid::Drop.public_instance_methods).map(&:to_s))
+      @invokable_methods.include?(method_name.to_s)
     end
   end
 end

lib/liquid/strainer.rb

@@ -2,19 +2,15 @@ require 'set'
 
 module Liquid
 
-  parent_object = if defined? BlankObject
-    BlankObject
-  else
-    Object
-  end
-
   # Strainer is the parent class for the filters system.
   # New filters are mixed into the strainer class which is then instantiated for each liquid template render run.
   #
   # The Strainer only allows method calls defined in filters given to it via Strainer.global_filter,
   # Context#add_filters or Template.register_filter
-  class Strainer < parent_object #:nodoc:
+  class Strainer #:nodoc:
     @@filters = {}
+    @@known_filters = Set.new
+    @@known_methods = Set.new
 
     def initialize(context)
       @context = context
@@ -22,9 +18,20 @@ module Liquid
 
     def self.global_filter(filter)
       raise ArgumentError, "Passed filter is not a module" unless filter.is_a?(Module)
+      add_known_filter(filter)
       @@filters[filter.name] = filter
     end
 
+    def self.add_known_filter(filter)
+      unless @@known_filters.include?(filter)
+        @@method_blacklist ||= Set.new(Strainer.instance_methods.map(&:to_s))
+        new_methods = filter.instance_methods.map(&:to_s)
+        new_methods.reject!{ |m| @@method_blacklist.include?(m) }
+        @@known_methods.merge(new_methods)
+        @@known_filters.add(filter)
+      end
+    end
+
     def self.create(context)
       strainer = Strainer.new(context)
       @@filters.each { |k,m| strainer.extend(m) }
@@ -32,21 +39,15 @@ module Liquid
     end
 
     def invoke(method, *args)
-      if has_method?(method)
+      if invokable?(method)
         send(method, *args)
       else
         args.first
       end
     end
 
-    private
+    def invokable?(method)
-
+      @@known_methods.include?(method.to_s) && respond_to?(method)
-    def has_method?(method)
-      methods_to_check = self.methods - self.class.public_instance_methods
-      methods_to_check.any? do |instance_method|
-        instance_method.to_s == method.to_s
-      end
     end
-
   end
 end

test/liquid/drop_test.rb

@@ -115,6 +115,13 @@ class DropsTest < Test::Unit::TestCase
     assert_equal '  ', output
   end
 
+  def test_object_methods_not_allowed
+    [:dup, :clone, :singleton_class, :eval, :class_eval, :inspect].each do |method|
+      output = Liquid::Template.parse(" {{ product.#{method} }} ").render('product' => ProductDrop.new)
+      assert_equal '  ', output
+    end
+  end
+
   def test_scope
     assert_equal '1', Liquid::Template.parse( '{{ context.scopes }}'  ).render('context' => ContextDrop.new)
     assert_equal '2', Liquid::Template.parse( '{%for i in dummy%}{{ context.scopes }}{%endfor%}'  ).render('context' => ContextDrop.new, 'dummy' => [1])

test/liquid/strainer_test.rb

@@ -14,22 +14,21 @@ class StrainerTest < Test::Unit::TestCase
     private :private_filter
   end
 
-  module TestingFilter
-    def test1
-      "test1"
-    end
-
-    def test2
-      "test2"
-    end
-  end
-
   Strainer.global_filter(AccessScopeFilters)
-  Strainer.global_filter(TestingFilter)
+
+  def test_strainer
+    strainer = Strainer.create(nil)
+    assert_equal 5, strainer.invoke('size', 'input')
+    assert_equal "public", strainer.invoke("public_filter")
+  end
 
   def test_strainer_only_invokes_public_filter_methods
     strainer = Strainer.create(nil)
-    assert_equal "public", strainer.invoke("public_filter")
+    assert_equal false, strainer.invokable?('__test__')
+    assert_equal false, strainer.invokable?('test')
+    assert_equal false, strainer.invokable?('instance_eval')
+    assert_equal false, strainer.invokable?('__send__')
+    assert_equal true, strainer.invokable?('size') # from the standard lib
   end
 
   def test_strainer_returns_nil_if_no_filter_method_found
@@ -43,12 +42,6 @@ class StrainerTest < Test::Unit::TestCase
     assert_equal "password", strainer.invoke("undef_the_method", "password")
   end
 
-  def test_strainer_allows_multiple_filters
-    strainer = Strainer.create(nil)
-    assert_equal "test1", strainer.invoke("test1")
-    assert_equal "test2", strainer.invoke("test2")
-  end
-
   def test_strainer_only_allows_methods_defined_in_filters
     strainer = Strainer.create(nil)
     assert_equal "1 + 1", strainer.invoke("instance_eval", "1 + 1")