Support CAS properly from Memcached::Rails.

lib/memcached/memcached.rb

@@ -264,7 +264,10 @@ Please note that when pipelining is enabled, setter and deleter methods do not r
   def cas(key, ttl=nil, marshal=true, flags=FLAGS)
     raise ClientError, "CAS not enabled for this Memcached instance" unless options[:support_cas]
 
-    value = get(key, marshal)
+    value, flags, ret = Lib.memcached_get_rvalue(@struct, key)
+    check_return_code(ret, key)
+    value = Marshal.load(value) if marshal
+
     value = yield value
     value = marshal ? Marshal.dump(value) : value.to_s
 
@@ -273,6 +276,8 @@ Please note that when pipelining is enabled, setter and deleter methods do not r
       key
     )
   end
+  
+  alias :compare_and_swap :cas
 
 ### Deleters
 

lib/memcached/rails.rb

@@ -25,6 +25,12 @@ class Memcached
       super(key, !raw)
     rescue NotFound
     end
+
+    # Wraps Memcached#cas so that it doesn't raise. Doesn't set anything if no value is present.
+    def cas(key, ttl=nil, raw=false, &block)
+      super(key, ttl, !raw, &block)
+    rescue NotFound    
+    end
     
     # Wraps Memcached#get with multiple arguments.
     def get_multi(*keys)

test/unit/rails_test.rb

@@ -47,6 +47,36 @@ class RailsTest < Test::Unit::TestCase
     assert_equal @value, result
   end
   
+  def test_cas
+    cache = Memcached::Rails.new(:servers => @servers, :namespace => @namespace, :support_cas => true)
+    value2 = OpenStruct.new(:d => 3, :e => 4, :f => GenericClass)
+
+    # Existing set
+    cache.set key, @value
+    cache.cas(key) do |current|
+      assert_equal @value, current
+      value2
+    end
+    assert_equal value2, cache.get(key)
+
+    # Missing set
+    cache.delete key
+    assert_nothing_raised do
+      cache.cas(key) { @called = true }
+    end
+    assert_nil cache.get(key)
+    assert_nil @called
+
+    # Conflicting set
+    cache.set key, @value
+    assert_raises(Memcached::ConnectionDataExists) do
+      cache.cas(key) do |current|
+        cache.set key, value2
+        current
+      end
+    end
+  end  
+  
   def test_get_missing
     @cache.delete key rescue nil
     result = @cache.get key