fix for a very subtle race condition

hard to test

Author: Nick Kallen

lib/cash/accessor.rb

@@ -38,23 +38,25 @@ def get(keys, options = {}, &block)
       end
 
       def add(key, value, options = {})
-        repository.add(cache_key(key), value, options[:ttl] || 0, options[:raw])
+        if repository.add(cache_key(key), value, options[:ttl] || 0, options[:raw]) == "NOT_STORED\r\n"
+          yield
+        end
       end
 
       def set(key, value, options = {})
         repository.set(cache_key(key), value, options[:ttl] || 0, options[:raw])
       end
 
       def incr(key, delta = 1, ttl = 0)
-        repository.incr(cache_key(key), delta) || begin
-          repository.add(cache_key(key), (result = yield).to_s, ttl, true)
+        repository.incr(cache_key = cache_key(key), delta) || begin
+          repository.add(cache_key, (result = yield).to_s, ttl, true) { repository.incr(cache_key) }
           result
         end
       end
 
       def decr(key, delta = 1, ttl = 0)
-        repository.decr(cache_key(key), delta) || begin
-          repository.add(cache_key(key), (result = yield).to_s, ttl, true)
+        repository.decr(cache_key = cache_key(key), delta) || begin
+          repository.add(cache_key, (result = yield).to_s, ttl, true) { repository.decr(cache_key) }
           result
         end
       end

spec/cash/accessor_spec.rb

@@ -101,6 +101,22 @@ module Cash
         end
       end
     end
+    
+    describe '#add' do
+      describe 'when the value already exists' do
+        it 'yields to the block' do
+          Story.set("count", 1)
+          Story.add("count", 1) { "yield me" }.should == "yield me"
+        end
+      end
+      
+      describe 'when the value does not already exist' do
+        it 'adds the key to the cache' do
+          Story.add("count", 1)
+          Story.get("count").should == 1
+        end
+      end
+    end
 
     describe '#decr' do
       describe 'when there is a cache hit' do