Prevent OptionsCache from leaking memory

Rakefile

@@ -3,7 +3,7 @@ Bundler::GemHelper.install_tasks
 
 require 'rspec/core/rake_task'
 RSpec::Core::RakeTask.new(:base_spec) do |task|
-  task.pattern = 'spec/multi_json_spec.rb'
+  task.pattern = 'spec/multi_json_spec.rb,spec/options_cache_spec.rb'
 end
 
 namespace :adapters do

lib/multi_json/options_cache.rb

@@ -9,7 +9,21 @@ def reset
 
     def fetch(type, key)
       cache = instance_variable_get("@#{type}_cache")
-      cache.key?(key) ? cache[key] : cache[key] = yield
+      cache.key?(key) ? cache[key] : write(cache, key, &Proc.new)
+    end
+
+    private
+
+    # Normally MultiJson is used with a few option sets for both dump/load
+    # methods. When options are generated dynamically though, every call would
+    # cause a cache miss and the cache would grow indefinitely. To prevent
+    # this, we just reset the cache every time the number of keys outgrows
+    # 1000.
+    MAX_CACHE_SIZE = 1000
+
+    def write(cache, key)
+      cache.clear if cache.length >= MAX_CACHE_SIZE
+      cache[key] = yield
     end
   end
 end

spec/options_cache_spec.rb

@@ -0,0 +1,20 @@
+require "spec_helper"
+
+describe MultiJson::OptionsCache do
+  before { described_class.reset }
+
+  it "doesn't leak memory" do
+    described_class::MAX_CACHE_SIZE.succ.times do |i|
+      described_class.fetch(:dump, :key => i) do
+        { :foo => i }
+      end
+
+      described_class.fetch(:load, :key => i) do
+        { :foo => i }
+      end
+    end
+
+    expect(described_class.instance_variable_get(:@dump_cache).length).to eq(1)
+    expect(described_class.instance_variable_get(:@load_cache).length).to eq(1)
+  end
+end