Cache: change to a Singleton object so it is only loaded from disk once

spec/rscons/cache_spec.rb

@@ -5,14 +5,11 @@ module Rscons
     end
 
     def build_from(cache)
-      JSON.should_receive(:load).and_return(cache)
-      Cache.new
-    end
-
-    describe ".clear" do
-      it "removes the cache file" do
-        FileUtils.should_receive(:rm_f).with(Cache::CACHE_FILE)
-        Cache.clear
+      JSON.stub(:load) do
+        cache
+      end
+      Cache.instance.tap do |cache|
+        cache.send(:initialize!)
       end
     end
 
@@ -21,11 +18,21 @@ module Rscons
         it "prints a warning and defaults to an empty hash" do
           JSON.should_receive(:load).and_return("string")
           $stderr.should_receive(:puts).with(/Warning:.*was.corrupt/)
-          Cache.new.instance_variable_get(:@cache).is_a?(Hash).should be_true
+          c = Cache.instance
+          c.send(:initialize!)
+          c.instance_variable_get(:@cache).is_a?(Hash).should be_true
         end
       end
     end
 
+    describe "#clear" do
+      it "removes the cache file" do
+        FileUtils.should_receive(:rm_f).with(Cache::CACHE_FILE)
+        JSON.stub(:load) {{}}
+        Cache.instance.clear
+      end
+    end
+
     describe "#write" do
       it "should fill in 'version' and write to file" do
         cache = {}

spec/rscons/environment_spec.rb

@@ -140,7 +140,8 @@ module Rscons
         env.Program("a.out", "main.c")
 
         cache = "cache"
-        Cache.should_receive(:new).and_return(cache)
+        Cache.should_receive(:instance).and_return(cache)
+        cache.should_receive(:clear_checksum_cache!)
         env.should_receive(:run_builder).with(anything, "a.out", ["main.c"], cache, {}).and_return(true)
         cache.should_receive(:write)
 
@@ -153,7 +154,8 @@ module Rscons
         env.Object("main.o", "other.cc")
 
         cache = "cache"
-        Cache.should_receive(:new).and_return(cache)
+        Cache.should_receive(:instance).and_return(cache)
+        cache.should_receive(:clear_checksum_cache!)
         env.should_receive(:run_builder).with(anything, "main.o", ["other.cc"], cache, {}).and_return("main.o")
         env.should_receive(:run_builder).with(anything, "a.out", ["main.o"], cache, {}).and_return("a.out")
         cache.should_receive(:write)
@@ -167,7 +169,8 @@ module Rscons
         env.Object("main.o", "other.cc")
 
         cache = "cache"
-        Cache.should_receive(:new).and_return(cache)
+        Cache.should_receive(:instance).and_return(cache)
+        cache.should_receive(:clear_checksum_cache!)
         env.should_receive(:run_builder).with(anything, "main.o", ["other.cc"], cache, {}).and_return(false)
         cache.should_receive(:write)