Ignore load path caches generated by a different ruby version

Fix: #384

Until now it was assume that if the `$LOAD_PATH` was identical,
then the content of the paths would be too.

However from one minor ruby version to another, the layout of the
stdlib can change. So if the newer version is installed in the same
place than the previous one, it might cause the load path cache to
be invalid.

So we store `RUBY_DESCRIPTION` in the cache and make sure it matches
before reusing the cache.

lib/bootsnap/load_path_cache/store.rb

@@ -7,6 +7,7 @@
 module Bootsnap
   module LoadPathCache
     class Store
+      VERSION_KEY = '__bootsnap_ruby_version__'
       NestedTransactionError = Class.new(StandardError)
       SetOutsideTransactionNotAllowed = Class.new(StandardError)
 
@@ -62,15 +63,20 @@ def commit_transaction
 
       def load_data
         @data = begin
-          File.open(@store_path, encoding: Encoding::BINARY) do |io|
+          data = File.open(@store_path, encoding: Encoding::BINARY) do |io|
             MessagePack.load(io)
           end
+          if data.is_a?(Hash) && data[VERSION_KEY] == RUBY_DESCRIPTION
+            data
+          else
+            default_data
+          end
         # handle malformed data due to upgrade incompatibility
         rescue Errno::ENOENT, MessagePack::MalformedFormatError, MessagePack::UnknownExtTypeError, EOFError
-          {}
+          default_data
         rescue ArgumentError => error
           if error.message =~ /negative array size/
-            {}
+            default_data
           else
             raise
           end
@@ -93,6 +99,10 @@ def dump_data
         retry
       rescue SystemCallError
       end
+
+      def default_data
+        { VERSION_KEY => RUBY_DESCRIPTION }
+      end
     end
   end
 end

test/load_path_cache/store_test.rb

@@ -80,6 +80,30 @@ def test_ignore_read_only_filesystem
         store.transaction { store.set('a', 1) }
         refute(File.exist?(@path))
       end
+
+      def test_bust_cache_on_ruby_change
+        store.transaction { store.set('a', 'b') }
+
+        assert_equal 'b', Store.new(@path).get('a')
+
+        stub_const(Object, :RUBY_DESCRIPTION, "foobar") do
+          assert_nil Store.new(@path).get('a')
+        end
+      end
+
+      private
+
+      def stub_const(owner, const_name, stub_value)
+        original_value = owner.const_get(const_name)
+        owner.send(:remove_const, const_name)
+        owner.const_set(const_name, stub_value)
+        begin
+          yield
+        ensure
+          owner.send(:remove_const, const_name)
+          owner.const_set(const_name, original_value)
+        end
+      end
     end
   end
 end