Accept ultra_safe as an initializer option

Allow stores to select atomic-save behavior when they are created
while preserving the existing accessor.

Author: nobu

lib/pstore.rb

@@ -373,7 +373,13 @@ class Error < StandardError
   #   store = PStore.new(path, true)
   #   store = PStore.new(path, thread_safe: true)
   #
-  def initialize(file, _thread_safe = false, thread_safe: _thread_safe)
+  # If keyword argument +ultra_safe+ is given as +true+, the object is
+  # set to +ultra_safe+ mode.
+  #
+  #   store = PStore.new(path, ultra_safe: true)
+  #
+  def initialize(file, _thread_safe = false, thread_safe: _thread_safe,
+                 ultra_safe: false)
     dir = File::dirname(file)
     unless File::directory? dir
       raise PStore::Error, format("directory %s does not exist", dir)
@@ -383,7 +389,7 @@ def initialize(file, _thread_safe = false, thread_safe: _thread_safe)
     end
     @filename = File.path(file)
     @abort = false
-    @ultra_safe = false
+    @ultra_safe = ultra_safe
     @thread_safe = thread_safe
     @lock = Thread::Mutex.new
   end

test/test_pstore.rb

@@ -135,6 +135,12 @@ def test_thread_safe_argument
     assert_equal false, PStore.new(@pstore_file, true, thread_safe: false).instance_variable_get(:@thread_safe)
   end
 
+  def test_ultra_safe_argument
+    assert_equal false, PStore.new(@pstore_file).ultra_safe
+    assert_equal true, PStore.new(@pstore_file, ultra_safe: true).ultra_safe
+    assert_equal false, PStore.new(@pstore_file, ultra_safe: false).ultra_safe
+  end
+
   def test_nested_transaction_raises_error
     assert_raise(PStore::Error) do
       @pstore.transaction { @pstore.transaction { } }