Change Mutant::Isolation.call to use block form of IO.pipe

* The block form automatically closes the reader and writer if they have
  not already been closed. This prevents resource leakage if the method
  exits due to an exception.
* Refactor specs to have common setup performed in a before block.

lib/mutant/isolation.rb

@@ -38,10 +38,10 @@ module Mutant
       #
       # @api private
       def self.call(&block)
-        reader, writer = IO.pipe.map(&:binmode)
-
+        IO.pipe(binmode: true) do |reader, writer|
+          begin
             pid = Process.fork do
-          File.open('/dev/null', 'w') do |file|
+              File.open(File::NULL, 'w') do |file|
                 $stderr.reopen(file)
                 reader.close
                 writer.write(Marshal.dump(block.call))
@@ -51,11 +51,13 @@ module Mutant
 
             writer.close
             Marshal.load(reader.read)
-      rescue => exception
-        raise Error, exception
           ensure
             Process.waitpid(pid) if pid
           end
+        end
+      rescue => exception
+        raise Error, exception
+      end
 
     end # Fork
 

spec/unit/mutant/isolation_spec.rb

@@ -64,11 +64,17 @@ RSpec.describe Mutant::Isolation::Fork do
       end
     end
 
-    it 'uses primitives in correct order when fork succeeds' do
-      reader, writer = double('reader'), double('writer')
-      expect(IO).to receive(:pipe).ordered.and_return([reader, writer])
-      expect(reader).to receive(:binmode).and_return(reader).ordered
-      expect(writer).to receive(:binmode).and_return(writer).ordered
+    context 'uses primitives in correct order' do
+      let(:reader) { double('reader') }
+      let(:writer) { double('writer') }
+
+      before do
+        expect(IO).to receive(:pipe).with(binmode: true).ordered do |&block|
+          block.call([reader, writer])
+        end
+      end
+
+      it 'when fork succeeds' do
         pid = double('PID')
         expect(Process).to receive(:fork).ordered.and_yield.and_return(pid)
         file = double('file')
@@ -84,17 +90,13 @@ RSpec.describe Mutant::Isolation::Fork do
         expect(object.call { :foo }).to be(:foo)
       end
 
-    it 'uses primitives in correct order when fork fails' do
-      reader, writer = double('reader'), double('writer')
-      expect(IO).to receive(:pipe).ordered.and_return([reader, writer])
-      expect(reader).to receive(:binmode).and_return(reader).ordered
-      expect(writer).to receive(:binmode).and_return(writer).ordered
+      it 'when fork fails' do
         expect(Process).to receive(:fork).ordered.and_return(nil)
         expect(Process).to_not receive(:waitpid)
-
         expect(writer).to receive(:close).ordered
         expect(reader).to receive(:read).ordered.and_return(Marshal.dump(:foo))
         expect(object.call).to be(:foo)
       end
     end
   end
+end