File handles: Make spec more reliable, and satisfy Rubocop

lib/prawn/images.rb

@@ -149,9 +149,7 @@ def verify_and_read_image(io_or_path)
       # String or Pathname
       io_or_path = Pathname.new(io_or_path)
       raise ArgumentError, "#{io_or_path} not found" unless io_or_path.file?
-      File.open(io_or_path, 'rb') do |file_io|
-        file_io.read
-      end
+      File.binread(io_or_path)
     end
 
     def image_position(w, h, options)

spec/prawn/images_spec.rb

@@ -47,32 +47,44 @@
     file = File.open(filename, 'rb')
     _info = pdf.image(file)
 
-    expect(file).not_to be_closed
+    expect(file).to_not be_closed
   end
 
   it 'accepts Pathname objects' do
     info = pdf.image(Pathname.new(filename))
     expect(info.height).to eq(453)
   end
 
-  it 'closes opened files again after getting Pathname objects (lsof version)' do
-    if system('lsof -v > /dev/null 2&>1') && system('grep --version > /dev/null 2&>1')
+  it 'closes opened files again after getting Pathname objects' do
+    # lsof version
+    system_has_lsof = system('lsof -v > /dev/null 2>&1')
+    system_has_grep = system('grep --version > /dev/null 2>&1')
+    if system_has_lsof && system_has_grep
       # linux and macOS will actually run this spec
+      gc_was_disabled = GC.disable # make sure GC doesn't remove File instance and close file
+
       open_files_before = `lsof -c ruby | grep "#{filename}"`
       _info = pdf.image(Pathname.new(filename))
       expect(`lsof -c ruby | grep "#{filename}"`).to eq(open_files_before)
+
+      GC.enable unless gc_was_disabled
     else
       # can't test this on every OS
-      expect(true).to be_truthy
+      expect(system_has_lsof && system_has_grep).to be_falsy
     end
   end
 
-  it 'closes opened files again after getting Pathname objects (file double version)' do
-    file_double = double('file double')
-    expect(file_double).to receive(:read).and_return(File.new(filename).read)
-    expect(File).to receive(:open).and_yield(file_double)
+  context 'with File message spy' do
+    before do
+      file_path = Pathname.new(filename)
+      file_content = File.new(filename, 'rb').read
+      allow(File).to receive(:binread).with(file_path).and_return(file_content)
+    end
 
-    _info = pdf.image(Pathname.new(filename))
+    it 'closes opened files again after getting Pathname objects' do
+      _info = pdf.image(Pathname.new(filename))
+      expect(File).to have_received(:binread)
+    end
   end
 
   context 'setting the length of the bytestream' do