Suppress STDERR in the given block

suppress_stderr.rb

# Suppress $stderr in the given block
#
# @author Masa Sakano, under Licence of CC BY-SA 4.0
#
# @example Simply suppressing $stderr (but not STDERR)
#   suppress_stderr{ warn "Not displayed."; a=5 }  #=> 5
#
# @example Backup-ping STDERR in a block, suppressing $stderr (and warn)
#   suppress_stderr('/tmp/backup.log'){ warn "Not displayed."; a=5 }  #=> 5
#
# @param backup_file [String] Filename to backup $stderr
# @return [Object] Returned value of the given block (yield)
# @yield [] Block to execute, in which $stderr is suppressed.
def suppress_stderr(backup_file='/dev/null')
  stderr_orig = $stderr.dup  # dup() is essential
  $stderr.sync
  $stderr = open(backup_file, 'w')  # $stderr.reopen() does not work well.
  begin
    ret = yield
    $stderr.sync
  ensure
    $stderr.close
  end
  return ret
ensure
  $stderr = (stderr_orig || $stderr)  # in case "ensured" before the first sentence is completed.
end

# Code snippet to easily suppress STDERR
#
def example_easy_suppress_stderr
  warn_level = $VERBOSE
  begin
    $VERBOSE = nil    # Suppress the warning in the following line(s).
    warn "This is not printed"  # warn does nothing when $VERBOSE.nil?
  ensure
    $VERBOSE = warn_level
  end
end

########## Unit tests ##########

if $0 == __FILE__
  require 'minitest/autorun'
  require 'tempfile'
  $stderr.sync=true
  $stderr.puts "This is meant to be displayed."
  suppress_stderr(){$stderr.puts "Not displayed."}
  suppress_stderr(){warn         "Not displayed."}
  class TestUnitFoo < MiniTest::Test
    def test_defs  # $VERBOSE is set true in UnitTest
      orig_io = $stderr.dup
      assert_equal(5, suppress_stderr(){ 5})  # Return value test
      assert_output('',"bar\n"){$stderr.print "bar\n"}
      assert_output('',''){suppress_stderr(){$stderr.print "bar\n"}}
      assert_output('',''){suppress_stderr(){warn "bar\n"}}
      _, err = capture_io{warn "bar\n"}
      assert_equal "bar\n", err
      _, _   = capture_io{suppress_stderr(){STDERR.print "STDERR is meant not to be suppressed, or even captured by Unit-Test, like this.\n"}}
      Tempfile.open(File.basename($0)+'tmp'){ |iotmp|
        fname = iotmp.path  # Test of Backup file
        assert_output('',''){suppress_stderr(fname){$stderr.print "bar\n"}}
        assert_equal "bar\n", IO.read(fname)
      }
      assert_raises(RangeError){suppress_stderr(){raise RangeError}}  # Exception passed to the caller.
      assert_equal orig_io.inspect, $stderr.inspect  # All recovered; inspect is the only way to compare them.
      assert_output('',''){example_easy_suppress_stderr}
    end
  end	# class TestUnitFoo < MiniTest::Unit::TestCase
end	# if $0 == __FILE__