don't print to stdout unconditionally on ExitOnError

[alessio]

If flagset.output was set to stderr, the error would still be
printed to stdout instead of stderr.

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[mckern]

Can you clarify intent a little bit? "If flagset.output was set to stderr, the error would still be printed to stderr instead of stdout" still seems like desired and explicit behavior to me; what are the circumstances where a fatal error should print to stdout instead of stderr?

[alessio]

"If flagset.output was set to stderr, the error would still be printed to stderr instead of stdout"

It was a dumb typo, thanks for pointing it out. I've just fixed the PR's description. Obviously the problem is that on ExitOnError f.Parse() would print the error message to STDOUT instead of f.Output().

[alessio]

CC'ing @therealmitchconnors

[cornfeedhobo]

@alessio I'm going to take a look, but if you can think of a test for this, that'd be great!

[alessio]

@cornfeedhobo how about the following:

func TestParseErrorHandling(t *testing.T) {
	if os.Getenv("PFLAG_CRASH_TEST") == "1" {
		f := NewFlagSet(t.Name(), ExitOnError)
		args := []string{"--invalid-arg"}

		f.Parse(args)
		t.Fatal("this error should not be triggered")

		return
	}

	cmd := exec.Command(os.Args[0], "-test.run="+t.Name())
	mockStderr := bytes.NewBufferString("")
	mockStdout := bytes.NewBufferString("")
	cmd.Env = append(os.Environ(), "PFLAG_CRASH_TEST=1")
	cmd.Stderr = mockStderr
	cmd.Stdout = mockStdout
	err := cmd.Run()

	if e, ok := err.(*exec.ExitError); ok && !e.Success() {
		want := `unknown flag: --invalid-arg
Usage of TestParseErrorHandling:
unknown flag: --invalid-arg
`
		if got := mockStderr.String(); got != want {
			t.Errorf("got '%s', want '%s'", got, want)
		}

		if len(mockStdout.String()) != 0 {
			t.Error("stdout should be empty")
		}

		return
	}

	t.Fatal("this error should not be triggered")
}

[dmitryrn]

Printing to STDOUT without new line prevents Goland to print test output correctly https://youtrack.jetbrains.com/issue/GO-7215 :(