deep-exit: detect exit-triggering flag usage

[uloamaka]

This PR resolves #1523,
Packages using flag.ExitOnError or flag.Parse() can exit on invalid flags or --help, which violates the principle of reusability the deep-exit rule enforces.

This PR enhances the deep-exit rule to detect indirect program termination caused by the Go flag package.

Specifically, it adds checks for:

• flag.NewFlagSet created with flag.ExitOnError
• Calls to flag.Parse() outside of main() or init()

These usages can terminate programs unexpectedly, reducing reusability of packages that expose such functions.

Updates:

• testdata/deep_exit.go

• I added tests

[uloamaka]

@alexandear Can you please help review this PR.

[chavacava]

The fact we disable the rule in our own code makes me think on the pertinence of checking calls to flag.Parse outside the main function.

[ccoVeille]

That's because the right way to do is to use either this

func main() {

	// (...)
	flag.BoolVar(&setExitStatus, "set_exit_status", false, exitStatusUsage)
	flag.IntVar(&maxOpenFiles, "max_open_files", 0, maxOpenFilesUsage)
	flag.Parse()

	// (...)
}

or this

func main() {
	err := run()
	// (...)
}

func run() error {
	fl := flag.NewFlagSet(os.Args[0], flag.ContinueOnError)
	fl.BoolVar(&setExitStatus, "set_exit_status", false, exitStatusUsage)
	fl.IntVar(&maxOpenFiles, "max_open_files", 0, maxOpenFilesUsage)
	err := fl.Parse(os.Args[1:])
	if err != nil {
		return err
	}
	// (...)

	return nil
}

You can take a look at the code I wrote for gh-reaction

But maybe, the error message for flag.Parse should be way clearer, about the fact there is a need to use flag.ContinueOnError

Please note also, that the same code, using flag.NewFlagSet(os.Args[0], flag.ExitOnError) or flag.NewFlagSet(os.Args[0], flag.PanicOnError) should be reported as deep-exit, but it would be a next move, and somehow a niche maybe

EDIT: it's not a niche :P https://github.com/search?q=language%3Ago%20%2Fflag%5C.NewFlagSet.*flag%5C.(Exit%7CPanic)OnError%2F%20-is%3Afork%20-is%3Aarchived&type=code

[chavacava]

Thank you @uloamaka for the PR and @alexandear @ccoVeille for the feedback