Fix file reading in cmd/jwt

[travisnewhouse]

File reading by cmd/jwt is failing. Inside of loadData(), the os.File was being closed before its contents were read by io.ReadAll(). This leads errors like the following.

$ go run github.com/golang-jwt/jwt/v5/cmd/[email protected] -alg EdDSA -key privateKey.pem -sign jwt.json 
Error: couldn't read token: read /tmp/access-jwt.json: file already closed
exit status 1

The solution is to defer the close operation until the end of the function, which is the way the code had been written prior to PR #438.

[oxisto]

I think we need to do something like

defer func() {
if err := f.Close(); err != nil {
 // Somehow "handle" the error
}
}()

to silence the linter

[equalsgibson]

One way that it can be handled to allow returning the error value to the caller, as well as other errors, is to use a helper function like this one with a named return value;

// Helper func:  Pass in the function directly rather than calling the function
func joinErr(err *error, op func() error) {
  *err = errors.Join(*err, op())
}

// Helper func:  Read input from specified file or stdin
func loadData(p string) (_ []byte, err error) {
	if p == "" {
		return nil, fmt.Errorf("no path specified")
	}

	var rdr io.Reader
	switch p {
	case "-":
		rdr = os.Stdin
	case "+":
		return []byte("{}"), nil
	default:
		f, err := os.Open(p)
		if err != nil {
			return nil, err
		}
		rdr = f
		defer joinErr(&err, f.Close)
	}
	return io.ReadAll(rdr)
}

VictoriaMetrics has a good blogpost on this as well as Uber having run into a similar issue and creating an "Invoker" interface.

[equalsgibson]

As a note, the function needs to use a named return value, which is a first for me (I've not really used named returns before), otherwise the deferred function updates the error value that is scoped to the function (and is already discarded)

Details here; https://gophers.slack.com/archives/C02A8LZKT/p1754303913406599