filesystem_freebsd: Fix label values

[phyber]

Fixes #1727

We must know the length of the various filesystem C strings before
turning them from a byte array into a Go string, otherwise our Go
strings could contain null bytes, corrupting the label values.

[discordianfish]

Hrmmm isn't that something that Go should take care of? Surprised about that behavior.
/cc @SuperQ @pgier

[phyber]

I was surprised here as well, but it looks like intended behaviour. Documentation on the string() cast is kind of hard to find (I found the Go docs hard to browse and couldn't find the relevant source), but it looks like it will just take any bytes and dump them out into a string type instead of stopping at null. I guess this makes sense from a speed point of view, but is surprising in a language like Go.

The closest thing describing this behaviour is the spec which simply says: A string value is a (possibly empty) sequence of bytes. It makes no mention of null being disallowed in a string.

I've added a simple program showing the behaviour below.

$ cat main.go && go run main.go | tee output.log | xxd && cat output.log
package main

import (
        "bytes"
        "fmt"
)

func main() {
        b := []byte{65, 66, 67, 0, 68, 0, 69, 0, 70, 0, 71, 0}
        n := bytes.Index(b, []byte{0})

        bad := string(b)
        good := string(b[:n])

        fmt.Println(bad)
        fmt.Println(good)
}

00000000: 4142 4300 4400 4500 4600 4700 0a41 4243  ABC.D.E.F.G..ABC
00000010: 0a                                       .

ABCDEFG # Null bytes hidden in output on terminal
ABC     # Good string after cast was stopped at first null byte

[discordianfish]

Thanks for elaborating! Yeah okay then this makes sense. Could you move this into a function though?

[phyber]

I'll have this simplified and broken out into its own function in a while. helpers.go looks like a good place for such a function to live, and I'll add a few basic tests for it too.

[phyber]

This should be ready for another review now.

[SuperQ]

Minor nit, otherwise LGTM. Thanks!

[phyber]

Copyright year fixed. Sorry for missing that :)

[discordianfish]

Awesome, thanks!