os/user: fix signedness of UIDs before stringifying

[chipaca]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

at least 1.6 and 1.9 (haven't checked elsewhere)

Does this issue reproduce with the latest release?

that's 1.9, right?

What operating system and processor architecture are you using (go env)?

issue is with 32 bit Linux, only (so GOARCH of 386 and arm, and ppc)

What did you do?

given

package main

import (
	"fmt"
	"os/user"
	"syscall"
)

func main() {
	fmt.Printf("%x\n", syscall.Getuid())
	u, e := user.Current()
	fmt.Printf("%v %#v\n", e, u)
}

and

sudo useradd -u $(((1<<32)-2)) test123

and

CGO_ENABLED=1 GOARCH=386 go build userhi.go

What did you expect to see?

fffffffe
<nil> &user.User{Uid:"4294967294", Gid:"1001", Username:"test123", Name:"", HomeDir:"/home/test123"}

What did you see instead?

-1
user: unknown userid -1 (*user.User)(nil)

[bradfitz]

We can fix os/user's User.Uid, but the Go 1 compatibility promise prevents us from changing the syscall.Getuid signature. You'll have to convert there. But it's the syscall package anyway, so people are expected to roll up their sleeves a bit.

[chipaca]

note the same applies to gid

[chipaca]

also, in case you're wondering, uids and gids of 0xffffffff aren't allowed because -1 is used as a flag to mean 'no change' for chown(2)

[gopherbot]

Change https://golang.org/cl/77930 mentions this issue: os/user: handle large 32-bit uid/gid values when stringifying User.Uid/Gid

[bradfitz]

I handled gid in https://go-review.googlesource.com/#/c/go/+/77930 too.