os/user: “GroupIds requires cgo” when building for linux/amd64 with tag osusergo

Question

I'm trying to build a static executable that uses the os/user module to lookup the groups that a particular user is a member of on linux/amd64 .

According to the documentation , setting the osusergo build tag will allow me to use the non-cgo version of this module, but the flag doesn't seem to be doing anything.

Here's how I'm building the executable:

export CGO_ENABLED=0
export GOOS=linux
export GOARCH=amd64
go build -tags osusergo -o bin/agent

Here's my Go version:

$ go version
go version go1.14.4 linux/amd64

Here's an example of how I'm using the os/user module:

import (
    "os/user"
)

func GetUserGroupIds(uid string) ([]string, error) {
    u, err := user.LookupId(uid)
    if err != nil {
        return nil, err
    }
    gids, err := u.GroupIds()
    if err != nil {
        return nil, err
    }
    return gids, nil
}

When looking up the groups that a particular user is a member of, I receive the following error: user: GroupIds requires cgo .

Am I not using -tags osusergo correctly?

Answer 1

After reading the source code, it turns out the listGroups(u *User) function of the os/user module doesn't have a native Go implementation:

// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// +build dragonfly darwin freebsd !android,linux netbsd openbsd
// +build cgo,!osusergo

package user

import (
    "fmt"
    "strconv"
    "unsafe"
)

/*
#include <unistd.h>
#include <sys/types.h>
*/
import "C"

const maxGroups = 2048

func listGroups(u *User) ([]string, error) {
    ug, err := strconv.Atoi(u.Gid)
    if err != nil {
        return nil, fmt.Errorf("user: list groups for %s: invalid gid %q", u.Username, u.Gid)
    }
    userGID := C.gid_t(ug)
    nameC := make([]byte, len(u.Username)+1)
    copy(nameC, u.Username)

    n := C.int(256)
    gidsC := make([]C.gid_t, n)
    rv := getGroupList((*C.char)(unsafe.Pointer(&nameC[0])), userGID, &gidsC[0], &n)
    if rv == -1 {
        // Mac is the only Unix that does not set n properly when rv == -1, so
        // we need to use different logic for Mac vs. the other OS's.
        if err := groupRetry(u.Username, nameC, userGID, &gidsC, &n); err != nil {
            return nil, err
        }
    }
    gidsC = gidsC[:n]
    gids := make([]string, 0, n)
    for _, g := range gidsC[:n] {
        gids = append(gids, strconv.Itoa(int(g)))
    }
    return gids, nil
}