19 files changed, 2118 insertions, 0 deletions

diff --git a/src/os/user/cgo_listgroups_unix.go b/src/os/user/cgo_listgroups_unix.go
new file mode 100644
index 0000000..5963695
--- /dev/null
+++ b/src/os/user/cgo_listgroups_unix.go
@@ -0,0 +1,57 @@
+// 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.
+
+//go:build (cgo || darwin) && !osusergo && (darwin || dragonfly || freebsd || (linux && !android) || netbsd || openbsd || (solaris && !illumos))
+
+package user
+
+import (
+	"fmt"
+	"strconv"
+	"unsafe"
+)
+
+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
+}
+
+// groupRetry retries getGroupList with much larger size for n. The result is
+// stored in gids.
+func groupRetry(username string, name []byte, userGID _C_gid_t, gids *[]_C_gid_t, n *_C_int) error {
+	// More than initial buffer, but now n contains the correct size.
+	if *n > maxGroups {
+		return fmt.Errorf("user: %q is a member of more than %d groups", username, maxGroups)
+	}
+	*gids = make([]_C_gid_t, *n)
+	rv := getGroupList((*_C_char)(unsafe.Pointer(&name[0])), userGID, &(*gids)[0], n)
+	if rv == -1 {
+		return fmt.Errorf("user: list groups for %s failed", username)
+	}
+	return nil
+}
diff --git a/src/os/user/cgo_lookup_cgo.go b/src/os/user/cgo_lookup_cgo.go
new file mode 100644
index 0000000..4f78dca
--- /dev/null
+++ b/src/os/user/cgo_lookup_cgo.go
@@ -0,0 +1,112 @@
+// Copyright 2011 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.
+
+//go:build cgo && !osusergo && unix && !android && !darwin
+
+package user
+
+import (
+	"syscall"
+)
+
+/*
+#cgo solaris CFLAGS: -D_POSIX_PTHREAD_SEMANTICS
+#cgo CFLAGS: -fno-stack-protector
+#include <unistd.h>
+#include <sys/types.h>
+#include <pwd.h>
+#include <grp.h>
+#include <stdlib.h>
+#include <string.h>
+
+static struct passwd mygetpwuid_r(int uid, char *buf, size_t buflen, int *found, int *perr) {
+	struct passwd pwd;
+	struct passwd *result;
+	memset (&pwd, 0, sizeof(pwd));
+	*perr = getpwuid_r(uid, &pwd, buf, buflen, &result);
+	*found = result != NULL;
+	return pwd;
+}
+
+static struct passwd mygetpwnam_r(const char *name, char *buf, size_t buflen, int *found, int *perr) {
+	struct passwd pwd;
+	struct passwd *result;
+	memset(&pwd, 0, sizeof(pwd));
+	*perr = getpwnam_r(name, &pwd, buf, buflen, &result);
+	*found = result != NULL;
+	return pwd;
+}
+
+static struct group mygetgrgid_r(int gid, char *buf, size_t buflen, int *found, int *perr) {
+	struct group grp;
+	struct group *result;
+	memset(&grp, 0, sizeof(grp));
+	*perr = getgrgid_r(gid, &grp, buf, buflen, &result);
+	*found = result != NULL;
+	return grp;
+}
+
+static struct group mygetgrnam_r(const char *name, char *buf, size_t buflen, int *found, int *perr) {
+	struct group grp;
+	struct group *result;
+	memset(&grp, 0, sizeof(grp));
+	*perr = getgrnam_r(name, &grp, buf, buflen, &result);
+	*found = result != NULL;
+	return grp;
+}
+*/
+import "C"
+
+type _C_char = C.char
+type _C_int = C.int
+type _C_gid_t = C.gid_t
+type _C_uid_t = C.uid_t
+type _C_size_t = C.size_t
+type _C_struct_group = C.struct_group
+type _C_struct_passwd = C.struct_passwd
+type _C_long = C.long
+
+func _C_pw_uid(p *_C_struct_passwd) _C_uid_t   { return p.pw_uid }
+func _C_pw_uidp(p *_C_struct_passwd) *_C_uid_t { return &p.pw_uid }
+func _C_pw_gid(p *_C_struct_passwd) _C_gid_t   { return p.pw_gid }
+func _C_pw_gidp(p *_C_struct_passwd) *_C_gid_t { return &p.pw_gid }
+func _C_pw_name(p *_C_struct_passwd) *_C_char  { return p.pw_name }
+func _C_pw_gecos(p *_C_struct_passwd) *_C_char { return p.pw_gecos }
+func _C_pw_dir(p *_C_struct_passwd) *_C_char   { return p.pw_dir }
+
+func _C_gr_gid(g *_C_struct_group) _C_gid_t  { return g.gr_gid }
+func _C_gr_name(g *_C_struct_group) *_C_char { return g.gr_name }
+
+func _C_GoString(p *_C_char) string { return C.GoString(p) }
+
+func _C_getpwnam_r(name *_C_char, buf *_C_char, size _C_size_t) (pwd _C_struct_passwd, found bool, errno syscall.Errno) {
+	var f, e _C_int
+	pwd = C.mygetpwnam_r(name, buf, size, &f, &e)
+	return pwd, f != 0, syscall.Errno(e)
+}
+
+func _C_getpwuid_r(uid _C_uid_t, buf *_C_char, size _C_size_t) (pwd _C_struct_passwd, found bool, errno syscall.Errno) {
+	var f, e _C_int
+	pwd = C.mygetpwuid_r(_C_int(uid), buf, size, &f, &e)
+	return pwd, f != 0, syscall.Errno(e)
+}
+
+func _C_getgrnam_r(name *_C_char, buf *_C_char, size _C_size_t) (grp _C_struct_group, found bool, errno syscall.Errno) {
+	var f, e _C_int
+	grp = C.mygetgrnam_r(name, buf, size, &f, &e)
+	return grp, f != 0, syscall.Errno(e)
+}
+
+func _C_getgrgid_r(gid _C_gid_t, buf *_C_char, size _C_size_t) (grp _C_struct_group, found bool, errno syscall.Errno) {
+	var f, e _C_int
+	grp = C.mygetgrgid_r(_C_int(gid), buf, size, &f, &e)
+	return grp, f != 0, syscall.Errno(e)
+}
+
+const (
+	_C__SC_GETPW_R_SIZE_MAX = C._SC_GETPW_R_SIZE_MAX
+	_C__SC_GETGR_R_SIZE_MAX = C._SC_GETGR_R_SIZE_MAX
+)
+
+func _C_sysconf(key _C_int) _C_long { return C.sysconf(key) }
diff --git a/src/os/user/cgo_lookup_syscall.go b/src/os/user/cgo_lookup_syscall.go
new file mode 100644
index 0000000..321df65
--- /dev/null
+++ b/src/os/user/cgo_lookup_syscall.go
@@ -0,0 +1,65 @@
+// Copyright 2011 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.
+
+//go:build !osusergo && darwin
+
+package user
+
+import (
+	"internal/syscall/unix"
+	"syscall"
+)
+
+type _C_char = byte
+type _C_int = int32
+type _C_gid_t = uint32
+type _C_uid_t = uint32
+type _C_size_t = uintptr
+type _C_struct_group = unix.Group
+type _C_struct_passwd = unix.Passwd
+type _C_long = int64
+
+func _C_pw_uid(p *_C_struct_passwd) _C_uid_t   { return p.Uid }
+func _C_pw_uidp(p *_C_struct_passwd) *_C_uid_t { return &p.Uid }
+func _C_pw_gid(p *_C_struct_passwd) _C_gid_t   { return p.Gid }
+func _C_pw_gidp(p *_C_struct_passwd) *_C_gid_t { return &p.Gid }
+func _C_pw_name(p *_C_struct_passwd) *_C_char  { return p.Name }
+func _C_pw_gecos(p *_C_struct_passwd) *_C_char { return p.Gecos }
+func _C_pw_dir(p *_C_struct_passwd) *_C_char   { return p.Dir }
+
+func _C_gr_gid(g *_C_struct_group) _C_gid_t  { return g.Gid }
+func _C_gr_name(g *_C_struct_group) *_C_char { return g.Name }
+
+func _C_GoString(p *_C_char) string { return unix.GoString(p) }
+
+func _C_getpwnam_r(name *_C_char, buf *_C_char, size _C_size_t) (pwd _C_struct_passwd, found bool, errno syscall.Errno) {
+	var result *_C_struct_passwd
+	errno = unix.Getpwnam(name, &pwd, buf, size, &result)
+	return pwd, result != nil, errno
+}
+
+func _C_getpwuid_r(uid _C_uid_t, buf *_C_char, size _C_size_t) (pwd _C_struct_passwd, found bool, errno syscall.Errno) {
+	var result *_C_struct_passwd
+	errno = unix.Getpwuid(uid, &pwd, buf, size, &result)
+	return pwd, result != nil, errno
+}
+
+func _C_getgrnam_r(name *_C_char, buf *_C_char, size _C_size_t) (grp _C_struct_group, found bool, errno syscall.Errno) {
+	var result *_C_struct_group
+	errno = unix.Getgrnam(name, &grp, buf, size, &result)
+	return grp, result != nil, errno
+}
+
+func _C_getgrgid_r(gid _C_gid_t, buf *_C_char, size _C_size_t) (grp _C_struct_group, found bool, errno syscall.Errno) {
+	var result *_C_struct_group
+	errno = unix.Getgrgid(gid, &grp, buf, size, &result)
+	return grp, result != nil, errno
+}
+
+const (
+	_C__SC_GETPW_R_SIZE_MAX = unix.SC_GETPW_R_SIZE_MAX
+	_C__SC_GETGR_R_SIZE_MAX = unix.SC_GETGR_R_SIZE_MAX
+)
+
+func _C_sysconf(key _C_int) _C_long { return unix.Sysconf(key) }
diff --git a/src/os/user/cgo_lookup_unix.go b/src/os/user/cgo_lookup_unix.go
new file mode 100644
index 0000000..3735971
--- /dev/null
+++ b/src/os/user/cgo_lookup_unix.go
@@ -0,0 +1,200 @@
+// Copyright 2011 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.
+
+//go:build (cgo || darwin) && !osusergo && unix && !android
+
+package user
+
+import (
+	"fmt"
+	"runtime"
+	"strconv"
+	"strings"
+	"syscall"
+	"unsafe"
+)
+
+func current() (*User, error) {
+	return lookupUnixUid(syscall.Getuid())
+}
+
+func lookupUser(username string) (*User, error) {
+	var pwd _C_struct_passwd
+	var found bool
+	nameC := make([]byte, len(username)+1)
+	copy(nameC, username)
+
+	err := retryWithBuffer(userBuffer, func(buf []byte) syscall.Errno {
+		var errno syscall.Errno
+		pwd, found, errno = _C_getpwnam_r((*_C_char)(unsafe.Pointer(&nameC[0])),
+			(*_C_char)(unsafe.Pointer(&buf[0])), _C_size_t(len(buf)))
+		return errno
+	})
+	if err != nil {
+		return nil, fmt.Errorf("user: lookup username %s: %v", username, err)
+	}
+	if !found {
+		return nil, UnknownUserError(username)
+	}
+	return buildUser(&pwd), err
+}
+
+func lookupUserId(uid string) (*User, error) {
+	i, e := strconv.Atoi(uid)
+	if e != nil {
+		return nil, e
+	}
+	return lookupUnixUid(i)
+}
+
+func lookupUnixUid(uid int) (*User, error) {
+	var pwd _C_struct_passwd
+	var found bool
+
+	err := retryWithBuffer(userBuffer, func(buf []byte) syscall.Errno {
+		var errno syscall.Errno
+		pwd, found, errno = _C_getpwuid_r(_C_uid_t(uid),
+			(*_C_char)(unsafe.Pointer(&buf[0])), _C_size_t(len(buf)))
+		return errno
+	})
+	if err != nil {
+		return nil, fmt.Errorf("user: lookup userid %d: %v", uid, err)
+	}
+	if !found {
+		return nil, UnknownUserIdError(uid)
+	}
+	return buildUser(&pwd), nil
+}
+
+func buildUser(pwd *_C_struct_passwd) *User {
+	u := &User{
+		Uid:      strconv.FormatUint(uint64(_C_pw_uid(pwd)), 10),
+		Gid:      strconv.FormatUint(uint64(_C_pw_gid(pwd)), 10),
+		Username: _C_GoString(_C_pw_name(pwd)),
+		Name:     _C_GoString(_C_pw_gecos(pwd)),
+		HomeDir:  _C_GoString(_C_pw_dir(pwd)),
+	}
+	// The pw_gecos field isn't quite standardized. Some docs
+	// say: "It is expected to be a comma separated list of
+	// personal data where the first item is the full name of the
+	// user."
+	u.Name, _, _ = strings.Cut(u.Name, ",")
+	return u
+}
+
+func lookupGroup(groupname string) (*Group, error) {
+	var grp _C_struct_group
+	var found bool
+
+	cname := make([]byte, len(groupname)+1)
+	copy(cname, groupname)
+
+	err := retryWithBuffer(groupBuffer, func(buf []byte) syscall.Errno {
+		var errno syscall.Errno
+		grp, found, errno = _C_getgrnam_r((*_C_char)(unsafe.Pointer(&cname[0])),
+			(*_C_char)(unsafe.Pointer(&buf[0])), _C_size_t(len(buf)))
+		return errno
+	})
+	if err != nil {
+		return nil, fmt.Errorf("user: lookup groupname %s: %v", groupname, err)
+	}
+	if !found {
+		return nil, UnknownGroupError(groupname)
+	}
+	return buildGroup(&grp), nil
+}
+
+func lookupGroupId(gid string) (*Group, error) {
+	i, e := strconv.Atoi(gid)
+	if e != nil {
+		return nil, e
+	}
+	return lookupUnixGid(i)
+}
+
+func lookupUnixGid(gid int) (*Group, error) {
+	var grp _C_struct_group
+	var found bool
+
+	err := retryWithBuffer(groupBuffer, func(buf []byte) syscall.Errno {
+		var errno syscall.Errno
+		grp, found, errno = _C_getgrgid_r(_C_gid_t(gid),
+			(*_C_char)(unsafe.Pointer(&buf[0])), _C_size_t(len(buf)))
+		return syscall.Errno(errno)
+	})
+	if err != nil {
+		return nil, fmt.Errorf("user: lookup groupid %d: %v", gid, err)
+	}
+	if !found {
+		return nil, UnknownGroupIdError(strconv.Itoa(gid))
+	}
+	return buildGroup(&grp), nil
+}
+
+func buildGroup(grp *_C_struct_group) *Group {
+	g := &Group{
+		Gid:  strconv.Itoa(int(_C_gr_gid(grp))),
+		Name: _C_GoString(_C_gr_name(grp)),
+	}
+	return g
+}
+
+type bufferKind _C_int
+
+var (
+	userBuffer  = bufferKind(_C__SC_GETPW_R_SIZE_MAX)
+	groupBuffer = bufferKind(_C__SC_GETGR_R_SIZE_MAX)
+)
+
+func (k bufferKind) initialSize() _C_size_t {
+	sz := _C_sysconf(_C_int(k))
+	if sz == -1 {
+		// DragonFly and FreeBSD do not have _SC_GETPW_R_SIZE_MAX.
+		// Additionally, not all Linux systems have it, either. For
+		// example, the musl libc returns -1.
+		return 1024
+	}
+	if !isSizeReasonable(int64(sz)) {
+		// Truncate.  If this truly isn't enough, retryWithBuffer will error on the first run.
+		return maxBufferSize
+	}
+	return _C_size_t(sz)
+}
+
+// retryWithBuffer repeatedly calls f(), increasing the size of the
+// buffer each time, until f succeeds, fails with a non-ERANGE error,
+// or the buffer exceeds a reasonable limit.
+func retryWithBuffer(startSize bufferKind, f func([]byte) syscall.Errno) error {
+	buf := make([]byte, startSize)
+	for {
+		errno := f(buf)
+		if errno == 0 {
+			return nil
+		} else if runtime.GOOS == "aix" && errno+1 == 0 {
+			// On AIX getpwuid_r appears to return -1,
+			// not ERANGE, on buffer overflow.
+		} else if errno != syscall.ERANGE {
+			return errno
+		}
+		newSize := len(buf) * 2
+		if !isSizeReasonable(int64(newSize)) {
+			return fmt.Errorf("internal buffer exceeds %d bytes", maxBufferSize)
+		}
+		buf = make([]byte, newSize)
+	}
+}
+
+const maxBufferSize = 1 << 20
+
+func isSizeReasonable(sz int64) bool {
+	return sz > 0 && sz <= maxBufferSize
+}
+
+// Because we can't use cgo in tests:
+func structPasswdForNegativeTest() _C_struct_passwd {
+	sp := _C_struct_passwd{}
+	*_C_pw_uidp(&sp) = 1<<32 - 2
+	*_C_pw_gidp(&sp) = 1<<32 - 3
+	return sp
+}
diff --git a/src/os/user/cgo_unix_test.go b/src/os/user/cgo_unix_test.go
new file mode 100644
index 0000000..6d16aa2
--- /dev/null
+++ b/src/os/user/cgo_unix_test.go
@@ -0,0 +1,23 @@
+// Copyright 2017 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.
+
+//go:build (darwin || dragonfly || freebsd || (!android && linux) || netbsd || openbsd || solaris) && cgo && !osusergo
+
+package user
+
+import (
+	"testing"
+)
+
+// Issue 22739
+func TestNegativeUid(t *testing.T) {
+	sp := structPasswdForNegativeTest()
+	u := buildUser(&sp)
+	if g, w := u.Uid, "4294967294"; g != w {
+		t.Errorf("Uid = %q; want %q", g, w)
+	}
+	if g, w := u.Gid, "4294967293"; g != w {
+		t.Errorf("Gid = %q; want %q", g, w)
+	}
+}
diff --git a/src/os/user/getgrouplist_syscall.go b/src/os/user/getgrouplist_syscall.go
new file mode 100644
index 0000000..41b64fc
--- /dev/null
+++ b/src/os/user/getgrouplist_syscall.go
@@ -0,0 +1,19 @@
+// 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.
+
+//go:build !osusergo && darwin
+
+package user
+
+import (
+	"internal/syscall/unix"
+)
+
+func getGroupList(name *_C_char, userGID _C_gid_t, gids *_C_gid_t, n *_C_int) _C_int {
+	err := unix.Getgrouplist(name, userGID, gids, n)
+	if err != nil {
+		return -1
+	}
+	return 0
+}
diff --git a/src/os/user/getgrouplist_unix.go b/src/os/user/getgrouplist_unix.go
new file mode 100644
index 0000000..fb482d3
--- /dev/null
+++ b/src/os/user/getgrouplist_unix.go
@@ -0,0 +1,22 @@
+// 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.
+
+//go:build cgo && !osusergo && (dragonfly || freebsd || (!android && linux) || netbsd || openbsd || (solaris && !illumos))
+
+package user
+
+/*
+#include <unistd.h>
+#include <sys/types.h>
+#include <grp.h>
+
+static int mygetgrouplist(const char* user, gid_t group, gid_t* groups, int* ngroups) {
+	return getgrouplist(user, group, groups, ngroups);
+}
+*/
+import "C"
+
+func getGroupList(name *_C_char, userGID _C_gid_t, gids *_C_gid_t, n *_C_int) _C_int {
+	return C.mygetgrouplist(name, userGID, gids, n)
+}
diff --git a/src/os/user/listgroups_stub.go b/src/os/user/listgroups_stub.go
new file mode 100644
index 0000000..4cf808b
--- /dev/null
+++ b/src/os/user/listgroups_stub.go
@@ -0,0 +1,19 @@
+// Copyright 2021 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.
+
+//go:build android || (js && !wasm)
+
+package user
+
+import (
+	"errors"
+)
+
+func init() {
+	groupListImplemented = false
+}
+
+func listGroups(*User) ([]string, error) {
+	return nil, errors.New("user: list groups not implemented")
+}
diff --git a/src/os/user/listgroups_unix.go b/src/os/user/listgroups_unix.go
new file mode 100644
index 0000000..ef366fa
--- /dev/null
+++ b/src/os/user/listgroups_unix.go
@@ -0,0 +1,109 @@
+// Copyright 2021 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.
+
+//go:build ((darwin || dragonfly || freebsd || (js && wasm) || (!android && linux) || netbsd || openbsd || solaris) && ((!cgo && !darwin) || osusergo)) || aix || illumos
+
+package user
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"strconv"
+)
+
+func listGroupsFromReader(u *User, r io.Reader) ([]string, error) {
+	if u.Username == "" {
+		return nil, errors.New("user: list groups: empty username")
+	}
+	primaryGid, err := strconv.Atoi(u.Gid)
+	if err != nil {
+		return nil, fmt.Errorf("user: list groups for %s: invalid gid %q", u.Username, u.Gid)
+	}
+
+	userCommas := []byte("," + u.Username + ",")  // ,john,
+	userFirst := userCommas[1:]                   // john,
+	userLast := userCommas[:len(userCommas)-1]    // ,john
+	userOnly := userCommas[1 : len(userCommas)-1] // john
+
+	// Add primary Gid first.
+	groups := []string{u.Gid}
+
+	rd := bufio.NewReader(r)
+	done := false
+	for !done {
+		line, err := rd.ReadBytes('\n')
+		if err != nil {
+			if err == io.EOF {
+				done = true
+			} else {
+				return groups, err
+			}
+		}
+
+		// Look for username in the list of users. If user is found,
+		// append the GID to the groups slice.
+
+		// There's no spec for /etc/passwd or /etc/group, but we try to follow
+		// the same rules as the glibc parser, which allows comments and blank
+		// space at the beginning of a line.
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 || line[0] == '#' ||
+			// If you search for a gid in a row where the group
+			// name (the first field) starts with "+" or "-",
+			// glibc fails to find the record, and so should we.
+			line[0] == '+' || line[0] == '-' {
+			continue
+		}
+
+		// Format of /etc/group is
+		// 	groupname:password:GID:user_list
+		// for example
+		// 	wheel:x:10:john,paul,jack
+		//	tcpdump:x:72:
+		listIdx := bytes.LastIndexByte(line, ':')
+		if listIdx == -1 || listIdx == len(line)-1 {
+			// No commas, or empty group list.
+			continue
+		}
+		if bytes.Count(line[:listIdx], colon) != 2 {
+			// Incorrect number of colons.
+			continue
+		}
+		list := line[listIdx+1:]
+		// Check the list for user without splitting or copying.
+		if !(bytes.Equal(list, userOnly) || bytes.HasPrefix(list, userFirst) || bytes.HasSuffix(list, userLast) || bytes.Contains(list, userCommas)) {
+			continue
+		}
+
+		// groupname:password:GID
+		parts := bytes.Split(line[:listIdx], colon)
+		if len(parts) != 3 || len(parts[0]) == 0 {
+			continue
+		}
+		gid := string(parts[2])
+		// Make sure it's numeric and not the same as primary GID.
+		numGid, err := strconv.Atoi(gid)
+		if err != nil || numGid == primaryGid {
+			continue
+		}
+
+		groups = append(groups, gid)
+	}
+
+	return groups, nil
+}
+
+func listGroups(u *User) ([]string, error) {
+	f, err := os.Open(groupFile)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	return listGroupsFromReader(u, f)
+}
diff --git a/src/os/user/listgroups_unix_test.go b/src/os/user/listgroups_unix_test.go
new file mode 100644
index 0000000..4fa8b1f
--- /dev/null
+++ b/src/os/user/listgroups_unix_test.go
@@ -0,0 +1,107 @@
+// Copyright 2021 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.
+
+//go:build ((darwin || dragonfly || freebsd || (js && wasm) || (!android && linux) || netbsd || openbsd || solaris) && ((!cgo && !darwin) || osusergo)) || aix || illumos
+
+package user
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+	"testing"
+)
+
+var testGroupFile = `# See the opendirectoryd(8) man page for additional
+# information about Open Directory.
+##
+nobody:*:-2:
+nogroup:*:-1:
+wheel:*:0:root
+emptyid:*::root
+invalidgid:*:notanumber:root
++plussign:*:20:root
+-minussign:*:21:root
+# Next line is invalid (empty group name)
+:*:22:root
+
+daemon:*:1:root
+    indented:*:7:root
+# comment:*:4:found
+     # comment:*:4:found
+kmem:*:2:root
+manymembers:x:777:jill,jody,john,jack,jov,user777
+` + largeGroup()
+
+func largeGroup() (res string) {
+	var b strings.Builder
+	b.WriteString("largegroup:x:1000:user1")
+	for i := 2; i <= 7500; i++ {
+		fmt.Fprintf(&b, ",user%d", i)
+	}
+	return b.String()
+}
+
+var listGroupsTests = []struct {
+	// input
+	in   string
+	user string
+	gid  string
+	// output
+	gids []string
+	err  bool
+}{
+	{in: testGroupFile, user: "root", gid: "0", gids: []string{"0", "1", "2", "7"}},
+	{in: testGroupFile, user: "jill", gid: "33", gids: []string{"33", "777"}},
+	{in: testGroupFile, user: "jody", gid: "34", gids: []string{"34", "777"}},
+	{in: testGroupFile, user: "john", gid: "35", gids: []string{"35", "777"}},
+	{in: testGroupFile, user: "jov", gid: "37", gids: []string{"37", "777"}},
+	{in: testGroupFile, user: "user777", gid: "7", gids: []string{"7", "777", "1000"}},
+	{in: testGroupFile, user: "user1111", gid: "1111", gids: []string{"1111", "1000"}},
+	{in: testGroupFile, user: "user1000", gid: "1000", gids: []string{"1000"}},
+	{in: testGroupFile, user: "user7500", gid: "7500", gids: []string{"1000", "7500"}},
+	{in: testGroupFile, user: "no-such-user", gid: "2345", gids: []string{"2345"}},
+	{in: "", user: "no-such-user", gid: "2345", gids: []string{"2345"}},
+	// Error cases.
+	{in: "", user: "", gid: "2345", err: true},
+	{in: "", user: "joanna", gid: "bad", err: true},
+}
+
+func TestListGroups(t *testing.T) {
+	for _, tc := range listGroupsTests {
+		u := &User{Username: tc.user, Gid: tc.gid}
+		got, err := listGroupsFromReader(u, strings.NewReader(tc.in))
+		if tc.err {
+			if err == nil {
+				t.Errorf("listGroups(%q): got nil; want error", tc.user)
+			}
+			continue // no more checks
+		}
+		if err != nil {
+			t.Errorf("listGroups(%q): got %v error, want nil", tc.user, err)
+			continue // no more checks
+		}
+		checkSameIDs(t, got, tc.gids)
+	}
+}
+
+func checkSameIDs(t *testing.T, got, want []string) {
+	t.Helper()
+	if len(got) != len(want) {
+		t.Errorf("ID list mismatch: got %v; want %v", got, want)
+		return
+	}
+	sort.Strings(got)
+	sort.Strings(want)
+	mismatch := -1
+	for i, g := range want {
+		if got[i] != g {
+			mismatch = i
+			break
+		}
+	}
+	if mismatch != -1 {
+		t.Errorf("ID list mismatch (at index %d): got %v; want %v", mismatch, got, want)
+	}
+}
diff --git a/src/os/user/lookup.go b/src/os/user/lookup.go
new file mode 100644
index 0000000..ed33d0c
--- /dev/null
+++ b/src/os/user/lookup.go
@@ -0,0 +1,70 @@
+// Copyright 2011 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.
+
+package user
+
+import "sync"
+
+const (
+	userFile  = "/etc/passwd"
+	groupFile = "/etc/group"
+)
+
+var colon = []byte{':'}
+
+// Current returns the current user.
+//
+// The first call will cache the current user information.
+// Subsequent calls will return the cached value and will not reflect
+// changes to the current user.
+func Current() (*User, error) {
+	cache.Do(func() { cache.u, cache.err = current() })
+	if cache.err != nil {
+		return nil, cache.err
+	}
+	u := *cache.u // copy
+	return &u, nil
+}
+
+// cache of the current user
+var cache struct {
+	sync.Once
+	u   *User
+	err error
+}
+
+// Lookup looks up a user by username. If the user cannot be found, the
+// returned error is of type UnknownUserError.
+func Lookup(username string) (*User, error) {
+	if u, err := Current(); err == nil && u.Username == username {
+		return u, err
+	}
+	return lookupUser(username)
+}
+
+// LookupId looks up a user by userid. If the user cannot be found, the
+// returned error is of type UnknownUserIdError.
+func LookupId(uid string) (*User, error) {
+	if u, err := Current(); err == nil && u.Uid == uid {
+		return u, err
+	}
+	return lookupUserId(uid)
+}
+
+// LookupGroup looks up a group by name. If the group cannot be found, the
+// returned error is of type UnknownGroupError.
+func LookupGroup(name string) (*Group, error) {
+	return lookupGroup(name)
+}
+
+// LookupGroupId looks up a group by groupid. If the group cannot be found, the
+// returned error is of type UnknownGroupIdError.
+func LookupGroupId(gid string) (*Group, error) {
+	return lookupGroupId(gid)
+}
+
+// GroupIds returns the list of group IDs that the user is a member of.
+func (u *User) GroupIds() ([]string, error) {
+	return listGroups(u)
+}
diff --git a/src/os/user/lookup_android.go b/src/os/user/lookup_android.go
new file mode 100644
index 0000000..0ae31fd
--- /dev/null
+++ b/src/os/user/lookup_android.go
@@ -0,0 +1,25 @@
+// 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.
+
+//go:build android
+
+package user
+
+import "errors"
+
+func lookupUser(string) (*User, error) {
+	return nil, errors.New("user: Lookup not implemented on android")
+}
+
+func lookupUserId(string) (*User, error) {
+	return nil, errors.New("user: LookupId not implemented on android")
+}
+
+func lookupGroup(string) (*Group, error) {
+	return nil, errors.New("user: LookupGroup not implemented on android")
+}
+
+func lookupGroupId(string) (*Group, error) {
+	return nil, errors.New("user: LookupGroupId not implemented on android")
+}
diff --git a/src/os/user/lookup_plan9.go b/src/os/user/lookup_plan9.go
new file mode 100644
index 0000000..c2aabd5
--- /dev/null
+++ b/src/os/user/lookup_plan9.go
@@ -0,0 +1,67 @@
+// Copyright 2013 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.
+
+package user
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+)
+
+// Partial os/user support on Plan 9.
+// Supports Current(), but not Lookup()/LookupId().
+// The latter two would require parsing /adm/users.
+
+func init() {
+	userImplemented = false
+	groupImplemented = false
+	groupListImplemented = false
+}
+
+var (
+	// unused variables (in this implementation)
+	// modified during test to exercise code paths in the cgo implementation.
+	userBuffer  = 0
+	groupBuffer = 0
+)
+
+func current() (*User, error) {
+	ubytes, err := os.ReadFile("/dev/user")
+	if err != nil {
+		return nil, fmt.Errorf("user: %s", err)
+	}
+
+	uname := string(ubytes)
+
+	u := &User{
+		Uid:      uname,
+		Gid:      uname,
+		Username: uname,
+		Name:     uname,
+		HomeDir:  os.Getenv("home"),
+	}
+
+	return u, nil
+}
+
+func lookupUser(username string) (*User, error) {
+	return nil, syscall.EPLAN9
+}
+
+func lookupUserId(uid string) (*User, error) {
+	return nil, syscall.EPLAN9
+}
+
+func lookupGroup(groupname string) (*Group, error) {
+	return nil, syscall.EPLAN9
+}
+
+func lookupGroupId(string) (*Group, error) {
+	return nil, syscall.EPLAN9
+}
+
+func listGroups(*User) ([]string, error) {
+	return nil, syscall.EPLAN9
+}
diff --git a/src/os/user/lookup_stubs.go b/src/os/user/lookup_stubs.go
new file mode 100644
index 0000000..89dfe45
--- /dev/null
+++ b/src/os/user/lookup_stubs.go
@@ -0,0 +1,83 @@
+// Copyright 2011 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.
+
+//go:build (!cgo && !darwin && !windows && !plan9) || android || (osusergo && !windows && !plan9)
+
+package user
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"strconv"
+)
+
+var (
+	// unused variables (in this implementation)
+	// modified during test to exercise code paths in the cgo implementation.
+	userBuffer  = 0
+	groupBuffer = 0
+)
+
+func current() (*User, error) {
+	uid := currentUID()
+	// $USER and /etc/passwd may disagree; prefer the latter if we can get it.
+	// See issue 27524 for more information.
+	u, err := lookupUserId(uid)
+	if err == nil {
+		return u, nil
+	}
+
+	homeDir, _ := os.UserHomeDir()
+	u = &User{
+		Uid:      uid,
+		Gid:      currentGID(),
+		Username: os.Getenv("USER"),
+		Name:     "", // ignored
+		HomeDir:  homeDir,
+	}
+	// On Android, return a dummy user instead of failing.
+	switch runtime.GOOS {
+	case "android":
+		if u.Uid == "" {
+			u.Uid = "1"
+		}
+		if u.Username == "" {
+			u.Username = "android"
+		}
+	}
+	// cgo isn't available, but if we found the minimum information
+	// without it, use it:
+	if u.Uid != "" && u.Username != "" && u.HomeDir != "" {
+		return u, nil
+	}
+	var missing string
+	if u.Username == "" {
+		missing = "$USER"
+	}
+	if u.HomeDir == "" {
+		if missing != "" {
+			missing += ", "
+		}
+		missing += "$HOME"
+	}
+	return u, fmt.Errorf("user: Current requires cgo or %s set in environment", missing)
+}
+
+func currentUID() string {
+	if id := os.Getuid(); id >= 0 {
+		return strconv.Itoa(id)
+	}
+	// Note: Windows returns -1, but this file isn't used on
+	// Windows anyway, so this empty return path shouldn't be
+	// used.
+	return ""
+}
+
+func currentGID() string {
+	if id := os.Getgid(); id >= 0 {
+		return strconv.Itoa(id)
+	}
+	return ""
+}
diff --git a/src/os/user/lookup_unix.go b/src/os/user/lookup_unix.go
new file mode 100644
index 0000000..608d9b2
--- /dev/null
+++ b/src/os/user/lookup_unix.go
@@ -0,0 +1,234 @@
+// 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.
+
+//go:build ((unix && !android) || (js && wasm)) && ((!cgo && !darwin) || osusergo)
+
+package user
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+)
+
+// lineFunc returns a value, an error, or (nil, nil) to skip the row.
+type lineFunc func(line []byte) (v any, err error)
+
+// readColonFile parses r as an /etc/group or /etc/passwd style file, running
+// fn for each row. readColonFile returns a value, an error, or (nil, nil) if
+// the end of the file is reached without a match.
+//
+// readCols is the minimum number of colon-separated fields that will be passed
+// to fn; in a long line additional fields may be silently discarded.
+func readColonFile(r io.Reader, fn lineFunc, readCols int) (v any, err error) {
+	rd := bufio.NewReader(r)
+
+	// Read the file line-by-line.
+	for {
+		var isPrefix bool
+		var wholeLine []byte
+
+		// Read the next line. We do so in chunks (as much as reader's
+		// buffer is able to keep), check if we read enough columns
+		// already on each step and store final result in wholeLine.
+		for {
+			var line []byte
+			line, isPrefix, err = rd.ReadLine()
+
+			if err != nil {
+				// We should return (nil, nil) if EOF is reached
+				// without a match.
+				if err == io.EOF {
+					err = nil
+				}
+				return nil, err
+			}
+
+			// Simple common case: line is short enough to fit in a
+			// single reader's buffer.
+			if !isPrefix && len(wholeLine) == 0 {
+				wholeLine = line
+				break
+			}
+
+			wholeLine = append(wholeLine, line...)
+
+			// Check if we read the whole line (or enough columns)
+			// already.
+			if !isPrefix || bytes.Count(wholeLine, []byte{':'}) >= readCols {
+				break
+			}
+		}
+
+		// There's no spec for /etc/passwd or /etc/group, but we try to follow
+		// the same rules as the glibc parser, which allows comments and blank
+		// space at the beginning of a line.
+		wholeLine = bytes.TrimSpace(wholeLine)
+		if len(wholeLine) == 0 || wholeLine[0] == '#' {
+			continue
+		}
+		v, err = fn(wholeLine)
+		if v != nil || err != nil {
+			return
+		}
+
+		// If necessary, skip the rest of the line
+		for ; isPrefix; _, isPrefix, err = rd.ReadLine() {
+			if err != nil {
+				// We should return (nil, nil) if EOF is reached without a match.
+				if err == io.EOF {
+					err = nil
+				}
+				return nil, err
+			}
+		}
+	}
+}
+
+func matchGroupIndexValue(value string, idx int) lineFunc {
+	var leadColon string
+	if idx > 0 {
+		leadColon = ":"
+	}
+	substr := []byte(leadColon + value + ":")
+	return func(line []byte) (v any, err error) {
+		if !bytes.Contains(line, substr) || bytes.Count(line, colon) < 3 {
+			return
+		}
+		// wheel:*:0:root
+		parts := strings.SplitN(string(line), ":", 4)
+		if len(parts) < 4 || parts[0] == "" || parts[idx] != value ||
+			// If the file contains +foo and you search for "foo", glibc
+			// returns an "invalid argument" error. Similarly, if you search
+			// for a gid for a row where the group name starts with "+" or "-",
+			// glibc fails to find the record.
+			parts[0][0] == '+' || parts[0][0] == '-' {
+			return
+		}
+		if _, err := strconv.Atoi(parts[2]); err != nil {
+			return nil, nil
+		}
+		return &Group{Name: parts[0], Gid: parts[2]}, nil
+	}
+}
+
+func findGroupId(id string, r io.Reader) (*Group, error) {
+	if v, err := readColonFile(r, matchGroupIndexValue(id, 2), 3); err != nil {
+		return nil, err
+	} else if v != nil {
+		return v.(*Group), nil
+	}
+	return nil, UnknownGroupIdError(id)
+}
+
+func findGroupName(name string, r io.Reader) (*Group, error) {
+	if v, err := readColonFile(r, matchGroupIndexValue(name, 0), 3); err != nil {
+		return nil, err
+	} else if v != nil {
+		return v.(*Group), nil
+	}
+	return nil, UnknownGroupError(name)
+}
+
+// returns a *User for a row if that row's has the given value at the
+// given index.
+func matchUserIndexValue(value string, idx int) lineFunc {
+	var leadColon string
+	if idx > 0 {
+		leadColon = ":"
+	}
+	substr := []byte(leadColon + value + ":")
+	return func(line []byte) (v any, err error) {
+		if !bytes.Contains(line, substr) || bytes.Count(line, colon) < 6 {
+			return
+		}
+		// kevin:x:1005:1006::/home/kevin:/usr/bin/zsh
+		parts := strings.SplitN(string(line), ":", 7)
+		if len(parts) < 6 || parts[idx] != value || parts[0] == "" ||
+			parts[0][0] == '+' || parts[0][0] == '-' {
+			return
+		}
+		if _, err := strconv.Atoi(parts[2]); err != nil {
+			return nil, nil
+		}
+		if _, err := strconv.Atoi(parts[3]); err != nil {
+			return nil, nil
+		}
+		u := &User{
+			Username: parts[0],
+			Uid:      parts[2],
+			Gid:      parts[3],
+			Name:     parts[4],
+			HomeDir:  parts[5],
+		}
+		// The pw_gecos field isn't quite standardized. Some docs
+		// say: "It is expected to be a comma separated list of
+		// personal data where the first item is the full name of the
+		// user."
+		u.Name, _, _ = strings.Cut(u.Name, ",")
+		return u, nil
+	}
+}
+
+func findUserId(uid string, r io.Reader) (*User, error) {
+	i, e := strconv.Atoi(uid)
+	if e != nil {
+		return nil, errors.New("user: invalid userid " + uid)
+	}
+	if v, err := readColonFile(r, matchUserIndexValue(uid, 2), 6); err != nil {
+		return nil, err
+	} else if v != nil {
+		return v.(*User), nil
+	}
+	return nil, UnknownUserIdError(i)
+}
+
+func findUsername(name string, r io.Reader) (*User, error) {
+	if v, err := readColonFile(r, matchUserIndexValue(name, 0), 6); err != nil {
+		return nil, err
+	} else if v != nil {
+		return v.(*User), nil
+	}
+	return nil, UnknownUserError(name)
+}
+
+func lookupGroup(groupname string) (*Group, error) {
+	f, err := os.Open(groupFile)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return findGroupName(groupname, f)
+}
+
+func lookupGroupId(id string) (*Group, error) {
+	f, err := os.Open(groupFile)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return findGroupId(id, f)
+}
+
+func lookupUser(username string) (*User, error) {
+	f, err := os.Open(userFile)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return findUsername(username, f)
+}
+
+func lookupUserId(uid string) (*User, error) {
+	f, err := os.Open(userFile)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return findUserId(uid, f)
+}
diff --git a/src/os/user/lookup_unix_test.go b/src/os/user/lookup_unix_test.go
new file mode 100644
index 0000000..78b3392
--- /dev/null
+++ b/src/os/user/lookup_unix_test.go
@@ -0,0 +1,259 @@
+// 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.
+
+//go:build unix && !android && !cgo && !darwin
+
+package user
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+var groupTests = []struct {
+	in   string
+	name string
+	gid  string
+}{
+	{testGroupFile, "nobody", "-2"},
+	{testGroupFile, "kmem", "2"},
+	{testGroupFile, "notinthefile", ""},
+	{testGroupFile, "comment", ""},
+	{testGroupFile, "plussign", ""},
+	{testGroupFile, "+plussign", ""},
+	{testGroupFile, "-minussign", ""},
+	{testGroupFile, "minussign", ""},
+	{testGroupFile, "emptyid", ""},
+	{testGroupFile, "invalidgid", ""},
+	{testGroupFile, "indented", "7"},
+	{testGroupFile, "# comment", ""},
+	{testGroupFile, "largegroup", "1000"},
+	{testGroupFile, "manymembers", "777"},
+	{"", "emptyfile", ""},
+}
+
+func TestFindGroupName(t *testing.T) {
+	for _, tt := range groupTests {
+		got, err := findGroupName(tt.name, strings.NewReader(tt.in))
+		if tt.gid == "" {
+			if err == nil {
+				t.Errorf("findGroupName(%s): got nil error, expected err", tt.name)
+				continue
+			}
+			switch terr := err.(type) {
+			case UnknownGroupError:
+				if terr.Error() != "group: unknown group "+tt.name {
+					t.Errorf("findGroupName(%s): got %v, want %v", tt.name, terr, tt.name)
+				}
+			default:
+				t.Errorf("findGroupName(%s): got unexpected error %v", tt.name, terr)
+			}
+		} else {
+			if err != nil {
+				t.Fatalf("findGroupName(%s): got unexpected error %v", tt.name, err)
+			}
+			if got.Gid != tt.gid {
+				t.Errorf("findGroupName(%s): got gid %v, want %s", tt.name, got.Gid, tt.gid)
+			}
+			if got.Name != tt.name {
+				t.Errorf("findGroupName(%s): got name %s, want %s", tt.name, got.Name, tt.name)
+			}
+		}
+	}
+}
+
+var groupIdTests = []struct {
+	in   string
+	gid  string
+	name string
+}{
+	{testGroupFile, "-2", "nobody"},
+	{testGroupFile, "2", "kmem"},
+	{testGroupFile, "notinthefile", ""},
+	{testGroupFile, "comment", ""},
+	{testGroupFile, "7", "indented"},
+	{testGroupFile, "4", ""},
+	{testGroupFile, "20", ""}, // row starts with a plus
+	{testGroupFile, "21", ""}, // row starts with a minus
+	{"", "emptyfile", ""},
+}
+
+func TestFindGroupId(t *testing.T) {
+	for _, tt := range groupIdTests {
+		got, err := findGroupId(tt.gid, strings.NewReader(tt.in))
+		if tt.name == "" {
+			if err == nil {
+				t.Errorf("findGroupId(%s): got nil error, expected err", tt.gid)
+				continue
+			}
+			switch terr := err.(type) {
+			case UnknownGroupIdError:
+				if terr.Error() != "group: unknown groupid "+tt.gid {
+					t.Errorf("findGroupId(%s): got %v, want %v", tt.name, terr, tt.name)
+				}
+			default:
+				t.Errorf("findGroupId(%s): got unexpected error %v", tt.name, terr)
+			}
+		} else {
+			if err != nil {
+				t.Fatalf("findGroupId(%s): got unexpected error %v", tt.name, err)
+			}
+			if got.Gid != tt.gid {
+				t.Errorf("findGroupId(%s): got gid %v, want %s", tt.name, got.Gid, tt.gid)
+			}
+			if got.Name != tt.name {
+				t.Errorf("findGroupId(%s): got name %s, want %s", tt.name, got.Name, tt.name)
+			}
+		}
+	}
+}
+
+const testUserFile = `   # Example user file
+root:x:0:0:root:/root:/bin/bash
+daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
+bin:x:2:3:bin:/bin:/usr/sbin/nologin
+     indented:x:3:3:indented:/dev:/usr/sbin/nologin
+sync:x:4:65534:sync:/bin:/bin/sync
+negative:x:-5:60:games:/usr/games:/usr/sbin/nologin
+man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
+allfields:x:6:12:mansplit,man2,man3,man4:/home/allfields:/usr/sbin/nologin
++plussign:x:8:10:man:/var/cache/man:/usr/sbin/nologin
+-minussign:x:9:10:man:/var/cache/man:/usr/sbin/nologin
+
+malformed:x:27:12 # more:colons:after:comment
+
+struid:x:notanumber:12 # more:colons:after:comment
+
+# commented:x:28:12:commented:/var/cache/man:/usr/sbin/nologin
+      # commentindented:x:29:12:commentindented:/var/cache/man:/usr/sbin/nologin
+
+struid2:x:30:badgid:struid2name:/home/struid:/usr/sbin/nologin
+`
+
+var userIdTests = []struct {
+	in   string
+	uid  string
+	name string
+}{
+	{testUserFile, "-5", "negative"},
+	{testUserFile, "2", "bin"},
+	{testUserFile, "100", ""}, // not in the file
+	{testUserFile, "8", ""},   // plus sign, glibc doesn't find it
+	{testUserFile, "9", ""},   // minus sign, glibc doesn't find it
+	{testUserFile, "27", ""},  // malformed
+	{testUserFile, "28", ""},  // commented out
+	{testUserFile, "29", ""},  // commented out, indented
+	{testUserFile, "3", "indented"},
+	{testUserFile, "30", ""}, // the Gid is not valid, shouldn't match
+	{"", "1", ""},
+}
+
+func TestInvalidUserId(t *testing.T) {
+	_, err := findUserId("notanumber", strings.NewReader(""))
+	if err == nil {
+		t.Fatalf("findUserId('notanumber'): got nil error")
+	}
+	if want := "user: invalid userid notanumber"; err.Error() != want {
+		t.Errorf("findUserId('notanumber'): got %v, want %s", err, want)
+	}
+}
+
+func TestLookupUserId(t *testing.T) {
+	for _, tt := range userIdTests {
+		got, err := findUserId(tt.uid, strings.NewReader(tt.in))
+		if tt.name == "" {
+			if err == nil {
+				t.Errorf("findUserId(%s): got nil error, expected err", tt.uid)
+				continue
+			}
+			switch terr := err.(type) {
+			case UnknownUserIdError:
+				if want := "user: unknown userid " + tt.uid; terr.Error() != want {
+					t.Errorf("findUserId(%s): got %v, want %v", tt.name, terr, want)
+				}
+			default:
+				t.Errorf("findUserId(%s): got unexpected error %v", tt.name, terr)
+			}
+		} else {
+			if err != nil {
+				t.Fatalf("findUserId(%s): got unexpected error %v", tt.name, err)
+			}
+			if got.Uid != tt.uid {
+				t.Errorf("findUserId(%s): got uid %v, want %s", tt.name, got.Uid, tt.uid)
+			}
+			if got.Username != tt.name {
+				t.Errorf("findUserId(%s): got name %s, want %s", tt.name, got.Username, tt.name)
+			}
+		}
+	}
+}
+
+func TestLookupUserPopulatesAllFields(t *testing.T) {
+	u, err := findUsername("allfields", strings.NewReader(testUserFile))
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := &User{
+		Username: "allfields",
+		Uid:      "6",
+		Gid:      "12",
+		Name:     "mansplit",
+		HomeDir:  "/home/allfields",
+	}
+	if !reflect.DeepEqual(u, want) {
+		t.Errorf("findUsername: got %#v, want %#v", u, want)
+	}
+}
+
+var userTests = []struct {
+	in   string
+	name string
+	uid  string
+}{
+	{testUserFile, "negative", "-5"},
+	{testUserFile, "bin", "2"},
+	{testUserFile, "notinthefile", ""},
+	{testUserFile, "indented", "3"},
+	{testUserFile, "plussign", ""},
+	{testUserFile, "+plussign", ""},
+	{testUserFile, "minussign", ""},
+	{testUserFile, "-minussign", ""},
+	{testUserFile, "   indented", ""},
+	{testUserFile, "commented", ""},
+	{testUserFile, "commentindented", ""},
+	{testUserFile, "malformed", ""},
+	{testUserFile, "# commented", ""},
+	{"", "emptyfile", ""},
+}
+
+func TestLookupUser(t *testing.T) {
+	for _, tt := range userTests {
+		got, err := findUsername(tt.name, strings.NewReader(tt.in))
+		if tt.uid == "" {
+			if err == nil {
+				t.Errorf("lookupUser(%s): got nil error, expected err", tt.uid)
+				continue
+			}
+			switch terr := err.(type) {
+			case UnknownUserError:
+				if want := "user: unknown user " + tt.name; terr.Error() != want {
+					t.Errorf("lookupUser(%s): got %v, want %v", tt.name, terr, want)
+				}
+			default:
+				t.Errorf("lookupUser(%s): got unexpected error %v", tt.name, terr)
+			}
+		} else {
+			if err != nil {
+				t.Fatalf("lookupUser(%s): got unexpected error %v", tt.name, err)
+			}
+			if got.Uid != tt.uid {
+				t.Errorf("lookupUser(%s): got uid %v, want %s", tt.name, got.Uid, tt.uid)
+			}
+			if got.Username != tt.name {
+				t.Errorf("lookupUser(%s): got name %s, want %s", tt.name, got.Username, tt.name)
+			}
+		}
+	}
+}
diff --git a/src/os/user/lookup_windows.go b/src/os/user/lookup_windows.go
new file mode 100644
index 0000000..e64b8ae
--- /dev/null
+++ b/src/os/user/lookup_windows.go
@@ -0,0 +1,392 @@
+// Copyright 2012 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.
+
+package user
+
+import (
+	"fmt"
+	"internal/syscall/windows"
+	"internal/syscall/windows/registry"
+	"syscall"
+	"unsafe"
+)
+
+func isDomainJoined() (bool, error) {
+	var domain *uint16
+	var status uint32
+	err := syscall.NetGetJoinInformation(nil, &domain, &status)
+	if err != nil {
+		return false, err
+	}
+	syscall.NetApiBufferFree((*byte)(unsafe.Pointer(domain)))
+	return status == syscall.NetSetupDomainName, nil
+}
+
+func lookupFullNameDomain(domainAndUser string) (string, error) {
+	return syscall.TranslateAccountName(domainAndUser,
+		syscall.NameSamCompatible, syscall.NameDisplay, 50)
+}
+
+func lookupFullNameServer(servername, username string) (string, error) {
+	s, e := syscall.UTF16PtrFromString(servername)
+	if e != nil {
+		return "", e
+	}
+	u, e := syscall.UTF16PtrFromString(username)
+	if e != nil {
+		return "", e
+	}
+	var p *byte
+	e = syscall.NetUserGetInfo(s, u, 10, &p)
+	if e != nil {
+		return "", e
+	}
+	defer syscall.NetApiBufferFree(p)
+	i := (*syscall.UserInfo10)(unsafe.Pointer(p))
+	return windows.UTF16PtrToString(i.FullName), nil
+}
+
+func lookupFullName(domain, username, domainAndUser string) (string, error) {
+	joined, err := isDomainJoined()
+	if err == nil && joined {
+		name, err := lookupFullNameDomain(domainAndUser)
+		if err == nil {
+			return name, nil
+		}
+	}
+	name, err := lookupFullNameServer(domain, username)
+	if err == nil {
+		return name, nil
+	}
+	// domain worked neither as a domain nor as a server
+	// could be domain server unavailable
+	// pretend username is fullname
+	return username, nil
+}
+
+// getProfilesDirectory retrieves the path to the root directory
+// where user profiles are stored.
+func getProfilesDirectory() (string, error) {
+	n := uint32(100)
+	for {
+		b := make([]uint16, n)
+		e := windows.GetProfilesDirectory(&b[0], &n)
+		if e == nil {
+			return syscall.UTF16ToString(b), nil
+		}
+		if e != syscall.ERROR_INSUFFICIENT_BUFFER {
+			return "", e
+		}
+		if n <= uint32(len(b)) {
+			return "", e
+		}
+	}
+}
+
+// lookupUsernameAndDomain obtains the username and domain for usid.
+func lookupUsernameAndDomain(usid *syscall.SID) (username, domain string, e error) {
+	username, domain, t, e := usid.LookupAccount("")
+	if e != nil {
+		return "", "", e
+	}
+	if t != syscall.SidTypeUser {
+		return "", "", fmt.Errorf("user: should be user account type, not %d", t)
+	}
+	return username, domain, nil
+}
+
+// findHomeDirInRegistry finds the user home path based on the uid.
+func findHomeDirInRegistry(uid string) (dir string, e error) {
+	k, e := registry.OpenKey(registry.LOCAL_MACHINE, `SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList\`+uid, registry.QUERY_VALUE)
+	if e != nil {
+		return "", e
+	}
+	defer k.Close()
+	dir, _, e = k.GetStringValue("ProfileImagePath")
+	if e != nil {
+		return "", e
+	}
+	return dir, nil
+}
+
+// lookupGroupName accepts the name of a group and retrieves the group SID.
+func lookupGroupName(groupname string) (string, error) {
+	sid, _, t, e := syscall.LookupSID("", groupname)
+	if e != nil {
+		return "", e
+	}
+	// https://msdn.microsoft.com/en-us/library/cc245478.aspx#gt_0387e636-5654-4910-9519-1f8326cf5ec0
+	// SidTypeAlias should also be treated as a group type next to SidTypeGroup
+	// and SidTypeWellKnownGroup:
+	// "alias object -> resource group: A group object..."
+	//
+	// Tests show that "Administrators" can be considered of type SidTypeAlias.
+	if t != syscall.SidTypeGroup && t != syscall.SidTypeWellKnownGroup && t != syscall.SidTypeAlias {
+		return "", fmt.Errorf("lookupGroupName: should be group account type, not %d", t)
+	}
+	return sid.String()
+}
+
+// listGroupsForUsernameAndDomain accepts username and domain and retrieves
+// a SID list of the local groups where this user is a member.
+func listGroupsForUsernameAndDomain(username, domain string) ([]string, error) {
+	// Check if both the domain name and user should be used.
+	var query string
+	joined, err := isDomainJoined()
+	if err == nil && joined && len(domain) != 0 {
+		query = domain + `\` + username
+	} else {
+		query = username
+	}
+	q, err := syscall.UTF16PtrFromString(query)
+	if err != nil {
+		return nil, err
+	}
+	var p0 *byte
+	var entriesRead, totalEntries uint32
+	// https://msdn.microsoft.com/en-us/library/windows/desktop/aa370655(v=vs.85).aspx
+	// NetUserGetLocalGroups() would return a list of LocalGroupUserInfo0
+	// elements which hold the names of local groups where the user participates.
+	// The list does not follow any sorting order.
+	//
+	// If no groups can be found for this user, NetUserGetLocalGroups() should
+	// always return the SID of a single group called "None", which
+	// also happens to be the primary group for the local user.
+	err = windows.NetUserGetLocalGroups(nil, q, 0, windows.LG_INCLUDE_INDIRECT, &p0, windows.MAX_PREFERRED_LENGTH, &entriesRead, &totalEntries)
+	if err != nil {
+		return nil, err
+	}
+	defer syscall.NetApiBufferFree(p0)
+	if entriesRead == 0 {
+		return nil, fmt.Errorf("listGroupsForUsernameAndDomain: NetUserGetLocalGroups() returned an empty list for domain: %s, username: %s", domain, username)
+	}
+	entries := (*[1024]windows.LocalGroupUserInfo0)(unsafe.Pointer(p0))[:entriesRead:entriesRead]
+	var sids []string
+	for _, entry := range entries {
+		if entry.Name == nil {
+			continue
+		}
+		sid, err := lookupGroupName(windows.UTF16PtrToString(entry.Name))
+		if err != nil {
+			return nil, err
+		}
+		sids = append(sids, sid)
+	}
+	return sids, nil
+}
+
+func newUser(uid, gid, dir, username, domain string) (*User, error) {
+	domainAndUser := domain + `\` + username
+	name, e := lookupFullName(domain, username, domainAndUser)
+	if e != nil {
+		return nil, e
+	}
+	u := &User{
+		Uid:      uid,
+		Gid:      gid,
+		Username: domainAndUser,
+		Name:     name,
+		HomeDir:  dir,
+	}
+	return u, nil
+}
+
+var (
+	// unused variables (in this implementation)
+	// modified during test to exercise code paths in the cgo implementation.
+	userBuffer  = 0
+	groupBuffer = 0
+)
+
+func current() (*User, error) {
+	t, e := syscall.OpenCurrentProcessToken()
+	if e != nil {
+		return nil, e
+	}
+	defer t.Close()
+	u, e := t.GetTokenUser()
+	if e != nil {
+		return nil, e
+	}
+	pg, e := t.GetTokenPrimaryGroup()
+	if e != nil {
+		return nil, e
+	}
+	uid, e := u.User.Sid.String()
+	if e != nil {
+		return nil, e
+	}
+	gid, e := pg.PrimaryGroup.String()
+	if e != nil {
+		return nil, e
+	}
+	dir, e := t.GetUserProfileDirectory()
+	if e != nil {
+		return nil, e
+	}
+	username, domain, e := lookupUsernameAndDomain(u.User.Sid)
+	if e != nil {
+		return nil, e
+	}
+	return newUser(uid, gid, dir, username, domain)
+}
+
+// lookupUserPrimaryGroup obtains the primary group SID for a user using this method:
+// https://support.microsoft.com/en-us/help/297951/how-to-use-the-primarygroupid-attribute-to-find-the-primary-group-for
+// The method follows this formula: domainRID + "-" + primaryGroupRID
+func lookupUserPrimaryGroup(username, domain string) (string, error) {
+	// get the domain RID
+	sid, _, t, e := syscall.LookupSID("", domain)
+	if e != nil {
+		return "", e
+	}
+	if t != syscall.SidTypeDomain {
+		return "", fmt.Errorf("lookupUserPrimaryGroup: should be domain account type, not %d", t)
+	}
+	domainRID, e := sid.String()
+	if e != nil {
+		return "", e
+	}
+	// If the user has joined a domain use the RID of the default primary group
+	// called "Domain Users":
+	// https://support.microsoft.com/en-us/help/243330/well-known-security-identifiers-in-windows-operating-systems
+	// SID: S-1-5-21domain-513
+	//
+	// The correct way to obtain the primary group of a domain user is
+	// probing the user primaryGroupID attribute in the server Active Directory:
+	// https://msdn.microsoft.com/en-us/library/ms679375(v=vs.85).aspx
+	//
+	// Note that the primary group of domain users should not be modified
+	// on Windows for performance reasons, even if it's possible to do that.
+	// The .NET Developer's Guide to Directory Services Programming - Page 409
+	// https://books.google.bg/books?id=kGApqjobEfsC&lpg=PA410&ots=p7oo-eOQL7&dq=primary%20group%20RID&hl=bg&pg=PA409#v=onepage&q&f=false
+	joined, err := isDomainJoined()
+	if err == nil && joined {
+		return domainRID + "-513", nil
+	}
+	// For non-domain users call NetUserGetInfo() with level 4, which
+	// in this case would not have any network overhead.
+	// The primary group should not change from RID 513 here either
+	// but the group will be called "None" instead:
+	// https://www.adampalmer.me/iodigitalsec/2013/08/10/windows-null-session-enumeration/
+	// "Group 'None' (RID: 513)"
+	u, e := syscall.UTF16PtrFromString(username)
+	if e != nil {
+		return "", e
+	}
+	d, e := syscall.UTF16PtrFromString(domain)
+	if e != nil {
+		return "", e
+	}
+	var p *byte
+	e = syscall.NetUserGetInfo(d, u, 4, &p)
+	if e != nil {
+		return "", e
+	}
+	defer syscall.NetApiBufferFree(p)
+	i := (*windows.UserInfo4)(unsafe.Pointer(p))
+	return fmt.Sprintf("%s-%d", domainRID, i.PrimaryGroupID), nil
+}
+
+func newUserFromSid(usid *syscall.SID) (*User, error) {
+	username, domain, e := lookupUsernameAndDomain(usid)
+	if e != nil {
+		return nil, e
+	}
+	gid, e := lookupUserPrimaryGroup(username, domain)
+	if e != nil {
+		return nil, e
+	}
+	uid, e := usid.String()
+	if e != nil {
+		return nil, e
+	}
+	// If this user has logged in at least once their home path should be stored
+	// in the registry under the specified SID. References:
+	// https://social.technet.microsoft.com/wiki/contents/articles/13895.how-to-remove-a-corrupted-user-profile-from-the-registry.aspx
+	// https://support.asperasoft.com/hc/en-us/articles/216127438-How-to-delete-Windows-user-profiles
+	//
+	// The registry is the most reliable way to find the home path as the user
+	// might have decided to move it outside of the default location,
+	// (e.g. C:\users). Reference:
+	// https://answers.microsoft.com/en-us/windows/forum/windows_7-security/how-do-i-set-a-home-directory-outside-cusers-for-a/aed68262-1bf4-4a4d-93dc-7495193a440f
+	dir, e := findHomeDirInRegistry(uid)
+	if e != nil {
+		// If the home path does not exist in the registry, the user might
+		// have not logged in yet; fall back to using getProfilesDirectory().
+		// Find the username based on a SID and append that to the result of
+		// getProfilesDirectory(). The domain is not relevant here.
+		dir, e = getProfilesDirectory()
+		if e != nil {
+			return nil, e
+		}
+		dir += `\` + username
+	}
+	return newUser(uid, gid, dir, username, domain)
+}
+
+func lookupUser(username string) (*User, error) {
+	sid, _, t, e := syscall.LookupSID("", username)
+	if e != nil {
+		return nil, e
+	}
+	if t != syscall.SidTypeUser {
+		return nil, fmt.Errorf("user: should be user account type, not %d", t)
+	}
+	return newUserFromSid(sid)
+}
+
+func lookupUserId(uid string) (*User, error) {
+	sid, e := syscall.StringToSid(uid)
+	if e != nil {
+		return nil, e
+	}
+	return newUserFromSid(sid)
+}
+
+func lookupGroup(groupname string) (*Group, error) {
+	sid, err := lookupGroupName(groupname)
+	if err != nil {
+		return nil, err
+	}
+	return &Group{Name: groupname, Gid: sid}, nil
+}
+
+func lookupGroupId(gid string) (*Group, error) {
+	sid, err := syscall.StringToSid(gid)
+	if err != nil {
+		return nil, err
+	}
+	groupname, _, t, err := sid.LookupAccount("")
+	if err != nil {
+		return nil, err
+	}
+	if t != syscall.SidTypeGroup && t != syscall.SidTypeWellKnownGroup && t != syscall.SidTypeAlias {
+		return nil, fmt.Errorf("lookupGroupId: should be group account type, not %d", t)
+	}
+	return &Group{Name: groupname, Gid: gid}, nil
+}
+
+func listGroups(user *User) ([]string, error) {
+	sid, err := syscall.StringToSid(user.Uid)
+	if err != nil {
+		return nil, err
+	}
+	username, domain, err := lookupUsernameAndDomain(sid)
+	if err != nil {
+		return nil, err
+	}
+	sids, err := listGroupsForUsernameAndDomain(username, domain)
+	if err != nil {
+		return nil, err
+	}
+	// Add the primary group of the user to the list if it is not already there.
+	// This is done only to comply with the POSIX concept of a primary group.
+	for _, sid := range sids {
+		if sid == user.Gid {
+			return sids, nil
+		}
+	}
+	return append(sids, user.Gid), nil
+}
diff --git a/src/os/user/user.go b/src/os/user/user.go
new file mode 100644
index 0000000..0307d2a
--- /dev/null
+++ b/src/os/user/user.go
@@ -0,0 +1,95 @@
+// Copyright 2011 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.
+
+/*
+Package user allows user account lookups by name or id.
+
+For most Unix systems, this package has two internal implementations of
+resolving user and group ids to names, and listing supplementary group IDs.
+One is written in pure Go and parses /etc/passwd and /etc/group. The other
+is cgo-based and relies on the standard C library (libc) routines such as
+getpwuid_r, getgrnam_r, and getgrouplist.
+
+When cgo is available, and the required routines are implemented in libc
+for a particular platform, cgo-based (libc-backed) code is used.
+This can be overridden by using osusergo build tag, which enforces
+the pure Go implementation.
+*/
+package user
+
+import (
+	"strconv"
+)
+
+// These may be set to false in init() for a particular platform and/or
+// build flags to let the tests know to skip tests of some features.
+var (
+	userImplemented      = true
+	groupImplemented     = true
+	groupListImplemented = true
+)
+
+// User represents a user account.
+type User struct {
+	// Uid is the user ID.
+	// On POSIX systems, this is a decimal number representing the uid.
+	// On Windows, this is a security identifier (SID) in a string format.
+	// On Plan 9, this is the contents of /dev/user.
+	Uid string
+	// Gid is the primary group ID.
+	// On POSIX systems, this is a decimal number representing the gid.
+	// On Windows, this is a SID in a string format.
+	// On Plan 9, this is the contents of /dev/user.
+	Gid string
+	// Username is the login name.
+	Username string
+	// Name is the user's real or display name.
+	// It might be blank.
+	// On POSIX systems, this is the first (or only) entry in the GECOS field
+	// list.
+	// On Windows, this is the user's display name.
+	// On Plan 9, this is the contents of /dev/user.
+	Name string
+	// HomeDir is the path to the user's home directory (if they have one).
+	HomeDir string
+}
+
+// Group represents a grouping of users.
+//
+// On POSIX systems Gid contains a decimal number representing the group ID.
+type Group struct {
+	Gid  string // group ID
+	Name string // group name
+}
+
+// UnknownUserIdError is returned by LookupId when a user cannot be found.
+type UnknownUserIdError int
+
+func (e UnknownUserIdError) Error() string {
+	return "user: unknown userid " + strconv.Itoa(int(e))
+}
+
+// UnknownUserError is returned by Lookup when
+// a user cannot be found.
+type UnknownUserError string
+
+func (e UnknownUserError) Error() string {
+	return "user: unknown user " + string(e)
+}
+
+// UnknownGroupIdError is returned by LookupGroupId when
+// a group cannot be found.
+type UnknownGroupIdError string
+
+func (e UnknownGroupIdError) Error() string {
+	return "group: unknown groupid " + string(e)
+}
+
+// UnknownGroupError is returned by LookupGroup when
+// a group cannot be found.
+type UnknownGroupError string
+
+func (e UnknownGroupError) Error() string {
+	return "group: unknown group " + string(e)
+}
diff --git a/src/os/user/user_test.go b/src/os/user/user_test.go
new file mode 100644
index 0000000..0fa963d
--- /dev/null
+++ b/src/os/user/user_test.go
@@ -0,0 +1,160 @@
+// Copyright 2011 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.
+
+package user
+
+import (
+	"testing"
+)
+
+func checkUser(t *testing.T) {
+	t.Helper()
+	if !userImplemented {
+		t.Skip("user: not implemented; skipping tests")
+	}
+}
+
+func TestCurrent(t *testing.T) {
+	old := userBuffer
+	defer func() {
+		userBuffer = old
+	}()
+	userBuffer = 1 // force use of retry code
+	u, err := Current()
+	if err != nil {
+		t.Fatalf("Current: %v (got %#v)", err, u)
+	}
+	if u.HomeDir == "" {
+		t.Errorf("didn't get a HomeDir")
+	}
+	if u.Username == "" {
+		t.Errorf("didn't get a username")
+	}
+}
+
+func BenchmarkCurrent(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Current()
+	}
+}
+
+func compare(t *testing.T, want, got *User) {
+	if want.Uid != got.Uid {
+		t.Errorf("got Uid=%q; want %q", got.Uid, want.Uid)
+	}
+	if want.Username != got.Username {
+		t.Errorf("got Username=%q; want %q", got.Username, want.Username)
+	}
+	if want.Name != got.Name {
+		t.Errorf("got Name=%q; want %q", got.Name, want.Name)
+	}
+	if want.HomeDir != got.HomeDir {
+		t.Errorf("got HomeDir=%q; want %q", got.HomeDir, want.HomeDir)
+	}
+	if want.Gid != got.Gid {
+		t.Errorf("got Gid=%q; want %q", got.Gid, want.Gid)
+	}
+}
+
+func TestLookup(t *testing.T) {
+	checkUser(t)
+
+	want, err := Current()
+	if err != nil {
+		t.Fatalf("Current: %v", err)
+	}
+	// TODO: Lookup() has a fast path that calls Current() and returns if the
+	// usernames match, so this test does not exercise very much. It would be
+	// good to try and test finding a different user than the current user.
+	got, err := Lookup(want.Username)
+	if err != nil {
+		t.Fatalf("Lookup: %v", err)
+	}
+	compare(t, want, got)
+}
+
+func TestLookupId(t *testing.T) {
+	checkUser(t)
+
+	want, err := Current()
+	if err != nil {
+		t.Fatalf("Current: %v", err)
+	}
+	got, err := LookupId(want.Uid)
+	if err != nil {
+		t.Fatalf("LookupId: %v", err)
+	}
+	compare(t, want, got)
+}
+
+func checkGroup(t *testing.T) {
+	t.Helper()
+	if !groupImplemented {
+		t.Skip("user: group not implemented; skipping test")
+	}
+}
+
+func TestLookupGroup(t *testing.T) {
+	old := groupBuffer
+	defer func() {
+		groupBuffer = old
+	}()
+	groupBuffer = 1 // force use of retry code
+	checkGroup(t)
+	user, err := Current()
+	if err != nil {
+		t.Fatalf("Current(): %v", err)
+	}
+
+	g1, err := LookupGroupId(user.Gid)
+	if err != nil {
+		// NOTE(rsc): Maybe the group isn't defined. That's fine.
+		// On my OS X laptop, rsc logs in with group 5000 even
+		// though there's no name for group 5000. Such is Unix.
+		t.Logf("LookupGroupId(%q): %v", user.Gid, err)
+		return
+	}
+	if g1.Gid != user.Gid {
+		t.Errorf("LookupGroupId(%q).Gid = %s; want %s", user.Gid, g1.Gid, user.Gid)
+	}
+
+	g2, err := LookupGroup(g1.Name)
+	if err != nil {
+		t.Fatalf("LookupGroup(%q): %v", g1.Name, err)
+	}
+	if g1.Gid != g2.Gid || g1.Name != g2.Name {
+		t.Errorf("LookupGroup(%q) = %+v; want %+v", g1.Name, g2, g1)
+	}
+}
+
+func checkGroupList(t *testing.T) {
+	t.Helper()
+	if !groupListImplemented {
+		t.Skip("user: group list not implemented; skipping test")
+	}
+}
+
+func TestGroupIds(t *testing.T) {
+	checkGroupList(t)
+	user, err := Current()
+	if err != nil {
+		t.Fatalf("Current(): %v", err)
+	}
+	gids, err := user.GroupIds()
+	if err != nil {
+		t.Fatalf("%+v.GroupIds(): %v", user, err)
+	}
+	if !containsID(gids, user.Gid) {
+		t.Errorf("%+v.GroupIds() = %v; does not contain user GID %s", user, gids, user.Gid)
+	}
+}
+
+func containsID(ids []string, id string) bool {
+	for _, x := range ids {
+		if x == id {
+			return true
+		}
+	}
+	return false
+}