os/user: add Go implementation of LookupGroup, LookupGroupId

If cgo is not available, parse /etc/group in Go to find the name/gid
we need. This does not consult the Network Information System (NIS),
/etc/nsswitch.conf or any other libc extensions to /etc/group.

Fixes #18102.

Change-Id: I6ae4fe0e2c899396c45cdf243d5483113932657c
Reviewed-on: https://go-review.googlesource.com/33713Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/user/lookup_stubs.go

@@ -54,14 +54,6 @@ func lookupUserId(uid string) (*User, error) {
 	return nil, errors.New("user: LookupId requires cgo")
 }
 
-func lookupGroup(groupname string) (*Group, error) {
-	return nil, errors.New("user: LookupGroup requires cgo")
-}
-
-func lookupGroupId(string) (*Group, error) {
-	return nil, errors.New("user: LookupGroupId requires cgo")
-}
-
 func listGroups(*User) ([]string, error) {
 	return nil, errors.New("user: GroupIds requires cgo")
 }

src/os/user/lookup_unix.go

+// 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 darwin dragonfly freebsd !android,linux nacl netbsd openbsd solaris
+// +build !cgo
+
+package user
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"os"
+	"strings"
+)
+
+const groupFile = "/etc/group"
+
+var colon = []byte{':'}
+
+func init() {
+	groupImplemented = false
+}
+
+func findGroupId(id string, r io.Reader) (*Group, error) {
+	bs := bufio.NewScanner(r)
+	substr := []byte(":" + id)
+	for bs.Scan() {
+		lineBytes := bs.Bytes()
+		if !bytes.Contains(lineBytes, substr) || bytes.Count(lineBytes, colon) < 3 {
+			continue
+		}
+		text := strings.TrimSpace(removeComment(string(lineBytes)))
+		// wheel:*:0:root
+		parts := strings.SplitN(text, ":", 4)
+		if len(parts) < 4 {
+			continue
+		}
+		if parts[2] == id {
+			return &Group{Name: parts[0], Gid: parts[2]}, nil
+		}
+	}
+	if err := bs.Err(); err != nil {
+		return nil, err
+	}
+	return nil, UnknownGroupIdError(id)
+}
+
+func findGroupName(name string, r io.Reader) (*Group, error) {
+	bs := bufio.NewScanner(r)
+	substr := []byte(name + ":")
+	for bs.Scan() {
+		lineBytes := bs.Bytes()
+		if !bytes.Contains(lineBytes, substr) || bytes.Count(lineBytes, colon) < 3 {
+			continue
+		}
+		text := strings.TrimSpace(removeComment(string(lineBytes)))
+		// wheel:*:0:root
+		parts := strings.SplitN(text, ":", 4)
+		if len(parts) < 4 {
+			continue
+		}
+		if parts[0] == name && parts[2] != "" {
+			return &Group{Name: parts[0], Gid: parts[2]}, nil
+		}
+	}
+	if err := bs.Err(); err != nil {
+		return nil, err
+	}
+	return nil, UnknownGroupError(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)
+}
+
+// removeComment returns line, removing any '#' byte and any following
+// bytes.
+func removeComment(line string) string {
+	if i := strings.Index(line, "#"); i != -1 {
+		return line[:i]
+	}
+	return line
+}

src/os/user/lookup_unix_test.go

+// 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 darwin dragonfly freebsd !android,linux nacl netbsd openbsd solaris
+// +build !cgo
+
+package user
+
+import (
+	"strings"
+	"testing"
+)
+
+const testGroupFile = `# See the opendirectoryd(8) man page for additional 
+# information about Open Directory.
+##
+nobody:*:-2:
+nogroup:*:-1:
+wheel:*:0:root
+emptyid:*::root
+      
+daemon:*:1:root
+    indented:*:7:
+# comment:*:4:found
+     # comment:*:4:found
+kmem:*:2:root
+`
+
+var groupTests = []struct {
+	in   string
+	name string
+	gid  string
+}{
+	{testGroupFile, "nobody", "-2"},
+	{testGroupFile, "kmem", "2"},
+	{testGroupFile, "notinthefile", ""},
+	{testGroupFile, "comment", ""},
+	{testGroupFile, "emptyid", ""},
+	{testGroupFile, "indented", "7"},
+	{testGroupFile, "# comment", ""},
+	{"", "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", ""},
+	{"", "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)
+			}
+		}
+	}
+}