add os/user.Lookup

Signed-off-by: leongross <[email protected]>

Author: leongross

src/os/user/lookup_unix.go

@@ -0,0 +1,45 @@
+// Copyright 2024 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 linux && unix && !baremetal && !js && !wasi
+// +build linux,unix,!baremetal,!js,!wasi
+
+package user
+
+import (
+	"bufio"
+	"os"
+	"strings"
+)
+
+func lookupUser(username string) (*User, error) {
+	f, err := os.Open("/etc/passwd")
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	// parse file format <username>:<password>:<uid>:<gid>:<gecos>:<home>:<shell>
+	lines := bufio.NewScanner(f)
+	for lines.Scan() {
+		line := lines.Text()
+		fragments := strings.Split(line, ":")
+
+		if len(fragments) < 7 {
+			continue
+		}
+
+		if fragments[0] == username {
+			return &User{
+				Uid:      fragments[2],
+				Gid:      fragments[3],
+				Username: fragments[0],
+				Name:     fragments[4],
+				HomeDir:  fragments[5],
+			}, nil
+		}
+	}
+
+	return nil, UnknownUserError(username)
+}

src/os/user/user.go

@@ -31,11 +31,27 @@ type User struct {
 	HomeDir string
 }
 
+// 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)
+}
+
 // 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.
+// TODO: implement syscall.Getuid() and syscall.Getgid() to get the current user.
 func Current() (*User, error) {
 	return nil, errors.New("user: Current not implemented")
 }
+
+// Lookup looks up a user by username.
+//
+// If the user cannot be found, the returned error is of type UnknownUserError.
+// NOTE: This implementation does not support caching as the golang implementation does.
+func Lookup(username string) (*User, error) {
+	return lookupUser(username)
+}

src/os/user_test.go

@@ -0,0 +1,68 @@
+// Copyright 2024 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 !baremetal && !js && !wasip1
+
+package os_test
+
+import (
+	. "os/user"
+	"runtime"
+	"testing"
+)
+
+// NOTE: This test requires some users to be present in the CI environment.
+// If the test fails, it may be because the users are not present.
+// We can guarantee that the root user is present on all systems.
+// Currently we can also guarantee that the user with UID 1001 and GID 127 is present on all systems.
+func TestUserLookup(t *testing.T) {
+	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
+		t.Skip()
+	}
+
+	testCases := map[string]struct {
+		user    User
+		wantErr bool
+	}{
+		"root": {
+			wantErr: false,
+			user: User{
+				Uid:      "0",
+				Gid:      "0",
+				Username: "root",
+				Name:     "root",
+				HomeDir:  "/root",
+			},
+		},
+		"user-" + runtime.GOOS: {
+			wantErr: false,
+			user: User{
+				Uid:      "1001",
+				Gid:      "127",
+				Username: "user",
+				Name:     "user",
+				HomeDir:  "/home/user",
+			},
+		},
+		"error": {
+			wantErr: true,
+			user: User{
+				Uid:      "1000000",
+				Gid:      "1000000",
+				Username: "nonexistentuser",
+				Name:     "nonexistentuser",
+				HomeDir:  "/home/nonexistentuser",
+			},
+		},
+	}
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			user, err := Lookup(tc.user.Username)
+			if (err != nil && !tc.wantErr) || (err == nil && tc.wantErr) {
+				t.Fatalf("Lookup(%q) = %v; want %v, got error %v", tc.user.Username, user, tc.user, err)
+			}
+		})
+	}
+}