Implemente GenerateUIDInRange method

so that we can assign correct ids to real human users:

https://systemd.io/UIDS-GIDS/#special-distribution-uid-ranges

GenerateUID can return 65536 because it just takes all users in
/etc/passwd into account (even non-human ones). So it takes the "nobody"
user (uid 65535) into account.

Signed-off-by: Dimitris Karakasilis <[email protected]>

pkg/users/users.go

@@ -2,6 +2,8 @@
 package users
 
 import (
+	"errors"
+	"fmt"
 	"strconv"
 )
 
@@ -68,6 +70,7 @@ type UserList interface {
 	// GetAll returns all users in the list
 	GetAll() ([]User, error)
 	GenerateUID() int
+	GenerateUIDInRange(int, int) (int, error)
 	LastUID() int
 	SetPath(path string)
 	Load() error
@@ -99,3 +102,31 @@ func (list CommonUserList) GenerateUID() int {
 	}
 	return list.lastUID + 1
 }
+
+// Finds the lowest available uid in the specified range.
+// Returns an error if there is no available uid in that range.
+func (list CommonUserList) GenerateUIDInRange(minimum, maximum int) (int, error) {
+	userSet := make(map[int]struct{})
+	for _, user := range list.users {
+		uid, err := user.UID()
+		if err != nil {
+			return -1, fmt.Errorf("getting user's uid: %w", err)
+		}
+		userSet[uid] = struct{}{}
+	}
+
+	result := -1
+	for i := minimum; i <= maximum; i++ {
+		if _, found := userSet[i]; found {
+			continue // uid in use, skip it
+		}
+		result = i // found a free one, stop here
+		break
+	}
+
+	if result == -1 {
+		return result, errors.New("no available uid in range")
+	}
+
+	return result, nil
+}

pkg/users/users_linux_test.go

@@ -9,6 +9,60 @@ import (
 )
 
 var _ = Describe("LinuxUserList", func() {
+	Describe("GenerateUIDInRange", func() {
+		var file *os.File
+		var err error
+		var list LinuxUserList
+
+		BeforeEach(func() {
+			file, err = os.CreateTemp("", "passwd")
+			Expect(err).ToNot(HaveOccurred())
+			DeferCleanup(func() {
+				defer os.Remove(file.Name())
+			})
+
+			_, err = file.WriteString("root:x:0:0:root:/root:/bin/bash\n")
+			Expect(err).ToNot(HaveOccurred())
+			_, err = file.WriteString("foo:x:1000:1000:foo:/home/foo:/bin/bash\n")
+			Expect(err).ToNot(HaveOccurred())
+			_, err = file.WriteString("foo:x:1001:1000:foo:/home/foo:/bin/bash\n")
+			_, err = file.WriteString("foo:x:1001:1000:foo:/home/foo:/bin/bash\n")
+			Expect(err).ToNot(HaveOccurred())
+
+			list = LinuxUserList{}
+			list.SetPath(file.Name())
+			Expect(list.Load()).ToNot(HaveOccurred())
+		})
+
+		When("a uid is available in the range", func() {
+			var minimum, maximum int
+			BeforeEach(func() {
+				minimum = 1000
+				maximum = 2000
+			})
+
+			It("returns the minimum available uid", func() {
+				r, err := list.GenerateUIDInRange(minimum, maximum)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(r).To(Equal(1002))
+			})
+		})
+
+		When("there is no available uid", func() {
+			var minimum, maximum int
+			BeforeEach(func() {
+				minimum = 1000
+				maximum = 1001
+			})
+
+			It("returns an error", func() {
+				_, err := list.GenerateUIDInRange(minimum, maximum)
+				Expect(err).To(HaveOccurred())
+				Expect(err).To(MatchError("no available uid in range"))
+			})
+		})
+	})
+
 	It("parses a record", func() {
 		rootRecord := `root:x:0:0:root:/root:/bin/bash`