Rewrite chuser() for simplicity and correctness (#1203)

- Use unambiguous variable names (w/o package name conflict).
- Fail on invalid input such as the empty string or `:`.
- Do not change group without user, i.e. fail on `:group`.
- Parse input using mnemonic APIs.
- Do not juggle between integer types.
- Unset supplementary groups.
- Use set[ug]id(2) to follow the idiom of OpenBSD base programs.
  (cannot use setres[ug]id(2) as macOS does not have them.)

Includes/Supersedes #1202.
Fixes #927.

I only tested on OpenBSD (so far), but other systems should just work.

cmd/yggdrasil/chuser_unix_test.go

@@ -0,0 +1,80 @@
+//go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package main
+
+import (
+	"testing"
+	"os/user"
+)
+
+// Usernames must not contain a number sign.
+func TestEmptyString (t *testing.T) {
+	if chuser("") == nil {
+		t.Fatal("the empty string is not a valid user")
+	}
+}
+
+// Either omit delimiter and group, or omit both.
+func TestEmptyGroup (t *testing.T) {
+	if chuser("0:") == nil {
+		t.Fatal("the empty group is not allowed")
+	}
+}
+
+// Either user only or user and group.
+func TestGroupOnly (t *testing.T) {
+	if chuser(":0") == nil {
+		t.Fatal("group only is not allowed")
+	}
+}
+
+// Usenames must not contain the number sign.
+func TestInvalidUsername (t *testing.T) {
+	const username = "#user"
+	if chuser(username) == nil {
+		t.Fatalf("'%s' is not a valid username", username)
+	}
+}
+
+// User IDs must be non-negative.
+func TestInvalidUserid (t *testing.T) {
+	if chuser("-1") == nil {
+		t.Fatal("User ID cannot be negative")
+	}
+}
+
+// Change to the current user by ID.
+func TestCurrentUserid (t *testing.T) {
+	usr, err := user.Current()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if usr.Uid != "0" {
+		t.Skip("setgroups(2): Only the superuser may set new groups.")
+	}
+
+	if err = chuser(usr.Uid); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// Change to a common user by name.
+func TestCommonUsername (t *testing.T) {
+	usr, err := user.Current()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if usr.Uid != "0" {
+		t.Skip("setgroups(2): Only the superuser may set new groups.")
+	}
+
+	if err := chuser("nobody"); err != nil {
+		if _, ok := err.(user.UnknownUserError); ok {
+			t.Skip(err)
+		}
+		t.Fatal(err)
+	}
+}