greeter: Add support for Debian greetd user/group name (#1685)

* greeter: Detect user and group used by greetd

On most distros greetd runs as user and group "greeter",
but on Debian the user and group "_greetd" are used.

* greeter: Use correct group in sync command

* greeter: more generic group detection

---------

Co-authored-by: bbedward <bbedward@gmail.com>

core/internal/utils/group.go

@@ -0,0 +1,37 @@
+package utils
+
+import (
+	"os"
+	"strings"
+)
+
+func HasGroup(groupName string) bool {
+	return HasGroupIn(groupName, "/etc/group")
+}
+
+func HasGroupIn(groupName, path string) bool {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return false
+	}
+	return HasGroupData(groupName, string(data))
+}
+
+func HasGroupData(groupName, data string) bool {
+	prefix := groupName + ":"
+	for line := range strings.SplitSeq(data, "\n") {
+		if strings.HasPrefix(line, prefix) {
+			return true
+		}
+	}
+	return false
+}
+
+func FindGroupData(data string, candidates ...string) (string, bool) {
+	for _, candidate := range candidates {
+		if HasGroupData(candidate, data) {
+			return candidate, true
+		}
+	}
+	return "", false
+}

core/internal/utils/group_test.go

@@ -0,0 +1,142 @@
+package utils
+
+import "testing"
+
+const testGroupData = `root:x:0:brltty,root
+sys:x:3:bin,testuser
+mem:x:8:
+ftp:x:11:
+mail:x:12:
+log:x:19:
+smmsp:x:25:
+proc:x:26:
+games:x:50:
+lock:x:54:
+network:x:90:
+floppy:x:94:
+scanner:x:96:
+power:x:98:
+nobody:x:65534:
+adm:x:999:daemon
+wheel:x:998:testuser
+utmp:x:997:
+audio:x:996:brltty
+disk:x:995:
+input:x:994:brltty,testuser,greeter
+kmem:x:993:
+kvm:x:992:libvirt-qemu,qemu,testuser
+lp:x:991:cups,testuser
+optical:x:990:
+render:x:989:
+sgx:x:988:
+storage:x:987:
+tty:x:5:brltty
+uucp:x:986:brltty
+video:x:985:cosmic-greeter,greeter,testuser
+users:x:984:
+groups:x:983:
+systemd-journal:x:982:
+rfkill:x:981:
+bin:x:1:daemon
+daemon:x:2:bin
+http:x:33:
+dbus:x:81:
+systemd-coredump:x:980:
+systemd-network:x:979:
+systemd-oom:x:978:
+systemd-journal-remote:x:977:
+systemd-resolve:x:976:
+systemd-timesync:x:975:
+tss:x:974:
+uuidd:x:973:
+alpm:x:972:
+polkitd:x:102:
+testuser:x:1000:
+avahi:x:971:
+git:x:970:
+nvidia-persistenced:x:143:
+i2c:x:969:testuser
+seat:x:968:
+rtkit:x:133:
+brlapi:x:967:brltty
+gdm:x:120:
+brltty:x:966:
+colord:x:965:
+flatpak:x:964:
+geoclue:x:963:testuser
+gnome-remote-desktop:x:962:
+saned:x:961:
+usbmux:x:140:
+cosmic-greeter:x:960:
+greeter:x:959:testuser
+openvpn:x:958:
+nm-openvpn:x:957:
+named:x:40:
+_talkd:x:956:
+keyd:x:955:
+cups:x:209:testuser
+docker:x:954:testuser
+mysql:x:953:
+radicale:x:952:
+onepassword:x:1001:
+nixbld:x:951:nixbld01,nixbld02,nixbld03,nixbld04,nixbld05,nixbld06,nixbld07,nixbld08,nixbld09,nixbld10
+virtlogin:x:940:
+libvirt:x:939:testuser
+libvirt-qemu:x:938:
+qemu:x:937:
+dnsmasq:x:936:
+clock:x:935:
+dms-greeter:x:1002:greeter,testuser
+pcscd:x:934:
+test:x:1003:
+empower:x:933:
+`
+
+func TestHasGroupData(t *testing.T) {
+	tests := []struct {
+		group string
+		want  bool
+	}{
+		{"greeter", true},
+		{"root", true},
+		{"docker", true},
+		{"cosmic-greeter", true},
+		{"dms-greeter", true},
+		{"nonexistent", false},
+		{"greet", false},
+	}
+
+	for _, tt := range tests {
+		if got := HasGroupData(tt.group, testGroupData); got != tt.want {
+			t.Errorf("HasGroupData(%q) = %v, want %v", tt.group, got, tt.want)
+		}
+	}
+}
+
+func TestFindGroupData(t *testing.T) {
+	tests := []struct {
+		name       string
+		candidates []string
+		wantGroup  string
+		wantFound  bool
+	}{
+		{"first match wins", []string{"greeter", "greetd", "_greeter"}, "greeter", true},
+		{"fallback to second", []string{"greetd", "greeter"}, "greeter", true},
+		{"none found", []string{"_greetd", "greetd"}, "", false},
+		{"single match", []string{"docker"}, "docker", true},
+	}
+
+	for _, tt := range tests {
+		got, found := FindGroupData(testGroupData, tt.candidates...)
+		if got != tt.wantGroup || found != tt.wantFound {
+			t.Errorf("%s: FindGroupData(%v) = (%q, %v), want (%q, %v)",
+				tt.name, tt.candidates, got, found, tt.wantGroup, tt.wantFound)
+		}
+	}
+}
+
+func TestHasGroupDataEmpty(t *testing.T) {
+	if HasGroupData("greeter", "") {
+		t.Error("expected false for empty data")
+	}
+}