feat(sessions): implement local user session switching functionality

- Core user is logged in tty1 while user two is in tty3, you can now seamlessly switch bewteen them
New IPC options:
- `dms ipc call sessions list`
- `dms switch-user [target]`
- New Powermenu switch users option

core/cmd/dms/commands_session.go

@@ -0,0 +1,187 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"os/exec"
+	"sort"
+	"strings"
+
+	"github.com/AvengeMedia/DankMaterialShell/core/internal/log"
+	"github.com/spf13/cobra"
+)
+
+var switchUserCmd = &cobra.Command{
+	Use:   "switch-user [target]",
+	Short: "Switch to another active session on this seat",
+	Long: `Switch the active VT to another running session.
+
+With no target, prints the list of switchable sessions. Pass a username or a
+numeric session ID to switch directly. Requires the target to already be a
+running session on the same seat (use the greeter for a fresh login).`,
+	Args: cobra.MaximumNArgs(1),
+	Run:  runSwitchUser,
+}
+
+type sessionInfo struct {
+	ID      string
+	Name    string
+	Seat    string
+	TTY     string
+	Type    string
+	Class   string
+	Active  bool
+	State   string
+	Current bool
+}
+
+func runSwitchUser(cmd *cobra.Command, args []string) {
+	currentID := os.Getenv("XDG_SESSION_ID")
+	sessions, err := listSessions(currentID)
+	if err != nil {
+		log.Fatalf("%v", err)
+	}
+
+	switchable := make([]sessionInfo, 0, len(sessions))
+	for _, s := range sessions {
+		if s.Class != "user" || s.State == "closing" || s.Current {
+			continue
+		}
+		switchable = append(switchable, s)
+	}
+
+	if len(args) == 0 {
+		if len(switchable) == 0 {
+			fmt.Println("No other active sessions on this seat.")
+			return
+		}
+		printSessions(switchable)
+		return
+	}
+
+	target := args[0]
+	picked, err := pickSession(switchable, target)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		if len(switchable) == 0 {
+			fmt.Fprintln(os.Stderr, "No other active sessions on this seat. Only already-running sessions can be switched to.")
+		} else {
+			fmt.Fprintln(os.Stderr, "\nSwitchable sessions:")
+			printSessions(switchable)
+		}
+		os.Exit(1)
+	}
+
+	if err := activateSession(picked.ID); err != nil {
+		log.Fatalf("loginctl activate %s: %v", picked.ID, err)
+	}
+}
+
+func listSessions(currentID string) ([]sessionInfo, error) {
+	listOut, err := exec.Command("loginctl", "list-sessions", "--no-legend").Output()
+	if err != nil {
+		return nil, fmt.Errorf("loginctl list-sessions: %w", err)
+	}
+
+	var ids []string
+	scanner := bufio.NewScanner(strings.NewReader(string(listOut)))
+	for scanner.Scan() {
+		fields := strings.Fields(scanner.Text())
+		if len(fields) == 0 {
+			continue
+		}
+		ids = append(ids, fields[0])
+	}
+
+	out := make([]sessionInfo, 0, len(ids))
+	for _, id := range ids {
+		s, err := showSession(id)
+		if err != nil {
+			continue
+		}
+		s.Current = currentID != "" && s.ID == currentID
+		out = append(out, s)
+	}
+	sort.SliceStable(out, func(i, j int) bool {
+		if out[i].Name != out[j].Name {
+			return out[i].Name < out[j].Name
+		}
+		return out[i].ID < out[j].ID
+	})
+	return out, nil
+}
+
+func showSession(id string) (sessionInfo, error) {
+	out, err := exec.Command("loginctl", "show-session", id,
+		"-p", "Id", "-p", "Name", "-p", "Seat", "-p", "TTY",
+		"-p", "Type", "-p", "Class", "-p", "Active", "-p", "State").Output()
+	if err != nil {
+		return sessionInfo{}, err
+	}
+	fields := map[string]string{}
+	for _, line := range strings.Split(string(out), "\n") {
+		idx := strings.IndexByte(line, '=')
+		if idx <= 0 {
+			continue
+		}
+		fields[line[:idx]] = line[idx+1:]
+	}
+	if fields["Id"] == "" {
+		return sessionInfo{}, fmt.Errorf("session %s: no Id", id)
+	}
+	return sessionInfo{
+		ID:     fields["Id"],
+		Name:   fields["Name"],
+		Seat:   fields["Seat"],
+		TTY:    fields["TTY"],
+		Type:   fields["Type"],
+		Class:  fields["Class"],
+		Active: fields["Active"] == "yes",
+		State:  fields["State"],
+	}, nil
+}
+
+func pickSession(sessions []sessionInfo, target string) (sessionInfo, error) {
+	for _, s := range sessions {
+		if s.ID == target {
+			return s, nil
+		}
+	}
+	matches := make([]sessionInfo, 0, 2)
+	for _, s := range sessions {
+		if s.Name == target {
+			matches = append(matches, s)
+		}
+	}
+	if len(matches) == 1 {
+		return matches[0], nil
+	}
+	if len(matches) > 1 {
+		ids := make([]string, len(matches))
+		for i, m := range matches {
+			ids[i] = m.ID
+		}
+		return sessionInfo{}, fmt.Errorf("%s has multiple active sessions (%s); pass a session ID instead", target, strings.Join(ids, ", "))
+	}
+	return sessionInfo{}, fmt.Errorf("no switchable session matches %q", target)
+}
+
+func activateSession(id string) error {
+	return exec.Command("loginctl", "activate", id).Run()
+}
+
+func printSessions(sessions []sessionInfo) {
+	fmt.Printf("%-6s %-12s %-8s %-8s %-8s\n", "ID", "USER", "TYPE", "SEAT", "TTY")
+	for _, s := range sessions {
+		tty := s.TTY
+		if tty == "" {
+			tty = "-"
+		}
+		seat := s.Seat
+		if seat == "" {
+			seat = "-"
+		}
+		fmt.Printf("%-6s %-12s %-8s %-8s %-8s\n", s.ID, s.Name, s.Type, seat, tty)
+	}
+}