fix(greeter): avoid pinning auto-login session commands

- Store a desktop session identity instead of relying on raw Exec commands
- Resolve the current session desktop file when auto-login launches
- Preserve legacy memory compatibility while ignoring stale lastSessionExec
- Add regression coverage for stale /nix/store session paths
- Autologin users should rerun the process

core/internal/greeter/session_launcher.go

@@ -0,0 +1,122 @@
+package greeter
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"syscall"
+)
+
+func sessionDesktopIDFromPath(path string) string {
+	id := strings.TrimSpace(path)
+	if id == "" {
+		return ""
+	}
+	if strings.ContainsAny(id, "/\\") {
+		id = filepath.Base(id)
+	}
+	if id == "" {
+		return ""
+	}
+	if !strings.HasSuffix(id, ".desktop") {
+		id += ".desktop"
+	}
+	return id
+}
+
+func sessionDesktopIDFromMemory(mem greeterAutoLoginMemory) string {
+	if id := sessionDesktopIDFromPath(mem.LastSessionDesktopID); id != "" {
+		return id
+	}
+	return sessionDesktopIDFromPath(mem.LastSessionID)
+}
+
+func sessionDesktopDirs() []string {
+	seen := make(map[string]bool)
+	dirs := make([]string, 0, 8)
+
+	addBase := func(base string) {
+		base = strings.TrimSpace(base)
+		if base == "" {
+			return
+		}
+		for _, sub := range []string{"wayland-sessions", "xsessions"} {
+			dir := filepath.Join(base, sub)
+			if seen[dir] {
+				continue
+			}
+			seen[dir] = true
+			dirs = append(dirs, dir)
+		}
+	}
+
+	if dataHome := os.Getenv("XDG_DATA_HOME"); dataHome != "" {
+		addBase(dataHome)
+	} else if home, err := os.UserHomeDir(); err == nil && home != "" {
+		addBase(filepath.Join(home, ".local", "share"))
+	}
+
+	if dataDirs := os.Getenv("XDG_DATA_DIRS"); dataDirs != "" {
+		for _, dir := range strings.Split(dataDirs, ":") {
+			addBase(dir)
+		}
+	} else {
+		addBase("/usr/local/share")
+		addBase("/usr/share")
+	}
+
+	return dirs
+}
+
+func ResolveSessionExec(sessionID string) (string, error) {
+	return resolveSessionExecInDirs(sessionID, sessionDesktopDirs())
+}
+
+func resolveSessionExecInDirs(sessionID string, dirs []string) (string, error) {
+	id := sessionDesktopIDFromPath(sessionID)
+	if id == "" {
+		return "", fmt.Errorf("session id is empty")
+	}
+
+	for _, dir := range dirs {
+		path := filepath.Join(dir, id)
+		execLine, err := execFromDesktopFile(path)
+		if err == nil {
+			return execLine, nil
+		}
+		if !os.IsNotExist(err) {
+			return "", err
+		}
+	}
+
+	return "", fmt.Errorf("session desktop file %q was not found", id)
+}
+
+func LaunchSessionByID(sessionID string) error {
+	execLine, err := ResolveSessionExec(sessionID)
+	if err != nil {
+		return err
+	}
+	execLine = strings.TrimSpace(stripDesktopExecCodes(execLine))
+	if execLine == "" {
+		return fmt.Errorf("session %q has an empty Exec command", sessionID)
+	}
+
+	env := append(os.Environ(), "XDG_SESSION_TYPE=wayland")
+	return syscall.Exec("/bin/sh", []string{"sh", "-c", "exec " + execLine}, env)
+}
+
+func LaunchSessionFromMemory(cacheDir, homeDir string) error {
+	enabled, _, sessionID, err := resolveGreeterAutoLoginState(cacheDir, homeDir)
+	if err != nil {
+		return err
+	}
+	if !enabled {
+		return fmt.Errorf("greeter auto-login is disabled")
+	}
+	if sessionID == "" {
+		return fmt.Errorf("greeter auto-login has no remembered session")
+	}
+	return LaunchSessionByID(sessionID)
+}