login: add public sd_session_get_desktop() API

The desktop brand is stored as DESKTOP variable for sessions. It can be
set arbitrarily by the session owner and identifies the desktop
environment that is running on that session.

src/libsystemd/libsystemd.sym.m4

@@ -153,6 +153,11 @@ global:
         sd_machine_get_ifindices;
 } LIBSYSTEMD_214;
 
+LIBSYSTEMD_217 {
+global:
+        sd_session_get_desktop;
+} LIBSYSTEMD_216;
+
 m4_ifdef(`ENABLE_KDBUS',
 LIBSYSTEMD_FUTURE {
 global:
@@ -438,5 +443,5 @@ global:
         /* sd-path */
         sd_path_home;
         sd_path_search;
-} LIBSYSTEMD_216;
+} LIBSYSTEMD_217;
 )

src/libsystemd/sd-login/sd-login.c

@@ -485,6 +485,25 @@ _public_ int sd_session_get_class(const char *session, char **class) {
         return session_get_string(session, "CLASS", class);
 }
 
+_public_ int sd_session_get_desktop(const char *session, char **desktop) {
+        _cleanup_free_ char *escaped = NULL;
+        char *t;
+        int r;
+
+        assert_return(desktop, -EINVAL);
+
+        r = session_get_string(session, "DESKTOP", &escaped);
+        if (r < 0)
+                return r;
+
+        t = cunescape(escaped);
+        if (!t)
+                return -ENOMEM;
+
+        *desktop = t;
+        return 0;
+}
+
 _public_ int sd_session_get_display(const char *session, char **display) {
         return session_get_string(session, "DISPLAY", display);
 }