feat: add inherited sessions and desktop session teardown

programs/src/desktop/main.cpp

@@ -895,6 +895,11 @@ static bool existing_desktop_running() {
 }
 
 extern "C" void _start() {
+    // External applications and all of their descendants inherit this stable
+    // desktop-session ID. login.elf uses it to tear down the complete session
+    // after Log Out, Shutdown, or Reboot returns control to the supervisor.
+    montauk::setsession();
+
     if (existing_desktop_running()) {
         gui::dialogs::message_box(
             "Desktop Already Running",

programs/src/login/login.hpp

@@ -40,6 +40,8 @@ enum SessionMode {
     SESSION_CONSOLE,
 };
 
+void terminate_desktop_session(int session_id);
+
 struct LoginState {
     gui::Framebuffer fb;
     int screen_w, screen_h;

programs/src/login/login_input.cpp

@@ -63,6 +63,9 @@ void launch_session(LoginState* ls) {
     if (pid >= 0) {
         montauk::setuser(pid, ls->username);
         montauk::waitpid(pid);
+        if (ls->session_mode == SESSION_DESKTOP) {
+            terminate_desktop_session(pid);
+        }
     }
 
     // The desktop hands off a graceful power-off by posting a request and
@@ -99,6 +102,21 @@ gui::Rect login_field_rect(const LoginLayout& lo, LoginState* ls, int idx) {
 
 } // namespace
 
+void terminate_desktop_session(int session_id) {
+    if (session_id < 0) return;
+
+    // Killing is asynchronous because a member may currently be executing on
+    // another CPU or inside a syscall. Repeat until the kernel observes the
+    // session empty; no member can spawn another process after that point.
+    //
+
+    const int timeout_ms = 2000;
+    for (int waited = 0; waited < timeout_ms; waited++) {
+        if (montauk::killsession(session_id) <= 0) return;
+        montauk::sleep_ms(1);
+    }
+}
+
 LoginField get_field(LoginState* ls, int field) {
     if (ls->mode == MODE_FIRST_BOOT) {
         switch (field) {

programs/src/login/main.cpp

@@ -29,6 +29,7 @@ static void maybe_run_setup_session() {
     if (pid >= 0) {
         montauk::setuser(pid, user);
         montauk::waitpid(pid);
+        terminate_desktop_session(pid);
     }
 
     montauk::user::clear_session();