ui/clipboard: add asserts for update and request

Should an issue like CVE-2023-6683 ever appear again in the future,
it will be more obvious which assumption was violated.

Suggested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20240124105749.204610-2-f.ebner@proxmox.com>
(cherry picked from commit 9c41658)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

ui/clipboard.c

@@ -65,12 +65,24 @@ bool qemu_clipboard_check_serial(QemuClipboardInfo *info, bool client)
 
 void qemu_clipboard_update(QemuClipboardInfo *info)
 {
+    uint32_t type;
     QemuClipboardNotify notify = {
         .type = QEMU_CLIPBOARD_UPDATE_INFO,
         .info = info,
     };
     assert(info->selection < QEMU_CLIPBOARD_SELECTION__COUNT);
 
+    for (type = 0; type < QEMU_CLIPBOARD_TYPE__COUNT; type++) {
+        /*
+         * If data is missing, the clipboard owner's 'request' callback needs to
+         * be set. Otherwise, there is no way to get the clipboard data and
+         * qemu_clipboard_request() cannot be called.
+         */
+        if (info->types[type].available && !info->types[type].data) {
+            assert(info->owner && info->owner->request);
+        }
+    }
+
     notifier_list_notify(&clipboard_notifiers, &notify);
 
     if (cbinfo[info->selection] != info) {
@@ -132,6 +144,8 @@ void qemu_clipboard_request(QemuClipboardInfo *info,
         !info->owner)
         return;
 
+    assert(info->owner->request);
+
     info->types[type].requested = true;
     info->owner->request(info, type);
 }