char: forbid direct chardevice access for hotswap devices

qemu_chr_fe_get_driver() is unsafe, frontends with hotswap support
should not access CharDriver ptr directly as CharDriver might change.
Signed-off-by: NAnton Nefedov <anton.nefedov@virtuozzo.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1499342940-56739-5-git-send-email-anton.nefedov@virtuozzo.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

chardev/char-fe.c

@@ -179,9 +179,16 @@ void qemu_chr_fe_printf(CharBackend *be, const char *fmt, ...)
 
 Chardev *qemu_chr_fe_get_driver(CharBackend *be)
 {
+    /* this is unsafe for the users that support chardev hotswap */
+    assert(be->chr_be_change == NULL);
     return be->chr;
 }
 
+bool qemu_chr_fe_backend_connected(CharBackend *be)
+{
+    return !!be->chr;
+}
+
 bool qemu_chr_fe_init(CharBackend *b, Chardev *s, Error **errp)
 {
     int tag = 0;

include/chardev/char-fe.h

@@ -46,9 +46,19 @@ void qemu_chr_fe_deinit(CharBackend *b, bool del);
  *
  * Returns the driver associated with a CharBackend or NULL if no
  * associated Chardev.
+ * Note: avoid this function as the driver should never be accessed directly,
+ *       especially by the frontends that support chardevice hotswap.
+ *       Consider qemu_chr_fe_backend_connected() to check for driver existence
  */
 Chardev *qemu_chr_fe_get_driver(CharBackend *be);
 
+/**
+ * @qemu_chr_fe_backend_connected:
+ *
+ * Returns true if there is a chardevice associated with @be.
+ */
+bool qemu_chr_fe_backend_connected(CharBackend *be);
+
 /**
  * @qemu_chr_fe_set_handlers:
  * @b: a CharBackend