From: Thomas Huth <thuth@redhat.com>

QEMU currently crashes when there is a [vnc] section in the config
file that does not have an "id = ..." line:

 $ echo "[vnc]" > /tmp/qemu.conf
 $ ./qemu-system-x86_64 -readconfig /tmp/qemu.conf
 qemu-system-x86_64: ../../devel/qemu/ui/vnc.c:4347: vnc_init_func:
  Assertion `id' failed.
 Aborted (core dumped)

The required "id" is only set up automatically while parsing the command
line, but not when reading the options from the config file.
Thus let's move code that automatically adds the id (if it does not
exist yet) to the init function that needs the id for the first time,
replacing the assert() statement there.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2836
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250821145130.845104-1-thuth@redhat.com>
(cherry picked from commit 38dd513263d814dc3cf554b899c118a46ca77577)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/ui/vnc.c b/ui/vnc.c
index a6bf8442d5..6a26f05daa 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -4265,8 +4265,9 @@ void vnc_display_add_client(const char *id, int csock, bool skipauth)
     }
 }
 
-static void vnc_auto_assign_id(QemuOptsList *olist, QemuOpts *opts)
+static char *vnc_auto_assign_id(QemuOpts *opts)
 {
+    QemuOptsList *olist = qemu_find_opts("vnc");
     int i = 2;
     char *id;
 
@@ -4276,23 +4277,18 @@ static void vnc_auto_assign_id(QemuOptsList *olist, QemuOpts *opts)
         id = g_strdup_printf("vnc%d", i++);
     }
     qemu_opts_set_id(opts, id);
+
+    return id;
 }
 
 void vnc_parse(const char *str)
 {
     QemuOptsList *olist = qemu_find_opts("vnc");
     QemuOpts *opts = qemu_opts_parse_noisily(olist, str, !is_help_option(str));
-    const char *id;
 
     if (!opts) {
         exit(1);
     }
-
-    id = qemu_opts_id(opts);
-    if (!id) {
-        /* auto-assign id if not present */
-        vnc_auto_assign_id(olist, opts);
-    }
 }
 
 int vnc_init_func(void *opaque, QemuOpts *opts, Error **errp)
@@ -4300,7 +4296,11 @@ int vnc_init_func(void *opaque, QemuOpts *opts, Error **errp)
     Error *local_err = NULL;
     char *id = (char *)qemu_opts_id(opts);
 
-    assert(id);
+    if (!id) {
+        /* auto-assign id if not present */
+        id = vnc_auto_assign_id(opts);
+    }
+
     vnc_display_init(id, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
-- 
2.47.3