[PATCH] qom/object: Display more helpful message when an interface is missing

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

When adding new devices implementing QOM interfaces, we might
forgot to add the Kconfig dependency that pulls the required
objects in when building.

Since QOM dependencies are resolved at runtime, we don't get any
link-time failures, and QEMU aborts while starting:

  $ qemu ...
  Segmentation fault (core dumped)

  (gdb) bt
  #0  0x00007ff6e96b1e35 in raise () from /lib64/libc.so.6
  #1  0x00007ff6e969c895 in abort () from /lib64/libc.so.6
  #2  0x00005572bc5051cf in type_initialize (ti=0x5572be6f1200) at qom/object.c:323
  #3  0x00005572bc505074 in type_initialize (ti=0x5572be6f1800) at qom/object.c:301
  #4  0x00005572bc505074 in type_initialize (ti=0x5572be6e48e0) at qom/object.c:301
  #5  0x00005572bc506939 in object_class_by_name (typename=0x5572bc56109a) at qom/object.c:959
  #6  0x00005572bc503dd5 in cpu_class_by_name (typename=0x5572bc56109a, cpu_model=0x5572be6d9930) at hw/core/cpu.c:286

Since the caller has access to the qdev parent/interface names,
we can simply display them to avoid starting a debugger:

  $ qemu ...
  qemu: missing interface 'fancy-if' for object 'fancy-dev'
  Aborted (core dumped)

This commit is similar to e02bdf1cecd2 ("Display more helpful message
when an object type is missing").

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
Cc: Cornelia Huck <cohuck@redhat.com>
Cc: Stefano Garzarella <sgarzare@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
---
 qom/object.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/qom/object.c b/qom/object.c
index 0d971ca897..36123fb330 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -317,6 +317,11 @@ static void type_initialize(TypeImpl *ti)
 
         for (i = 0; i < ti->num_interfaces; i++) {
             TypeImpl *t = type_get_by_name(ti->interfaces[i].typename);
+            if (!t) {
+                error_report("missing interface '%s' for object '%s'",
+                             ti->interfaces[i].typename, parent->name);
+                abort();
+            }
             for (e = ti->class->interfaces; e; e = e->next) {
                 TypeImpl *target_type = OBJECT_CLASS(e->data)->type;
 
-- 
2.21.1