[PATCH] migration: Add canary to VMSTATE_END_OF_LIST

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

We fairly regularly forget VMSTATE_END_OF_LIST markers off descriptions;
given that the current check is only for ->name being NULL, sometimes
we get unlucky and the code apparently works and no one spots the error.

Explicitly add a flag, VMS_END that should be set, and assert it is
set during the traversal.

Note: This can't go in until we update the copy of vmstate.h in slirp.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 include/migration/vmstate.h | 7 ++++++-
 migration/vmstate.c         | 2 ++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
index 017c03675c..b50708e57a 100644
--- a/include/migration/vmstate.h
+++ b/include/migration/vmstate.h
@@ -147,6 +147,9 @@ enum VMStateFlags {
      * VMStateField.struct_version_id to tell which version of the
      * structure we are referencing to use. */
     VMS_VSTRUCT           = 0x8000,
+
+    /* Marker for end of list */
+    VMS_END = 0x10000
 };
 
 typedef enum {
@@ -1163,7 +1166,9 @@ extern const VMStateInfo vmstate_info_qlist;
     VMSTATE_UNUSED_BUFFER(_test, 0, _size)
 
 #define VMSTATE_END_OF_LIST()                                         \
-    {}
+    {                     \
+        .flags = VMS_END, \
+    }
 
 int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
                        void *opaque, int version_id);
diff --git a/migration/vmstate.c b/migration/vmstate.c
index 05f87cdddc..181ba08c7d 100644
--- a/migration/vmstate.c
+++ b/migration/vmstate.c
@@ -160,6 +160,7 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
         }
         field++;
     }
+    assert(field->flags == VMS_END);
     ret = vmstate_subsection_load(f, vmsd, opaque);
     if (ret != 0) {
         return ret;
@@ -413,6 +414,7 @@ int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
         }
         field++;
     }
+    assert(field->flags == VMS_END);
 
     if (vmdesc) {
         json_writer_end_array(vmdesc);
-- 
2.34.1