qom: Add check() argument to object_property_add_link()

There are currently three types of object_property_add_link() callers:

1. The link property may be set at any time.
2. The link property of a DeviceState instance may only be set before
   realize.
3. The link property may never be set, it is read-only.

Something similar can already be achieved with
object_property_add_str()'s set() argument.  Follow its example and add
a check() argument to object_property_add_link().

Also provide default check() functions for case #1 and #2.  Case #3 is
covered by passing a NULL function pointer.

Cc: Peter Crosthwaite <peter.crosthwaite@petalogix.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Anthony Liguori <aliguori@amazon.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
[AF: Tweaked documentation comment]
Signed-off-by: Andreas Färber <afaerber@suse.de>

2 files changed, 29 insertions, 0 deletions

diff --git a/include/hw/qdev-properties.h b/include/hw/qdev-properties.h
index 3c000eea75..c46e908d71 100644
--- a/include/hw/qdev-properties.h
+++ b/include/hw/qdev-properties.h
@@ -204,4 +204,15 @@ void qdev_property_add_static(DeviceState *dev, Property *prop, Error **errp);
  */
 void qdev_prop_set_after_realize(DeviceState *dev, const char *name,
                                  Error **errp);
+
+/**
+ * qdev_prop_allow_set_link_before_realize:
+ *
+ * Set the #Error object if an attempt is made to set the link after realize.
+ * This function should be used as the check() argument to
+ * object_property_add_link().
+ */
+void qdev_prop_allow_set_link_before_realize(Object *obj, const char *name,
+                                             Object *val, Error **errp);
+
 #endif
diff --git a/include/qom/object.h b/include/qom/object.h
index 9feb441986..a641dcde10 100644
--- a/include/qom/object.h
+++ b/include/qom/object.h
@@ -1073,11 +1073,22 @@ typedef enum {
 } ObjectPropertyLinkFlags;
 
 /**
+ * object_property_allow_set_link:
+ *
+ * The default implementation of the object_property_add_link() check()
+ * callback function.  It allows the link property to be set and never returns
+ * an error.
+ */
+void object_property_allow_set_link(Object *, const char *,
+                                    Object *, Error **);
+
+/**
  * object_property_add_link:
  * @obj: the object to add a property to
  * @name: the name of the property
  * @type: the qobj type of the link
  * @child: a pointer to where the link object reference is stored
+ * @check: callback to veto setting or NULL if the property is read-only
  * @flags: additional options for the link
  * @errp: if an error occurs, a pointer to an area to store the area
  *
@@ -1087,6 +1098,11 @@ typedef enum {
  *
  * Links form the graph in the object model.
  *
+ * The <code>@check()</code> callback is invoked when
+ * object_property_set_link() is called and can raise an error to prevent the
+ * link being set.  If <code>@check</code> is NULL, the property is read-only
+ * and cannot be set.
+ *
  * Ownership of the pointer that @child points to is transferred to the
  * link property.  The reference count for <code>*@child</code> is
  * managed by the property from after the function returns till the
@@ -1096,6 +1112,8 @@ typedef enum {
  */
 void object_property_add_link(Object *obj, const char *name,
                               const char *type, Object **child,
+                              void (*check)(Object *obj, const char *name,
+                                            Object *val, Error **errp),
                               ObjectPropertyLinkFlags flags,
                               Error **errp);