Add object_property_get_child().

Add object_property_get_child().

     Adding a direct accessor to a child property.

     In the existing implementation, object_property_get() must be used,
     with with a visitor, implementing the 'type_str' callback,
     receiving the child's canonical path.

     In the new implementation, the child is returned directly.
     Link properties are resolved in the same way, as they are
     when an absolute path is resolved.

     Errors relevant to the operation, QERR_OBJECT_PROPERTY_NOT_FOUND
     and QERR_OBJECT_PROPERTY_INVALID_TYPE were added.

     Also, in the new implementation, some common sense refactoring was done
     in the file 'qom/object.c' in the code extracting child and link 
properties.

     Signed-off-by: Alexander Barabash <alexander_barabash@mentor.com>

-        if (!strstart(prop->type, "child<", NULL)) {
+        if (!object_property_is_child(prop)) {
              continue;
          }

@@ -799,6 +873,27 @@ Object *object_get_root(void)
      return root;
  }

+Object *object_property_get_child(Object *obj, const char *name,
+                                  struct Error **errp) {
+    Object *result = NULL;
+    ObjectProperty *prop = object_property_find(obj, name);
+    bool is_invalid_type;
+
+    if (prop == NULL) {
+        REPORT_OBJECT_ERROR(errp, QERR_OBJECT_PROPERTY_NOT_FOUND, obj, 
name);
+        return NULL;
+    }
+
+    result = object_property_extract_child(prop, &is_invalid_type);
+    if (is_invalid_type) {
+        REPORT_OBJECT_ERROR(errp, QERR_OBJECT_PROPERTY_INVALID_TYPE,
+                            obj, name, "child");
+        return NULL;
+    }
+
+    return result;
+}
+
  static void object_get_child_property(Object *obj, Visitor *v, void 
*opaque,
                                        const char *name, Error **errp)
  {
@@ -829,7 +924,10 @@ void object_property_add_child(Object *obj, const 
char *name,
       */
      assert(!object_is_type(obj, type_interface));

-    type = g_strdup_printf("child<%s>", 
object_get_typename(OBJECT(child)));
+    type = g_strdup_printf(CHILD_PROPERTY_TYPE_PREFIX
+                           "%s"
+                           CHILD_PROPERTY_TYPE_SUFFIX,
+                           object_get_typename(OBJECT(child)));

      object_property_add(obj, name, type, object_get_child_property,
                          NULL, object_finalize_child_property, child, 
errp);
@@ -878,8 +976,7 @@ static void object_set_link_property(Object *obj, 
Visitor *v, void *opaque,
      if (strcmp(path, "") != 0) {
          Object *target;

-        /* Go from link<FOO> to FOO.  */
-        target_type = g_strndup(&type[5], strlen(type) - 6);
+        target_type = link_type_to_type(type);
          target = object_resolve_path_type(path, target_type, &ambiguous);

          if (ambiguous) {
@@ -907,7 +1004,9 @@ void object_property_add_link(Object *obj, const 
char *name,
  {
      gchar *full_type;

-    full_type = g_strdup_printf("link<%s>", type);
+    full_type = g_strdup_printf(LINK_PROPERTY_TYPE_PREFIX
+                                "%s"
+                                LINK_PROPERTY_TYPE_SUFFIX, type);

      object_property_add(obj, name, full_type,
                          object_get_link_property,
@@ -932,7 +1031,7 @@ gchar *object_get_canonical_path(Object *obj)
          g_assert(obj->parent != NULL);

          QTAILQ_FOREACH(prop, &obj->parent->properties, node) {
-            if (!strstart(prop->type, "child<", NULL)) {
+            if (!object_property_is_child(prop)) {
                  continue;
              }

@@ -980,15 +1079,7 @@ static Object *object_resolve_abs_path(Object *parent,
          return NULL;
      }

-    child = NULL;
-    if (strstart(prop->type, "link<", NULL)) {
-        Object **pchild = prop->opaque;
-        if (*pchild) {
-            child = *pchild;
-        }
-    } else if (strstart(prop->type, "child<", NULL)) {
-        child = prop->opaque;
-    }
+    child = object_property_extract_child(prop, NULL);

      if (!child) {
          return NULL;
@@ -1010,7 +1101,7 @@ static Object *object_resolve_partial_path(Object 
*parent,
      QTAILQ_FOREACH(prop, &parent->properties, node) {
          Object *found;

-        if (!strstart(prop->type, "child<", NULL)) {
+        if (!object_property_is_child(prop)) {
              continue;
          }

Besides the non-triviality of the patch, how is this different from
object_property_get_link?  Perhaps we should just rename that one to
object_property_get_obj, or add a function that is just a synonym.

> + */
> +Object *object_property_get_child(Object *obj, const char *name,
> +                                  struct Error **errp);
> +
> +/**
>   * object_property_set:
>   * @obj: the object
>   * @v: the visitor that will be used to write the property value.  This
> should
> diff --git a/qom/object.c b/qom/object.c
> index 2de6eaf..61e775b 100644
> --- a/qom/object.c
> +++ b/qom/object.c
> @@ -297,12 +297,86 @@ static void object_property_del_all(Object *obj)
>      }
>  }
> 
> +/*
> + * To ensure correct format checking,
> + * this function should be used only via REPORT_OBJECT_ERROR() macro.
> + *
> + * The first argument after 'obj' should be of type 'const char *'.
> + * It is ignored, and replaced by the canonical path of 'obj'.
> + */
> +static void report_object_error(Error **errp, const char *fmt, Object
> *obj, ...)
> +    GCC_FMT_ATTR(2, 4);
> +static void report_object_error(Error **errp, const char *fmt, Object
> *obj, ...)
> +{
> +    gchar *path;
> +    va_list ap;
> +
> +    if (errp != NULL) {
> +        path = object_get_canonical_path(obj);
> +        va_start(ap, obj);
> +        va_arg(ap, const char *); /* Ignore the dummy string. */

Why do you need it at all?

> +        error_set(errp, fmt, path, &ap);
> +        va_end(ap);
> +        g_free(path);
> +    }
> +}
> +#define REPORT_OBJECT_ERROR(errp, fmt, obj, ...)                 \
> +    do {                                                         \
> +        report_object_error(errp, fmt, obj, "", ## __VA_ARGS__); \
> +    } while (0)
> +#define CHILD_PROPERTY_TYPE_PREFIX "child<"
> +#define CHILD_PROPERTY_TYPE_SUFFIX ">"
> +#define LINK_PROPERTY_TYPE_PREFIX "link<"
> +#define LINK_PROPERTY_TYPE_SUFFIX ">"
> +
> +static bool object_property_is_child(ObjectProperty *prop)
> +{
> +    return (strstart(prop->type, CHILD_PROPERTY_TYPE_PREFIX, NULL) != 0);
> +}
> +
> +static bool object_property_is_link(ObjectProperty *prop)
> +{
> +    return (strstart(prop->type, LINK_PROPERTY_TYPE_PREFIX, NULL) != 0);
> +}
> +
> +/* Go from LINK_PROPERTY_TYPE_PREFIX FOO LINK_PROPERTY_TYPE_SUFFIX to
> FOO.  */
> +static gchar *link_type_to_type(const gchar *type)
> +{
> +    return g_strndup(&type[sizeof(LINK_PROPERTY_TYPE_PREFIX) - 1],
> +                     strlen(type)
> +                     - (sizeof(LINK_PROPERTY_TYPE_PREFIX) - 1)
> +                     - (sizeof(LINK_PROPERTY_TYPE_SUFFIX) - 1));
> +}
> +
> +static Object *object_property_extract_child(ObjectProperty *prop,
> +                                             bool *p_is_invalid_type)
> +{
> +    if (p_is_invalid_type != NULL) {
> +        *p_is_invalid_type = false;
> +    }
> +    if (object_property_is_child(prop)) {
> +        return (Object *)prop->opaque;
> +    } else if (object_property_is_link(prop)) {
> +        if (prop->opaque != NULL) {
> +            return *(Object **)prop->opaque;
> +        } else {
> +            return NULL;
> +        }
> +    } else {
> +        if (p_is_invalid_type != NULL) {
> +            *p_is_invalid_type = true;
> +        }
> +        return NULL;
> +    }
> +}
> +

object_property_get_child should never be on a fast path.  We should
avoid as much as possible peeking in the opaque.  Instead, you should
just use a visitor like object_property_get_link does.

Everything starting here should be a separate patch.  But if you remove
object_property_extract_child, I doubt it makes as much sense to
refactor this.

>  static void object_property_del_child(Object *obj, Object *child, Error
> **errp)
>  {
>      ObjectProperty *prop;
> 
>      QTAILQ_FOREACH(prop, &obj->properties, node) {
> -        if (!strstart(prop->type, "child<", NULL)) {
> +        if (!object_property_is_child(prop)) {
>              continue;
>          }
> 
> @@ -799,6 +873,27 @@ Object *object_get_root(void)
>      return root;
>  }
> 
> +Object *object_property_get_child(Object *obj, const char *name,
> +                                  struct Error **errp) {
> +    Object *result = NULL;
> +    ObjectProperty *prop = object_property_find(obj, name);
> +    bool is_invalid_type;
> +
> +    if (prop == NULL) {
> +        REPORT_OBJECT_ERROR(errp, QERR_OBJECT_PROPERTY_NOT_FOUND, obj,
> name);
> +        return NULL;
> +    }
> +
> +    result = object_property_extract_child(prop, &is_invalid_type);
> +    if (is_invalid_type) {
> +        REPORT_OBJECT_ERROR(errp, QERR_OBJECT_PROPERTY_INVALID_TYPE,
> +                            obj, name, "child");
> +        return NULL;
> +    }
> +
> +    return result;
> +}
> +
>  static void object_get_child_property(Object *obj, Visitor *v, void
> *opaque,
>                                        const char *name, Error **errp)
>  {
> @@ -829,7 +924,10 @@ void object_property_add_child(Object *obj, const
> char *name,
>       */
>      assert(!object_is_type(obj, type_interface));
> 
> -    type = g_strdup_printf("child<%s>",
> object_get_typename(OBJECT(child)));
> +    type = g_strdup_printf(CHILD_PROPERTY_TYPE_PREFIX
> +                           "%s"
> +                           CHILD_PROPERTY_TYPE_SUFFIX,
> +                           object_get_typename(OBJECT(child)));
> 
>      object_property_add(obj, name, type, object_get_child_property,
>                          NULL, object_finalize_child_property, child,
> errp);
> @@ -878,8 +976,7 @@ static void object_set_link_property(Object *obj,
> Visitor *v, void *opaque,
>      if (strcmp(path, "") != 0) {
>          Object *target;
> 
> -        /* Go from link<FOO> to FOO.  */
> -        target_type = g_strndup(&type[5], strlen(type) - 6);
> +        target_type = link_type_to_type(type);
>          target = object_resolve_path_type(path, target_type, &ambiguous);
> 
>          if (ambiguous) {
> @@ -907,7 +1004,9 @@ void object_property_add_link(Object *obj, const
> char *name,
>  {
>      gchar *full_type;
> 
> -    full_type = g_strdup_printf("link<%s>", type);
> +    full_type = g_strdup_printf(LINK_PROPERTY_TYPE_PREFIX
> +                                "%s"
> +                                LINK_PROPERTY_TYPE_SUFFIX, type);
> 
>      object_property_add(obj, name, full_type,
>                          object_get_link_property,
> @@ -932,7 +1031,7 @@ gchar *object_get_canonical_path(Object *obj)
>          g_assert(obj->parent != NULL);
> 
>          QTAILQ_FOREACH(prop, &obj->parent->properties, node) {
> -            if (!strstart(prop->type, "child<", NULL)) {
> +            if (!object_property_is_child(prop)) {
>                  continue;
>              }
> 
> @@ -980,15 +1079,7 @@ static Object *object_resolve_abs_path(Object
> *parent,
>          return NULL;
>      }
> 
> -    child = NULL;
> -    if (strstart(prop->type, "link<", NULL)) {
> -        Object **pchild = prop->opaque;
> -        if (*pchild) {
> -            child = *pchild;
> -        }
> -    } else if (strstart(prop->type, "child<", NULL)) {
> -        child = prop->opaque;
> -    }
> +    child = object_property_extract_child(prop, NULL);
> 
>      if (!child) {
>          return NULL;
> @@ -1010,7 +1101,7 @@ static Object *object_resolve_partial_path(Object
> *parent,
>      QTAILQ_FOREACH(prop, &parent->properties, node) {
>          Object *found;
> 
> -        if (!strstart(prop->type, "child<", NULL)) {
> +        if (!object_property_is_child(prop)) {
>              continue;
>          }
> 
> 
> 
> 

Paolo

On 02/17/2012 12:17 PM, Paolo Bonzini wrote:
> Besides the non-triviality of the patch, how is this different from
> object_property_get_link?  Perhaps we should just rename that one to
> object_property_get_obj, or add a function that is just a synonym.
As for the patch's (non-)triviality, I just wanted to check what counts 
as trivial here.
I agree that was a bit over-reaching.

The proposed object_property_get_child() may return either
the direct child with the specified name in the composition tree,
or the value of the link with the specified name, as
object_property_get_link() indeed does.

It has exactly the same semantics as:

Object *object_property_get_child(Object *obj, const char *name,
                                   struct Error **errp)
{
     Object * result;
     bool ambigious;
     gchar *child_path;

     /* If name contains a '/' in it, report an error and return NULL. */

     child_path = g_strdup_printf("%s/%s", 
object_get_canonical_path(obj), name);
     result = object_resolve_path(child_path, &ambigious);
     g_free(child_path);
     if (result == NULL) {
        /* Report an error. */
        return NULL;
     }
     if (ambigious) {
        /* Report an error. */
        return NULL;
     }
     return result;
}

but does it in a more straightforward way.

>
>> + */
>> +Object *object_property_get_child(Object *obj, const char *name,
>> +                                  struct Error **errp);
>> +
>> +/**
>>    * object_property_set:
>>    * @obj: the object
>>    * @v: the visitor that will be used to write the property value.  This
>> should
>> diff --git a/qom/object.c b/qom/object.c
>> index 2de6eaf..61e775b 100644
>> --- a/qom/object.c
>> +++ b/qom/object.c
>> @@ -297,12 +297,86 @@ static void object_property_del_all(Object *obj)
>>       }
>>   }
>>
>> +/*
>> + * To ensure correct format checking,
>> + * this function should be used only via REPORT_OBJECT_ERROR() macro.
>> + *
>> + * The first argument after 'obj' should be of type 'const char *'.
>> + * It is ignored, and replaced by the canonical path of 'obj'.
>> + */
>> +static void report_object_error(Error **errp, const char *fmt, Object
>> *obj, ...)
>> +    GCC_FMT_ATTR(2, 4);
>> +static void report_object_error(Error **errp, const char *fmt, Object
>> *obj, ...)
>> +{
>> +    gchar *path;
>> +    va_list ap;
>> +
>> +    if (errp != NULL) {
>> +        path = object_get_canonical_path(obj);
>> +        va_start(ap, obj);
>> +        va_arg(ap, const char *); /* Ignore the dummy string. */
> Why do you need it at all?

I think this is needed to avoid replicating code and have an easy tool 
to report
object's path in errors.

>
>> +        error_set(errp, fmt, path,&ap);
>> +        va_end(ap);
>> +        g_free(path);
>> +    }
>> +}
>> +#define REPORT_OBJECT_ERROR(errp, fmt, obj, ...)                 \
>> +    do {                                                         \
>> +        report_object_error(errp, fmt, obj, "", ## __VA_ARGS__); \
>> +    } while (0)
>> +#define CHILD_PROPERTY_TYPE_PREFIX "child<"
>> +#define CHILD_PROPERTY_TYPE_SUFFIX ">"
>> +#define LINK_PROPERTY_TYPE_PREFIX "link<"
>> +#define LINK_PROPERTY_TYPE_SUFFIX ">"
>> +
>> +static bool object_property_is_child(ObjectProperty *prop)
>> +{
>> +    return (strstart(prop->type, CHILD_PROPERTY_TYPE_PREFIX, NULL) != 0);
>> +}
>> +
>> +static bool object_property_is_link(ObjectProperty *prop)
>> +{
>> +    return (strstart(prop->type, LINK_PROPERTY_TYPE_PREFIX, NULL) != 0);
>> +}
>> +
>> +/* Go from LINK_PROPERTY_TYPE_PREFIX FOO LINK_PROPERTY_TYPE_SUFFIX to
>> FOO.  */
>> +static gchar *link_type_to_type(const gchar *type)
>> +{
>> +    return g_strndup(&type[sizeof(LINK_PROPERTY_TYPE_PREFIX) - 1],
>> +                     strlen(type)
>> +                     - (sizeof(LINK_PROPERTY_TYPE_PREFIX) - 1)
>> +                     - (sizeof(LINK_PROPERTY_TYPE_SUFFIX) - 1));
>> +}
>> +
>> +static Object *object_property_extract_child(ObjectProperty *prop,
>> +                                             bool *p_is_invalid_type)
>> +{
>> +    if (p_is_invalid_type != NULL) {
>> +        *p_is_invalid_type = false;
>> +    }
>> +    if (object_property_is_child(prop)) {
>> +        return (Object *)prop->opaque;
>> +    } else if (object_property_is_link(prop)) {
>> +        if (prop->opaque != NULL) {
>> +            return *(Object **)prop->opaque;
>> +        } else {
>> +            return NULL;
>> +        }
>> +    } else {
>> +        if (p_is_invalid_type != NULL) {
>> +            *p_is_invalid_type = true;
>> +        }
>> +        return NULL;
>> +    }
>> +}
>> +
> object_property_get_child should never be on a fast path.  We should
> avoid as much as possible peeking in the opaque.  Instead, you should
> just use a visitor like object_property_get_link does.

I am not sure what do you mean by "should never be on a fast path".
Anyway, I did not add any peeking in the opaque. I just moved the code
from object_resolve_abs_path() into a subroutine. Fairly enough, we could
avoid looking into the link property's opaque by calling 
object_property_get_link()
instead. I'll prepare a revision of the patch to this end.


>
> Everything starting here should be a separate patch.  But if you remove
> object_property_extract_child, I doubt it makes as much sense to
> refactor this.

Obviously, the refactoring was needed to add code. If we don't,
no refactoring is required.

>
>>   static void object_property_del_child(Object *obj, Object *child, Error
>> **errp)
>>   {
>>       ObjectProperty *prop;
>>
>>       QTAILQ_FOREACH(prop,&obj->properties, node) {
>> -        if (!strstart(prop->type, "child<", NULL)) {
>> +        if (!object_property_is_child(prop)) {
>>               continue;
>>           }
>>
>> @@ -799,6 +873,27 @@ Object *object_get_root(void)
>>       return root;
>>   }
>>
>> +Object *object_property_get_child(Object *obj, const char *name,
>> +                                  struct Error **errp) {
>> +    Object *result = NULL;
>> +    ObjectProperty *prop = object_property_find(obj, name);
>> +    bool is_invalid_type;
>> +
>> +    if (prop == NULL) {
>> +        REPORT_OBJECT_ERROR(errp, QERR_OBJECT_PROPERTY_NOT_FOUND, obj,
>> name);
>> +        return NULL;
>> +    }
>> +
>> +    result = object_property_extract_child(prop,&is_invalid_type);
>> +    if (is_invalid_type) {
>> +        REPORT_OBJECT_ERROR(errp, QERR_OBJECT_PROPERTY_INVALID_TYPE,
>> +                            obj, name, "child");
>> +        return NULL;
>> +    }
>> +
>> +    return result;
>> +}
>> +
>>   static void object_get_child_property(Object *obj, Visitor *v, void
>> *opaque,
>>                                         const char *name, Error **errp)
>>   {
>> @@ -829,7 +924,10 @@ void object_property_add_child(Object *obj, const
>> char *name,
>>        */
>>       assert(!object_is_type(obj, type_interface));
>>
>> -    type = g_strdup_printf("child<%s>",
>> object_get_typename(OBJECT(child)));
>> +    type = g_strdup_printf(CHILD_PROPERTY_TYPE_PREFIX
>> +                           "%s"
>> +                           CHILD_PROPERTY_TYPE_SUFFIX,
>> +                           object_get_typename(OBJECT(child)));
>>
>>       object_property_add(obj, name, type, object_get_child_property,
>>                           NULL, object_finalize_child_property, child,
>> errp);
>> @@ -878,8 +976,7 @@ static void object_set_link_property(Object *obj,
>> Visitor *v, void *opaque,
>>       if (strcmp(path, "") != 0) {
>>           Object *target;
>>
>> -        /* Go from link<FOO>  to FOO.  */
>> -        target_type = g_strndup(&type[5], strlen(type) - 6);
>> +        target_type = link_type_to_type(type);
>>           target = object_resolve_path_type(path, target_type,&ambiguous);
>>
>>           if (ambiguous) {
>> @@ -907,7 +1004,9 @@ void object_property_add_link(Object *obj, const
>> char *name,
>>   {
>>       gchar *full_type;
>>
>> -    full_type = g_strdup_printf("link<%s>", type);
>> +    full_type = g_strdup_printf(LINK_PROPERTY_TYPE_PREFIX
>> +                                "%s"
>> +                                LINK_PROPERTY_TYPE_SUFFIX, type);
>>
>>       object_property_add(obj, name, full_type,
>>                           object_get_link_property,
>> @@ -932,7 +1031,7 @@ gchar *object_get_canonical_path(Object *obj)
>>           g_assert(obj->parent != NULL);
>>
>>           QTAILQ_FOREACH(prop,&obj->parent->properties, node) {
>> -            if (!strstart(prop->type, "child<", NULL)) {
>> +            if (!object_property_is_child(prop)) {
>>                   continue;
>>               }
>>
>> @@ -980,15 +1079,7 @@ static Object *object_resolve_abs_path(Object
>> *parent,
>>           return NULL;
>>       }
>>
>> -    child = NULL;
>> -    if (strstart(prop->type, "link<", NULL)) {
>> -        Object **pchild = prop->opaque;
>> -        if (*pchild) {
>> -            child = *pchild;
>> -        }
>> -    } else if (strstart(prop->type, "child<", NULL)) {
>> -        child = prop->opaque;
>> -    }
>> +    child = object_property_extract_child(prop, NULL);
>>
>>       if (!child) {
>>           return NULL;
>> @@ -1010,7 +1101,7 @@ static Object *object_resolve_partial_path(Object
>> *parent,
>>       QTAILQ_FOREACH(prop,&parent->properties, node) {
>>           Object *found;
>>
>> -        if (!strstart(prop->type, "child<", NULL)) {
>> +        if (!object_property_is_child(prop)) {
>>               continue;
>>           }
>>
>>
>>
>>
> Paolo
Alex

On 02/19/2012 01:36 PM, Alexander Barabash wrote:
> 
> 
> The proposed object_property_get_child() may return either
> the direct child with the specified name in the composition tree,
> or the value of the link with the specified name, as
> object_property_get_link() indeed does.

Have you actually checked that object_property_get_link() doesn't work
for you?  It seems to me that your patch does the same as the one-liner:

#define object_property_get_child object_property_get_link

Now _this_ would be good for qemu-trivial. ;)

> but does it in a more straightforward way. 

I disagree that it is more straightforward.  It is more *direct*
perhaps, in that it doesn't go through visitors,
object_get_child_property and object_resolve_path, but it also violates
the encapsulation that properties provide quite heavily.

Peeking at the opaque values of properties should be done as sparingly
as possible.  Ideally once, since no QOM property code should be in a
fast path.  In this case, it is done in object_resolve_abs_path, and
that should be enough.

Paolo

On 02/20/2012 11:11 AM, Paolo Bonzini wrote:
> On 02/19/2012 01:36 PM, Alexander Barabash wrote:
>>
>> The proposed object_property_get_child() may return either
>> the direct child with the specified name in the composition tree,
>> or the value of the link with the specified name, as
>> object_property_get_link() indeed does.
> Have you actually checked that object_property_get_link() doesn't work
> for you?  It seems to me that your patch does the same as the one-liner:
>
> #define object_property_get_child object_property_get_link
>
> Now _this_ would be good for qemu-trivial. ;)

You are right. Please disregard my v2 patch.

I would only suggest that the documentation of object_property_add_child()
be amended to reflect the fact that the child object may be retrieved using
object_property_get_link().

>> but does it in a more straightforward way.
> I disagree that it is more straightforward.  It is more *direct*
> perhaps, in that it doesn't go through visitors,
> object_get_child_property and object_resolve_path, but it also violates
> the encapsulation that properties provide quite heavily.
>
> Peeking at the opaque values of properties should be done as sparingly
> as possible.  Ideally once, since no QOM property code should be in a
> fast path.  In this case, it is done in object_resolve_abs_path, and
> that should be enough.
>
> Paolo
Alex

On 02/20/2012 12:13 PM, Alexander Barabash wrote:
> I would only suggest that the documentation of object_property_add_child()
> be amended to reflect the fact that the child object may be retrieved using
> object_property_get_link().

Sure, feel free to post a patch for that or to add a synonym with
#define.  Alternatively, amending the documentation of
object_property_get_link or even renaming it would also be fine, I guess.

Paolo