Re: [Qemu-devel] [PATCH 12/21] qapi: Improve qobject input visitor error reporting

[Eric Blake]

On 02/23/2017 03:45 PM, Markus Armbruster wrote:
> Error messages refer to nodes of the QObject being visited by name.
> Trouble is the names are sometimes less than helpful:
> 

> Improve error messages by referring to nodes by path instead, as
> follows:
> 
> * The path of the root QObject is whatever @name argument got passed
>   to the visitor, except NULL gets mapped to "<anonymous>".
> 
> * The path of a root QDict's member is the member key.
> 
> * The path of a root QList's member is "[%zu]", where %zu is the list
>   index, starting at zero.
> 
> * The path of a non-root QDict's member is the path of the QDict
>   concatenated with "." and the member key.
> 
> * The path of a non-root QList's member is the path of the QList
>   concatenated with "[%zu]", where %zu is the list index.
> 

> Signed-off-by: Markus Armbruster <address@hidden>
> ---
>  include/qapi/visitor.h       |   6 ---
>  qapi/qobject-input-visitor.c | 121 
> +++++++++++++++++++++++++++++++------------
>  2 files changed, 87 insertions(+), 40 deletions(-)
> 

> +typedef struct StackObject {
> +    const char *name;            /* Name of @obj in its parent, if any */
> +    QObject *obj;                /* QDict or QList being visited */
>      void *qapi; /* sanity check that caller uses same pointer */
>  
> -    GHashTable *h;           /* If obj is dict: unvisited keys */
> -    const QListEntry *entry; /* If obj is list: unvisited tail */
> +    GHashTable *h;              /* If @obj is QDict: unvisited keys */
> +    const QListEntry *entry;    /* If @obj is QList: unvisited tail */
> +    unsigned index;             /* If @obj is QList: list index of @entry */

Could perhaps make the QDict vs. QList fields shared through a union if
we were tight on storage space or cache line pressure. I don't think
that's the case, though, so no need to change it.

> +static const char *full_name(QObjectInputVisitor *qiv, const char *name)
> +{
> +    StackObject *so;
> +    char buf[32];
> +
> +    if (qiv->errname) {
> +        g_string_truncate(qiv->errname, 0);
> +    } else {
> +        qiv->errname = g_string_new("");
> +    }
> +
> +    QSLIST_FOREACH(so , &qiv->stack, node) {
> +        if (qobject_type(so->obj) == QTYPE_QDICT) {
> +            g_string_prepend(qiv->errname, name);
> +            g_string_prepend_c(qiv->errname, '.');
> +        } else {
> +            snprintf(buf, sizeof(buf), "[%u]", so->index);
> +            g_string_prepend(qiv->errname, buf);
> +        }
> +        name = so->name;
> +    }

Building the string from back to front requires quite a bit of memory
reshuffling, as well as the temporary buffer for integer formatting. Is
there any way to build it from front to back? But this code is only
triggered on error paths, so I don't care if it is slow.  I'm fine with
what you have.

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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