[v2,06/10] qapi: untangle next_list

Right now, the semantics of next_list are complicated.  The caller must:

* call start_list

* call next_list for each element *including the first*

* on the first call to next_list, the second argument should point to
NULL and the result is the head of the list.  On subsequent calls,
the second argument should point to the last node (last result of
next_list) and next_list itself tacks the element at the tail of the
list.

This works for both input and output visitor, but having the visitor
write memory when it is only reading the list is ugly.  Plus, relying
on *list to detect the first call is tricky and undocumented.

We can initialize so->entry in next_list instead of start_list, leaving
it NULL in start_list.  This way next_list sees clearly whether it is
on the first call---as a bonus, it discriminates the cases based on
internal state of the visitor rather than external state.  We can
also pull the assignment of the list head from generated code up to
next_list.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 docs/qapi-code-gen.txt   |    4 ++--
 qapi/qmp-input-visitor.c |   22 +++++++++++++---------
 scripts/qapi-visit.py    |    4 ++--
 3 files changed, 17 insertions(+), 13 deletions(-)

On Thu, Mar 22, 2012 at 10:38:40PM +0100, Paolo Bonzini wrote:
> Right now, the semantics of next_list are complicated.  The caller must:
> 
> * call start_list
> 
> * call next_list for each element *including the first*
> 
> * on the first call to next_list, the second argument should point to
> NULL and the result is the head of the list.  On subsequent calls,
> the second argument should point to the last node (last result of
> next_list) and next_list itself tacks the element at the tail of the
> list.
> 
> This works for both input and output visitor, but having the visitor
> write memory when it is only reading the list is ugly.  Plus, relying
> on *list to detect the first call is tricky and undocumented.
> 
> We can initialize so->entry in next_list instead of start_list, leaving
> it NULL in start_list.  This way next_list sees clearly whether it is
> on the first call---as a bonus, it discriminates the cases based on
> internal state of the visitor rather than external state.  We can
> also pull the assignment of the list head from generated code up to
> next_list.

Nice cleanup. We'd originally moved assignment of the list head out of
next_list because we had no way of knowing whether it was the first call
at that point, so it was either always assign the current entry or
never, so we did the latter, but that complicated the calling code.

Now that we can make that distinction I think doing this change makes
sense.

Patch in general looks good as well.

Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>

> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  docs/qapi-code-gen.txt   |    4 ++--
>  qapi/qmp-input-visitor.c |   22 +++++++++++++---------
>  scripts/qapi-visit.py    |    4 ++--
>  3 files changed, 17 insertions(+), 13 deletions(-)
> 
> diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
> index 5831e37..ad11767 100644
> --- a/docs/qapi-code-gen.txt
> +++ b/docs/qapi-code-gen.txt
> @@ -194,11 +194,11 @@ Example:
> 
>      void visit_type_UserDefOneList(Visitor *m, UserDefOneList ** obj, const char *name, Error **errp)
>      {
> -        GenericList *i;
> +        GenericList *i, **prev = (GenericList **)obj;
> 
>          visit_start_list(m, name, errp);
> 
> -        for (i = visit_next_list(m, (GenericList **)obj, errp); i; i = visit_next_list(m, &i, errp)) {
> +        for (; (i = visit_next_list(m, prev, errp)) != NULL; prev = &i) {
>              UserDefOneList *native_i = (UserDefOneList *)i;
>              visit_type_UserDefOne(m, &native_i->value, NULL, errp);
>          }
> diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
> index ef9288f..413e333 100644
> --- a/qapi/qmp-input-visitor.c
> +++ b/qapi/qmp-input-visitor.c
> @@ -64,9 +64,7 @@ static const QObject *qmp_input_get_object(QmpInputVisitor *qiv,
>  static void qmp_input_push(QmpInputVisitor *qiv, const QObject *obj, Error **errp)
>  {
>      qiv->stack[qiv->nb_stack].obj = obj;
> -    if (qobject_type(obj) == QTYPE_QLIST) {
> -        qiv->stack[qiv->nb_stack].entry = qlist_first(qobject_to_qlist(obj));
> -    }
> +    qiv->stack[qiv->nb_stack].entry = NULL;
>      qiv->nb_stack++;
> 
>      if (qiv->nb_stack >= QIV_STACK_SIZE) {
> @@ -132,18 +130,24 @@ static GenericList *qmp_input_next_list(Visitor *v, GenericList **list,
>      QmpInputVisitor *qiv = to_qiv(v);
>      GenericList *entry;
>      StackObject *so = &qiv->stack[qiv->nb_stack - 1];
> +    bool first;
> +
> +    if (so->entry == NULL) {
> +        so->entry = qlist_first(qobject_to_qlist(so->obj));
> +        first = true;
> +    } else {
> +        so->entry = qlist_next(so->entry);
> +        first = false;
> +    }
> 
>      if (so->entry == NULL) {
>          return NULL;
>      }
> 
>      entry = g_malloc0(sizeof(*entry));
> -    if (*list) {
> -        so->entry = qlist_next(so->entry);
> -        if (so->entry == NULL) {
> -            g_free(entry);
> -            return NULL;
> -        }
> +    if (first) {
> +        *list = entry;
> +    } else {
>          (*list)->next = entry;
>      }
> 
> diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
> index a85fb76..1ff81f4 100644
> --- a/scripts/qapi-visit.py
> +++ b/scripts/qapi-visit.py
> @@ -86,14 +86,14 @@ def generate_visit_list(name, members):
> 
>  void visit_type_%(name)sList(Visitor *m, %(name)sList ** obj, const char *name, Error **errp)
>  {
> -    GenericList *i, **head = (GenericList **)obj;
> +    GenericList *i, **prev = (GenericList **)obj;
> 
>      if (error_is_set(errp)) {
>          return;
>      }
>      visit_start_list(m, name, errp);
> 
> -    for (*head = i = visit_next_list(m, head, errp); i; i = visit_next_list(m, &i, errp)) {
> +    for (; (i = visit_next_list(m, prev, errp)) != NULL; prev = &i) {
>          %(name)sList *native_i = (%(name)sList *)i;
>          visit_type_%(name)s(m, &native_i->value, NULL, errp);
>      }
> -- 
> 1.7.9.1
>