Re: [Qemu-devel] [PATCH v11 25/28] qapi: Simplify visits of optional fields

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> None of the visitor callbacks would set an error when testing
> if an optional field was present; make this part of the interface
> contract by eliminating the errp argument.  Then, for less code,
> reflect the determined boolean value back to the caller instead
> of making the caller read the boolean after the fact.
>
> The resulting generated code has a nice diff:
>
> |-    visit_optional(v, &has_fdset_id, "fdset-id", &err);
> |-    if (err) {
> |-        goto out;
> |-    }
> |-    if (has_fdset_id) {
> |+    if (visit_optional(v, &has_fdset_id, "fdset-id")) {
> |         visit_type_int(v, &fdset_id, "fdset-id", &err);
> |         if (err) {
> |             goto out;
> |         }
> |     }

Any particular reason not to do

        has_fdset_id = visit_optional(v, "fdset-id");
        if (has_fdset_id) {
            visit_type_int(v, &fdset_id, "fdset-id", &err);
            if (err) {
                goto out;
            }
        }

?

> Signed-off-by: Eric Blake <eblake@redhat.com>
>
> ---
> v11 (no v10): no change
> v9: no change
> v8: no change
> v7: rebase to no member.c_name()
> v6: rebase onto earlier testsuite and gen_err_check() improvements
> ---
>  include/qapi/visitor-impl.h |  5 ++---
>  include/qapi/visitor.h      | 10 ++++++++--
>  qapi/opts-visitor.c         |  2 +-
>  qapi/qapi-visit-core.c      |  6 +++---
>  qapi/qmp-input-visitor.c    |  3 +--
>  qapi/string-input-visitor.c |  3 +--
>  scripts/qapi.py             |  7 +------
>  7 files changed, 17 insertions(+), 19 deletions(-)
>
> diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
> index ac6c17e..81d1aa0 100644
> --- a/include/qapi/visitor-impl.h
> +++ b/include/qapi/visitor-impl.h
> @@ -44,9 +44,8 @@ struct Visitor
>      void (*type_any)(Visitor *v, QObject **obj, const char *name,
>                       Error **errp);
>
> -    /* May be NULL */
> -    void (*optional)(Visitor *v, bool *present, const char *name,
> -                     Error **errp);
> +    /* May be NULL; most useful for input visitors. */
> +    void (*optional)(Visitor *v, bool *present, const char *name);
>
>      void (*type_uint8)(Visitor *v, uint8_t *obj, const char *name, Error **errp);
>      void (*type_uint16)(Visitor *v, uint16_t *obj, const char *name, Error **errp);
> diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
> index 250e8e1..dd4f0b3 100644
> --- a/include/qapi/visitor.h
> +++ b/include/qapi/visitor.h
> @@ -39,8 +39,14 @@ void visit_end_implicit_struct(Visitor *v, Error **errp);
>  void visit_start_list(Visitor *v, const char *name, Error **errp);
>  GenericList *visit_next_list(Visitor *v, GenericList **list, Error **errp);
>  void visit_end_list(Visitor *v, Error **errp);
> -void visit_optional(Visitor *v, bool *present, const char *name,
> -                    Error **errp);
> +
> +/**
> + * Check if an optional member @name of an object needs visiting.
> + * For input visitors, set *@present according to whether the
> + * corresponding visit_type_*() needs calling; for other visitors,
> + * leave *@present unchanged.  Return *@present for convenience.
> + */
> +bool visit_optional(Visitor *v, bool *present, const char *name);
>
>  /**
>   * Determine the qtype of the item @name in the current object visit.
> diff --git a/qapi/opts-visitor.c b/qapi/opts-visitor.c
> index cd10392..ef5fb8b 100644
> --- a/qapi/opts-visitor.c
> +++ b/qapi/opts-visitor.c
> @@ -488,7 +488,7 @@ opts_type_size(Visitor *v, uint64_t *obj, const char *name, Error **errp)
>
>
>  static void
> -opts_optional(Visitor *v, bool *present, const char *name, Error **errp)
> +opts_optional(Visitor *v, bool *present, const char *name)
>  {
>      OptsVisitor *ov = DO_UPCAST(OptsVisitor, visitor, v);
>
> diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
> index 52c132a..daaa3af 100644
> --- a/qapi/qapi-visit-core.c
> +++ b/qapi/qapi-visit-core.c
> @@ -73,12 +73,12 @@ void visit_end_union(Visitor *v, bool data_present, Error **errp)
>      }
>  }
>
> -void visit_optional(Visitor *v, bool *present, const char *name,
> -                    Error **errp)
> +bool visit_optional(Visitor *v, bool *present, const char *name)
>  {
>      if (v->optional) {
> -        v->optional(v, present, name, errp);
> +        v->optional(v, present, name);
>      }
> +    return *present;
>  }

Slightly ugly: struct Visitor method optional returns void, but the
wrapper returns bool.

>
>  void visit_get_next_type(Visitor *v, QTypeCode *type, bool promote_int,
> diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
> index 4238faf..9f0ed69 100644
> --- a/qapi/qmp-input-visitor.c
> +++ b/qapi/qmp-input-visitor.c
> @@ -304,8 +304,7 @@ static void qmp_input_type_any(Visitor *v, QObject **obj, const char *name,
>      *obj = qobj;
>  }
>
> -static void qmp_input_optional(Visitor *v, bool *present, const char *name,
> -                               Error **errp)
> +static void qmp_input_optional(Visitor *v, bool *present, const char *name)
>  {
>      QmpInputVisitor *qiv = to_qiv(v);
>      QObject *qobj = qmp_input_get_object(qiv, name, true);
> diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c
> index bbd6a54..dee780a 100644
> --- a/qapi/string-input-visitor.c
> +++ b/qapi/string-input-visitor.c
> @@ -299,8 +299,7 @@ static void parse_type_number(Visitor *v, double *obj, const char *name,
>      *obj = val;
>  }
>
> -static void parse_optional(Visitor *v, bool *present, const char *name,
> -                           Error **errp)
> +static void parse_optional(Visitor *v, bool *present, const char *name)
>  {
>      StringInputVisitor *siv = DO_UPCAST(StringInputVisitor, visitor, v);
>
> diff --git a/scripts/qapi.py b/scripts/qapi.py
> index 1790e8f..b17f843 100644
> --- a/scripts/qapi.py
> +++ b/scripts/qapi.py
> @@ -1656,15 +1656,10 @@ def gen_visit_fields(members, prefix='', need_cast=False, skiperr=False):
>      for memb in members:
>          if memb.optional:
>              ret += mcgen('''
> -    visit_optional(v, &%(prefix)shas_%(c_name)s, "%(name)s", %(errp)s);
> +    if (visit_optional(v, &%(prefix)shas_%(c_name)s, "%(name)s")) {
>  ''',
>                           prefix=prefix, c_name=c_name(memb.name),
>                           name=memb.name, errp=errparg)
> -            ret += gen_err_check(skiperr=skiperr)
> -            ret += mcgen('''
> -    if (%(prefix)shas_%(c_name)s) {
> -''',
> -                         prefix=prefix, c_name=c_name(memb.name))
>              push_indent()
>
>          # Ugly: sometimes we need to cast away const