Re: [PATCH v2 4/6] qapi: Apply aliases in qobject-input-visitor

[Markus Armbruster]

Kevin Wolf <kwolf@redhat.com> writes:

> When looking for an object in a struct in the external representation,
> check not only the currently visited struct, but also whether an alias
> in the current StackObject matches and try to fetch the value from the
> alias then. Providing two values for the same object through different
> aliases is an error.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  qapi/qobject-input-visitor.c | 214 +++++++++++++++++++++++++++++++++--
>  1 file changed, 205 insertions(+), 9 deletions(-)
>
> diff --git a/qapi/qobject-input-visitor.c b/qapi/qobject-input-visitor.c
> index dd04ef0027..3ea5e5abd6 100644
> --- a/qapi/qobject-input-visitor.c
> +++ b/qapi/qobject-input-visitor.c
> @@ -95,6 +95,8 @@ struct QObjectInputVisitor {
>      QObject *root;
>      bool keyval;                /* Assume @root made with keyval_parse() */
>  
> +    QDict *empty_qdict;         /* Used for implicit objects */
> +
>      /* Stack of objects being visited (all entries will be either
>       * QDict or QList). */
>      QSLIST_HEAD(, StackObject) stack;
> @@ -167,9 +169,178 @@ static const char *full_name(QObjectInputVisitor *qiv, 
> const char *name)
>      return full_name_so(qiv, name, false, tos);
>  }
>  
> +static bool find_object_member(QObjectInputVisitor *qiv,
> +                               StackObject **so, const char **name,
> +                               bool *implicit_object, Error **errp);
> +
> +/*
> + * Check whether the alias member defined by @a is present in the
> + * input and can be used to obtain the value for the member @name in
> + * the currently visited object.
> + */
> +static bool alias_present(QObjectInputVisitor *qiv,
> +                          InputVisitorAlias *a, const char *name)
> +{
> +    StackObject *so = a->alias_so;
> +
> +    /*
> +     * The passed source @name is only relevant for wildcard aliases which
> +     * don't have a separate name, otherwise we use the alias name.
> +     */
> +    if (a->name) {
> +        name = a->name;
> +    }
> +
> +    /*
> +     * Check whether the alias member is present in the input
> +     * (possibly recursively because aliases are transitive).
> +     */
> +    if (!find_object_member(qiv, &so, &name, NULL, NULL)) {
> +        return false;
> +    }
> +
> +    /*
> +     * Every source can be used only once. If a value in the input would end 
> up
> +     * being used twice through aliases, we'll fail the second access.
> +     */
> +    if (!g_hash_table_contains(so->h, name)) {
> +        return false;
> +    }
> +
> +    return true;
> +}
> +
> +/*
> + * Check whether the member @name in the object visited by @so can be
> + * specified in the input by using the alias described by @a.
> + *
> + * If @name is only a prefix of the alias source, but doesn't match
> + * immediately, false is returned and @implicit_object is set to true
> + * if it is non-NULL.  In all other cases, @implicit_object is left
> + * unchanged.
> + */
> +static bool alias_source_matches(QObjectInputVisitor *qiv,
> +                                 StackObject *so, InputVisitorAlias *a,
> +                                 const char *name, bool *implicit_object)
> +{
> +    if (a->src[0] == NULL) {
> +        assert(a->name == NULL);
> +        return true;
> +    }
> +
> +    if (!strcmp(a->src[0], name)) {
> +        if (a->name && a->src[1] == NULL) {
> +            /*
> +             * We're matching an exact member, the source for this alias is
> +             * immediately in @so.
> +             */
> +            return true;
> +        } else if (implicit_object) {
> +            /*
> +             * We're only looking at a prefix of the source path for the 
> alias.
> +             * If the input contains no object of the requested name, we will
> +             * implicitly create an empty one so that the alias can still be
> +             * used.
> +             *
> +             * We want to create the implicit object only if the alias is
> +             * actually used, but we can't tell here for wildcard aliases 
> (only
> +             * a later visitor call will determine this). This means that
> +             * wildcard aliases must never have optional keys in their source
> +             * path.
> +             */
> +            if (!a->name || alias_present(qiv, a, a->name)) {
> +                *implicit_object = true;
> +            }
> +        }
> +    }
> +
> +    return false;
> +}
> +
> +/*
> + * Find the place in the input where the value for the object member
> + * @name in @so is specified, considering applicable aliases.
> + *
> + * If a value could be found, true is returned and @so and @name are
> + * updated to identify the key name and StackObject where the value
> + * can be found in the input.  (This is either unchanged or the
> + * alias_so/name of an alias.)  The value of @implicit_object on
> + * return is undefined in this case.
> + *
> + * If no value could be found in the input, false is returned.  This
> + * is not an error and @errp remains unchanged.  If @implicit_object
> + * is non-NULL, it is set to true if the given name is a prefix of the
> + * source path of an alias for which a value may be present in the
> + * input.  It is set to false otherwise.
> + *
> + * If an error occurs (e.g. two values are specified for the member
> + * through different names), false is returned and @errp is set.  The
> + * value of @implicit_object on return is undefined in this case.
> + */
> +static bool find_object_member(QObjectInputVisitor *qiv,
> +                               StackObject **so, const char **name,
> +                               bool *implicit_object, Error **errp)
> +{
> +    StackObject *cur_so = *so;
> +    QDict *qdict = qobject_to(QDict, cur_so->obj);
> +    const char *found = NULL;
> +    bool found_is_wildcard = false;
> +    InputVisitorAlias *a;
> +
> +    if (implicit_object) {
> +        *implicit_object = false;
> +    }
> +
> +    /* Directly present in the container */
> +    if (qdict_haskey(qdict, *name)) {
> +        found = *name;
> +    }
> +
> +    /*
> +     * Find aliases whose source path matches @name in this StackObject. We 
> can
> +     * then get the value with the key a->name from a->alias_so.
> +     */
> +    QSLIST_FOREACH(a, &cur_so->aliases, next) {

After looking at the complete loop, I understand:

    @found is non-null if we have input, either non-alias input found
    above, or alias input found below.

    @found_is_wildcard is true if that input is from a wildcard alias.

    If @found, *so is the alias's outer StackObject, i.e. where it was
    defined.  Else, @so is still the argument value.  In either case,
    @cur_so is the argument value.

    If found, it is the member name in so->obj.  It'll be stored to
    *name after the loop.

    Updating *so in the loop and *name after the loop feels a bit
    awkward.  Using a new variable @found_so to go with @found might
    come out nicer.  No need for @cur_so then.  See also "*so may
    already be messed up" below.

> +        if (a->name == NULL && found) {
> +            /*
> +             * Skip wildcard aliases if we already have a match. This is
> +             * not a conflict that should result in an error.
> +             */
> +            continue;

Ignore wildcard alias when we have other input.

If multiple different wildcard aliases apply, we silently ignore all but
the first one.  Can this happen?

> +        }
> +
> +        if (!alias_source_matches(qiv, cur_so, a, *name, implicit_object)) {

The input for this alias is deeper.  *implicit_object is now true if a
visit of @name needs to be forced to maybe resolve it.  See
qobject_input_try_get_object() below.

> +            continue;
> +        }

Cases:

1. This is a wildcard alias propagated to @cur_so, i.e. the member *name
of cur_so->obj can serve as input.

2. This is a non-wildcard alias, and the member of cur_so->obj named by
it can serve as input, if it exists.

The alias_present() below uses *name when a is a wildcard, else a->name.
So this checks whether the member we want for input exists.

> +
> +        if (!alias_present(qiv, a, *name)) {
> +            continue;

It doesn't exist; ignore the alias.

> +        }

It exists; the alias applies.

> +
> +        if (found && !found_is_wildcard) {

The alias applies, but we already found non-alias input or non-wildcard
alias input.  This is an error:

> +            error_setg(errp, "Value for parameter %s was already given "
> +                       "through an alias",
> +                       full_name_so(qiv, *name, false, *so));
> +            return false;

Note: *so may already be messed up here.  I guess (hope?) the callers
are fine with that.  The contract should spell it out, though.  The
issue goes away if you store to *so only after the loop.

> +        } else {

The alias applies, and nothing or only wildcard aliases applied before.

In the latter case, this alias is not a wildcard, because we skipped
those above.

Taken together:

* More than one input is an error regardless of where it comes from
 (direct or alias), except

* we silently ignore extra wildcard alias input.

The error makes sense.

Ignoring one wildcard alias when there is non-wildcard input makes
sense.

When multiple wildcard aliases clash, I think we use the first one, and
ignore the others.  I'm not sure that's what we want.

> +            found = a->name ?: *name;
> +            *so = a->alias_so;
> +            found_is_wildcard = !a->name;

Record the alias applies.  Update *so, but not *name.

> +        }
> +    }
> +
> +    /* Chained aliases: *so/found might be the source of another alias */
> +    if (found && (*so != cur_so || found != *name)) {
> +        find_object_member(qiv, so, &found, NULL, errp);

I'm sure there is a reason why we don't need to know whatever
@implicit_object can tell us, but it escapes me at the moment.

> +    }
> +
> +    *name = found;

We zap *name when nothing was found.  I guess the callers are fine with
that.  Nevertheless, the contract should spell it out.  Storing only if
found might be simpler.

Oh, the caller right above might actually rely on this behavior.  If
that's the case, more reason to spell it out in the contract.

> +    return found;
> +}
> +
>  static QObject *qobject_input_try_get_object(QObjectInputVisitor *qiv,
>                                               const char *name,
> -                                             bool consume)
> +                                             bool consume, Error **errp)
>  {
>      StackObject *tos;
>      QObject *qobj;
> @@ -187,10 +358,30 @@ static QObject 
> *qobject_input_try_get_object(QObjectInputVisitor *qiv,
>      assert(qobj);
>  
>      if (qobject_type(qobj) == QTYPE_QDICT) {
> -        assert(name);
> -        ret = qdict_get(qobject_to(QDict, qobj), name);
> -        if (tos->h && consume && ret) {
> -            bool removed = g_hash_table_remove(tos->h, name);
> +        StackObject *so = tos;
> +        const char *key = name;
> +        bool implicit_object;
> +
> +        assert(key);
> +        if (!find_object_member(qiv, &so, &key, &implicit_object, errp)) {
> +            if (implicit_object) {
> +                /*
> +                 * The member is not present in the input, but
> +                 * something inside of it might still be given through
> +                 * an alias. Pretend there was an empty object in the
> +                 * input.
> +                 */
> +                if (!qiv->empty_qdict) {
> +                    qiv->empty_qdict = qdict_new();
> +                }
> +                return QOBJECT(qiv->empty_qdict);
> +            } else {
> +                return NULL;
> +            }
> +        }
> +        ret = qdict_get(qobject_to(QDict, so->obj), key);
> +        if (so->h && consume && ret) {
> +            bool removed = g_hash_table_remove(so->h, key);
>              assert(removed);
>          }
>      } else {
> @@ -216,9 +407,10 @@ static QObject 
> *qobject_input_get_object(QObjectInputVisitor *qiv,
>                                           const char *name,
>                                           bool consume, Error **errp)
>  {
> -    QObject *obj = qobject_input_try_get_object(qiv, name, consume);
> +    ERRP_GUARD();
> +    QObject *obj = qobject_input_try_get_object(qiv, name, consume, errp);
>  
> -    if (!obj) {
> +    if (!obj && !*errp) {
>          error_setg(errp, QERR_MISSING_PARAMETER, full_name(qiv, name));
>      }
>      return obj;
> @@ -799,13 +991,16 @@ static bool qobject_input_type_size_keyval(Visitor *v, 
> const char *name,
>  static void qobject_input_optional(Visitor *v, const char *name, bool 
> *present)
>  {
>      QObjectInputVisitor *qiv = to_qiv(v);
> -    QObject *qobj = qobject_input_try_get_object(qiv, name, false);
> +    Error *local_err = NULL;
> +    QObject *qobj = qobject_input_try_get_object(qiv, name, false, 
> &local_err);
>  
> -    if (!qobj) {
> +    /* If there was an error, let the caller try and run into the error */
> +    if (!qobj && !local_err) {
>          *present = false;
>          return;
>      }
>  
> +    error_free(local_err);
>      *present = true;
>  }
>  
> @@ -820,6 +1015,7 @@ static void qobject_input_free(Visitor *v)
>          qobject_input_stack_object_free(tos);
>      }
>  
> +    qobject_unref(qiv->empty_qdict);
>      qobject_unref(qiv->root);
>      if (qiv->errname) {
>          g_string_free(qiv->errname, TRUE);