Re: [Qemu-devel] Error propagation in generated visitors and command marshallers

[Anthony Liguori <anthony@codemonkey.ws>]

On Wed, Apr 9, 2014 at 8:48 AM, Markus Armbruster <armbru@redhat.com> wrote:
> I stumbled over this while trying to purge error_is_set() from the code.
>
>
> Here's how we commonly use the Error API:
>
>     Error *err = NULL;
>
>     foo(arg, &err)
>     if (err) {
>         goto out;
>     }
>     bar(arg, &err)
>     if (err) {
>         goto out;
>     }
>
> This ensures that err is null on entry, both for foo() and for bar().
> Many functions rely on that, like this:
>
>     void foo(ArgType arg, Error **errp)
>     {
>         if (frobnicate(arg) < 0) {
>             error_setg(errp, "Can't frobnicate");
>                                 // This asserts errp != NULL
>         }
>     }
>
>
> Here's how some of our visitor code uses the Error API (for real code,
> check out generated qmp-marshal.c):
>
>     Error *err = NULL;
>     QmpInputVisitor *mi = qmp_input_visitor_new_strict(QOBJECT(args));
>     Visitor *v = qmp_input_get_visitor(mi);
>     char *foo = NULL;
>     char *bar = NULL;
>
>     visit_type_str(v, &foo, "foo", &err);
>     visit_type_str(v, &bar, "bar", &err);
>     if (err) {
>         goto out;
>     }
>
> Unlike above, this may pass a non-null errp to the second
> visit_type_str(), namely when the first one fails.
>
> The visitor functions guard against that, like this:
>
>     void visit_type_str(Visitor *v, char **obj, const char *name, Error **errp)
>     {
>         if (!error_is_set(errp)) {
>             v->type_str(v, obj, name, errp);
>         }
>     }
>
> As discussed before, error_is_set() is almost almost wrong, fragile or
> unclean.  What if errp is null?  Then we fail to stop visiting after an
> error.
>
> The function could be improved like this:
>
>     void visit_type_str(Visitor *v, char **obj, const char *name, Error **errp)
>     {
>         assert(errp);
>         if (!*errp) {
>             v->type_str(v, obj, name, errp);
>         }
>     }
>
>
> But: is it a good idea to have both patterns in the code?  Should we
> perhaps use the common pattern for visiting, too?  Like this:
>
>     visit_type_str(v, &foo, "foo", &err);
>     if (err) {
>         goto out;
>     }
>     visit_type_str(v, &bar, "bar", &err);
>     if (err) {
>         goto out;
>     }
>
> Then we can assume *errp is clear on function entry, like this:
>
>     void visit_type_str(Visitor *v, char **obj, const char *name, Error **errp)
>     {
>         v->type_str(v, obj, name, errp);
>     }
>
> Should execute roughly the same number of conditional branches.
>
> Tedious repetition of "if (err) goto out" in the caller, but that's what
> we do elsewhere, and unlike elsewhere, these one's are generated.
>
> Opinions?

The original visiting code was loosely based on ASN1 marshaling code
from Samba which used the "if error, bail out at the top" style of
error handling.

As use of Error has evolved in QEMU, I agree that the paradigm of
"bail out as soon as you see an error and fail fast" is better so I'd
vote for changing the generated code to do that.

Regards,

Anthony Liguori