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

[Kevin Wolf <kwolf@redhat.com>]

Am 09.04.2014 um 17:48 hat Markus Armbruster geschrieben:
> 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?

I agree, use the same style as everywhere else.

The pattern in the generated visitor that I find more annoying, though,
is that it has a lot of code like:

    if (!error_is_set(errp)) {
        /* long block of code here */
    }

And I believe there are even cases where this nests. There are also
error_propagate() calls that can (and do in the common case) propagate
NULL, this way selecting the first error, if any, but not stopping on
the first error. I always found it confusing to read that code.

Kevin