Kevin Wolf <kw...@redhat.com> writes: > 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.
I also find "if (error) bail_out" generally more readable than "if (!error) do_more_work". More so when nested. I'll see what I can do about it in the generator scripts. > 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. Can you point me to an instance in the generated code?