Re: Questionable aspects of QEMU Error's design

[Markus Armbruster <armbru@redhat.com>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Thu, 2 Apr 2020 at 07:11, Vladimir Sementsov-Ogievskiy
> <vsementsov@virtuozzo.com> wrote:
>> Somehow, in general, especially with long function names and long parameter lists I prefer
>>
>> ret = func(..);
>> if (ret < 0) {
>>      return ret;
>> }
>
> Personally I prefer the other approach -- this one has an extra line
> in the source and
> needs an extra local variable.

Me too, except when func(...) is so long that

    if (func(...) < 0) {

becomes illegible like

    if (func(... yadda, yadda,
             yadda, ...) < 0) {

Then an extra variable can improve things.

>> Are you sure that adding a lot of boolean functions is a good idea? I somehow feel better with more usual int functions with -errno on failure.
>>
>> Bool is a good return value for functions which are boolean by nature: checks, is something correspond to some criteria. But for reporting an error I'd prefer -errno.
>
> When would we want to return an errno? I thought the whole point of the
> Error* was that that was where information about the error was returned.
> If all your callsites are just going to do "if (ret < 0) { ... } then having
> the functions pick an errno value to return is just extra work.

0/-1 vs. true/false is a matter of convention.  Lacking convention, it's
a matter of taste.

0/-1 vs. 0/-errno depends on the function and its callers.  When -errno
enables callers to distinguish failures in a sane and simple way, use
it.  When -errno feels "natural", I'd say feel free to use it even when
all existing callers only check < 0.

When you return non-null/null or true/false on success/error, neglecting
to document that in a function contract can perhaps be tolerated; you're
using the return type the common, obvious way.  But when you return 0/-1
or 0/-errno, please spell it out.  I've seen too many "Operation not
permitted" that were actually -1 mistaken for -EPERM.  Also too many
functions that return -1 for some failures and -errno for others.