Re: [Qemu-devel] [PATCH v2 1/1] block: improve error handling in raw_open

[Markus Armbruster <armbru@redhat.com>]

Max Reitz <mreitz@redhat.com> writes:

> On 26.07.2016 19:18, Halil Pasic wrote:
>> 
>> 
>> On 07/26/2016 05:42 PM, Max Reitz wrote:
>>>> +++ b/block/raw-posix.c
>>>>> @@ -485,6 +485,7 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
>>>>>      s->fd = -1;
>>>>>      fd = qemu_open(filename, s->open_flags, 0644);
>>>>>      if (fd < 0) {
>>>>> +        error_setg_errno(errp, errno, "Could not open file");
>>> We don't guarantee that error_setg_errno() does not modify errno. (In
>>> practice it should not, but we don't guarantee that.)
>>>
>> 
>> 
>> Thank you very much for your review. I have double checked, and I
>> remembered correctly: error_setg_errno saves and restores the original
>> errno, so that is why I assumed it does not. 
>
> Oh, and about this: Yes, I remember, this was introduced after we had
> noticed that we had some old code that assumed that error_setg() would
> not modify errno. We had to decide between making error_setg*() save and
> restore errno (which we deemed a bit ugly) and fixing all of that old
> code (which we deemed hard). So we want for the former, but I don't
> think we actually guarantee that behavior (because you should never rely
> on any function not to modify errno).

Since we rely on this behavior, we should definitely spell it out in the
function contract.

> (The difference between us saving/restoring errno in practice and
> guaranteeing that feature is the lack of documentation thereof, i.e.,
> the comment for error_setg() in include/qapi/error.h doesn't mention
> this :-))

Let's fix it.