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Re: [Qemu-block] [Qemu-devel] [PATCH v2 1/1] block: improve error handli

From: Max Reitz
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v2 1/1] block: improve error handling in raw_open
Date: Tue, 26 Jul 2016 19:47:32 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

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. 
>> Therefore, the common pattern is to save the errno value before calling
>> this function, i.e. to put the function call...
>>>>          ret = -errno;
>> ...here.
>> With that fixed, the patch should be good.
>> Max
> But now that you have this pointed out, I understand, it is better to
> have no function calls between failure and saving the errno. I will post
> a v3 and keep this in mind for the future. Sorry for the extra work.
> Frankly, I'm a bit uncomfortable with asking (do not want to be pushy),
> but do you have an opinion on the 'error_report_err' issue (pointed
> out in the cover letter part)?

You are using drive_add with QMP? As far as I know, one can only do so
with human-monitor-command which returns the error string like so:

{"return": "Could not open file: No such file or directory\r\n"}

Apart from that, as for QMP, in theory you're supposed to use
blockdev-add and device_add, I think (although blockdev-add is still
considered experimental...). And blockdev-add will return the error
string like so:

{"error": {"class": "GenericError", "desc": "Could not open file: No
such file or directory"}}


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