Re: [Qemu-devel] Ignoring errno makes QMP errors suck

[Kevin Wolf <kwolf@redhat.com>]

Am 26.03.2012 14:46, schrieb Luiz Capitulino:
> On Mon, 26 Mar 2012 10:39:50 +0200
> Kevin Wolf <kwolf@redhat.com> wrote:
> 
>> Hi,
>>
>> I keep getting reports of problems, with nice error descriptions that
>> usually look very similar to what I produced here:
>>
>> {"execute":"blockdev-snapshot-sync","arguments":{"device":"ide0-hd0","snapshot-file":"/tmp/backing.qcow2"}}
>> {"error": {"class": "OpenFileFailed", "desc": "Could not open
>> '/tmp/backing.qcow2'", "data": {"filename": "/tmp/backing.qcow2"}}}
>>
>> Who can tell me what has happened here? Oh, yes, the command failed, I
>> would have guessed that from the "error" key. But the actual error
>> description is as useless as it gets. It doesn't tell me anything about
>> _why_ the snapshot couldn't be created. ("Permission denied" would have
>> been the helpful additional information in this case)
> 
> There's a function called qemu_fopen_err() in the screendump conversion series
> that return more specific errors. It will be trivial to add qemu_open_err()
> as soon as qemu_fopen_err() is merged.
> 
> We're adding a bunch of more precise errors (some map directly to errno). That's
> the easy part. The hard part is to convert everything to use them.
> 
> Note that while it's true that this shouldn't have leaked to QMP, good error
> reporting is a general problem in QEMU.

I guess my point is that we're actually moving backwards here. In HMP
things like this do print the right error message (using error_report).
And the return code is passed all the way down to where the QMP error is
generated, it's just ignored there.

The last time I checked there was no easy way to handle it there because
errno and strerror(-errno) aren't things allowed in QMP messages. This
is the problem for which I wanted to get some attention.

Does the patch that you mentioned add a generic way for adding an
(converted) errno to QMP errors? Or does it split up existing errors
into more and finer grained errors?

Kevin