[libvirt] [PATCHv2] Don't log an internal error when the guest hasn't updated balloon stats

[Eric Blake]

On 05/15/2014 01:22 AM, Ján Tomko wrote:
> If virDomainMemoryStats is called too soon after domain startup,
> QEMU returns:
> "error":{"class":"GenericError","desc":"guest hasn't updated any stats yet"}
> when we try to query balloon stats.
> 
> Check for this reply and log it as OPERATION_INVALID instead of
> INTERNAL_ERROR. This means the daemon only logs it at the debug level,
> without polluting system logs.
> 
> Reported by Laszlo Pal:
> https://www.redhat.com/archives/libvirt-users/2014-May/msg00023.html
> ---
> v1: https://www.redhat.com/archives/libvir-list/2014-May/msg00420.html
> v2:
>   return 0 in this case - even though balloon stats are not yet available,
>     we can still return 'rss' in qemuDomainMemoryStats
>   jump to cleanup if CheckError returns < 0
> 
>  src/qemu/qemu_monitor_json.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)

> +    if ((data = virJSONValueObjectGet(reply, "error"))) {
> +        const char *klass = virJSONValueObjectGetString(data, "class");
> +        const char *desc = virJSONValueObjectGetString(data, "desc");
>  
> -    if (ret < 0)
> +        if (STREQ_NULLABLE(klass, "GenericError") &&
> +            STREQ_NULLABLE(desc, "guest hasn't updated any stats yet")) {

Adding qemu.  Uggh - the qemu documentation of QMP states:

- The "desc" member is a human-readable error message. Clients should
  not attempt to parse this message.

because the contents of that field are NOT guaranteed to be stable.
We're stuck parsing that field for old versions of qemu, but this is one
case where upstream qemu (for future versions) should change the "class"
member of that particular error case to a distinct value other than
GenericError so that it is trivially obvious when this particular
condition has occurred, since it is a case where libvirt wants to treat
it as a non-error.

reluctant ACK, while hoping that we can do something more reliable in
the future.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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