[libvirt] [PATCH 1/3] Introduce flag representing if MAC address of interface was generated or not.

[Laine Stump]

(I found this mostly-composed message sitting behind a bunch of windows 
on my desktop when I decided it was time to reboot just so I could have 
an apparently clean slate...)

On 03/01/2011 03:58 AM, Michal Prívozník wrote:
> On 02/25/2011 05:42 PM, Eric Blake wrote:
>> On 02/24/2011 07:56 AM, Michal Privoznik wrote:
>>> ---
>>>   src/conf/domain_conf.c |    2 ++
>>>   src/conf/domain_conf.h |    1 +
>>>   2 files changed, 3 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
>>> index b97c1f0..454f631 100644
>>> --- a/src/conf/domain_conf.c
>>> +++ b/src/conf/domain_conf.c
>>> @@ -2593,8 +2593,10 @@ virDomainNetDefParseXML(virCapsPtr caps,
>>>                                    (const char *)macaddr);
>>>               goto error;
>>>           }
>>> +        def->mac_generated = false;
>>>       } else {
>>>           virCapabilitiesGenerateMac(caps, def->mac);
>>> +        def->mac_generated = true;
>>>       }
>>
>> Rather than sticking the flag into the domain definition, I'd rather
>> pass it via the flags argument to
>> virDomainDeviceDefParse/virDomainNetDefParseXML - that is, most callers
>> will want:
>>
>> virDomainDeviceDefParse(..., 0)
>>
>> but qemuDomainDetachDevice (in qemu_driver.c) will want:
>>
>> virDomainDeviceDefParse(..., VIR_DOMAIN_PARSE_NO_GENERATE)
>>
>> (or maybe VIR_DOMAIN_XML_INACTIVE vs.
>> VIR_DOMAIN_XML_INACTIVE|VIR_DOMAIN_PARSE_NO_GENERATE)
>>
>> That is, it's more than just detaching an interface mac addresses that
>> has an issue.  It's anywhere that parsing a device XML snippet to be
>> used to match an existing device that we want to suppress generation of
>> additional items in the parse (this includes generating a random mac for
>> an interface, but may include other generated items).  So adding a new
>> flag that can be used to tell ALL parse routines to avoid generating
>> extra data is more useful than changing the domain definition to say
>> whether one piece of information was generated.
>>


Personally, I think that a function called "...Parse" should do just 
that - parse what is provided an return it in a different format. There 
are several cases in libvirt of parse functions that have side effects 
like this and, while they are convenient, I think it would be cleaner 
for the parsers to stick to what they say their duty is, and for the 
callers to do this generation if needed.


>
> Yes, i firstly thought in similar way. But I don't really need to 
> suppress generation of random values. I need information whether user 
> gave say mac in input XML or not.


Yes, you don't need to suppress them, but you also don't need to 
generate them. You just need to know whether or not the user supplied one.


> Same apply for pci address (by the way, we don't support detaching by 
> it now). The idea behind is:
> if (<mac was generated> && <pci addr was generated>) {
>    if (domain->nnets == 1)
>        device_clone_mac(domain->nnet[0], device)
>    else
>        error("don't know which detach")
> } else {
>    search for device
> }
>
> If i suppress generation, I wouldn't know if it was generated anyway, 
> because mac is stored in statically allocated array. One (tricky) way 
> out of this is to set mac before to some special mac (e.g. all 0's) 
> and then check for change. But I'd rather avoid it. Changing array to 
> dynamically allocated is huge amount of work to be done, because it is 
> hard to find pieces of code touching it, and omitting even one line 
> may lead into sigegv.


In the end, I think that the totally proper way of handling this would 
be to either:

1) add a flag to the def called mac_specified (or mac_is_valid or 
whatever), that would be 0 when the def is created, and set to 1 any 
time a value is given to it,

or

2) as you say, make the mac a dynamically allocated piece of data, so 
that a NULL pointer would be an implicit "mac_specified == FALSE".

The same goes for any other piece of data in any other def that is 
marked as optional in the RNG.

But that is all a bit pie-in-the-sky, since the existing code works, and 
"fixing" it could possibly actually break something :-)

When I recently had the same problem (adding a mac address to a 
virNetworkDef), I decided to make the mac address a fixed size array to 
simplify memory management and follow the precedent of the mac address 
in virDomainNetDef, and add a mac_specified boolean just after it. 
Consumers of virNetworkDef who really do need a mac address can generate 
one themselves if one isn't given, but those consumers that don't need 
one aren't burdened with it. (This has extra importance with objects 
that are saved as config, because you otherwise end up with something in 
the saved config that the user neither specified nor wants).


>
> But I agree, sticking the flag into domain definition is not very 
> clean either.
>
> One way out of this is adding pointer to flag as (another) function 
> parameter. Would this be acceptable?

I think that's even more obfuscated. If having the extra flag in the Def 
struct is confusing/unclean, then my vote would go for dynamically 
allocating mac when it's there (that's a potential source of null 
dereferences and memory leaks, though; everything has a confusion factor 
and a price).