Re: [Qemu-devel] [PATCH v2 1/3] backup: QEMU Backup Tool

[Stefan Hajnoczi]

On Wed, Aug 30, 2017 at 3:57 PM, Ishani
<address@hidden> wrote:
> ----- On Aug 30, 2017, at 9:51 AM, Fam Zheng address@hidden wrote:
>
>> On Tue, 08/29 22:13, Ishani Chugh wrote:
>>> +class BackupTool(object):
>>> +    """BackupTool Class"""
>>> +    def __init__(self, config_file=os.path.expanduser('~') +
>>> +                 '/.config/qemu/qemu-backup-config'):
>>> +        if "QEMU_BACKUP_CONFIG" in os.environ:
>>> +            self.config_file = os.environ["QEMU_BACKUP_CONFIG"]
>>> +        else:
>>> +            self.config_file = config_file
>>> +            try:
>>> +                if not os.path.isdir(os.path.dirname(self.config_file)):
>>> +                    os.makedirs(os.path.dirname(self.config_file))
>>> +            except:
>>> +                print("Cannot create config directory", file=sys.stderr)
>>> +                sys.exit(1)
>>> +        self.config = configparser.ConfigParser()
>>> +        self.config.read(self.config_file)
>>
>> I suggest adding versioning to the config file, so that a future update to 
>> this
>> tool can make an incompatible change without breaking older tool:
>>
>> [general]
>> version=0.1
>>
>> [guest_1]
>> ...
>>
>> [guest_2]
>> ...
>>
>> And only continue if the version is known.
>>
>> Fam
>
> I could not understand the intention behind adding versioning to config file.
> Can you please elaborate a little on what is meant by incompatible change?

If a new feature is added to qemu-backup in the future then it's
possible a user might have 2 machines, one with the old qemu-backup
version and one with the new qemu-backup.

When the new qemu-backup adds things to the configuration file the old
qemu-backup wouldn't know (because the code only queries the
configuration parameters it knows about).  Running the old qemu-backup
again could conflict with the new configuration.

Fam is suggesting putting a version number into the configuration
file.  This way the old qemu-backup can refuse to run with a newer
configuration file.

It also makes it possible for a new qemu-backup to detect that the
last run was with an older version so it can upgrade the configuration
file, if necessary.

Stefan