[Libguestfs] [PATCH nbdkit v2 03/10] python: Implement nbdkit API version 2.

[Eric Blake]

On 11/22/19 3:20 PM, Nir Soffer wrote:

>>> +# There are several variants of the API.  nbdkit will call this
>>> +# function first to determine which one you want to use.  This is the
>>> +# latest version at the time this example was written.
>>> +def api_version():
>>> +    return 2
>>
>> Matches the C counterpart of #define NBDKIT_API_VERSION 2 at the top of
>> a plugin.
> 
> This is the same thing I was thinking about. This makes it more clear
> that the api
> version is constant, and not something the plugin should change while
> it is being used.

Hmm - api_version() really is constant for the entire life of nbdkit. We 
call it exactly once.  Figuring out how to read a Python variable 
instead of calling a function would be slightly more in line with the 
fact that in C code it is a #define constant rather than a function 
pointer callback.  But it is that much more glue code to figure out how 
to check for a python global variable, compared to the glue code we 
already have for calling a python function.

> 
> Same for all can_xxx functions, NBD does not support changing any of
> these anyway
> after negotiation.

While can_xxx functions are somewhat dynamic (we only call them once per 
connection, but connection A can be readonly while connection B is 
read-write, changing the can_write result, for example).  So those have 
to remain functions.


>> and for zero (once fast zero is supported later in the series), it could
>> look like:
>>
>> def zero(h, count, offset, may_trim=False, fua=False, fast=False):
> 
> This is nicer - but not extensible.

Why not?  Any future flag additions would still appear as new key=value 
kwargs.

> 
>> The user could also write:
>>
>> def zero(h, count, offset, **flags)
> 
> This is less nice, but easy to extend without adding v3. But I would
> call it **options
> instead.

I really need to post my patches where I played with kwargs, for 
contrast to this version.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org