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