Re: [Libguestfs] Extending the nbdkit python plugin
On Thu, Nov 21, 2019 at 4:52 PM Richard W.M. Jones <rjones(a)redhat.com> wrote:
On Thu, Nov 21, 2019 at 02:17:43PM +0000, Daniel P. Berrangé wrote:
> I would define a new 'api_version()' method for your python
> plugins.
>
> Existing plugins don't implement that of course so when your
> C code invokes that it'll get an exception, and can assume
> the classic API contract for pwrite().
>
> If the plugin implements 'api_version()' returning '1' (or
> whatever), then this should inform the C code that the plugin
> has the 4 arg variant of pwrite().
I suspect this has to be the way to go. One reason is that
the V1 zero API is defined as:
def zero(h, count, offset, may_trim=False):
but in V2 we'd like to replace the fourth parameter with a flags
bitmask. I believe pure introspection can never detect this.
You can with:
def using_api_v2(module):
sig = inspect.signature(module.zero)
return "flags" in sig.parameters
See https://www.redhat.com/archives/libguestfs/2019-November/msg00130.html
using it to discover if ImageTransfer supports "format" argument.
But I would add:
API_VERSION = 2
And query it from the python plugin when importing the module.
Nir