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.