Re: [Libguestfs] Extending the nbdkit python plugin

We have an nbdkit plugin that lets you write NBD servers in Python. An example of an existing Python plugin is here: https://github.com/libguestfs/nbdkit/blob/master/plugins/python/example.p... This morning I tried to modify the plugin to use the newer nbdkit API (version 2). One of the things that would change would be passing flags parameters to some functions, eg: def pwrite (h, buf, offset): might become one of these possibilities (where flags is a bitmask): def pwrite (h, buf, offset, flags): def pwrite (h, buf, offset, flags=0): The problem is if we did this it would break all existing Python plugins. While we don't guarantee the nbdkit API for non-C languages, we do nevertheless have Python plugins that we care about such as the rhv-upload-plugin used by virt-v2v. Having a flag day which breaks all existing plugins is very awkward. I tried to simply pass the extra arguments from the C code to the Python code, and existing plugins break with: nbdkit: python[1]: error: ./test.py: pwrite: error: pwrite() takes 3 positional arguments but 4 were given One possibility is that we could introspect the plugin to find out how many parameters it takes. This is possible, but very difficult from C. (See https://stackoverflow.com/a/41188411 for how to do it from Python).

Another possibility is we could encourage existing Python plugins to add **kwargs to all functions where we might plausibly add extra parameters in future, ie. the above would become: def pwrite (h, buf, offset, **kwargs): This still requires all Python plugins to change, but at least they would remain backwards compatible with old and new nbdkit. However I couldn't actually work out how to make this work because: >>> def test(a, **kwargs): ... pass ... >>> test(1) >>> test(1,2) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: test() takes 1 positional argument but 2 were given Yet another possibility is the Python plugin itself should declare which version of the API it wants, and the C code can then pass the correct parameters. (This is in fact how nbdkit C plugins work). This pushes a bunch of work into the C code, but I guess we can deal with that.