Re: [Libguestfs] [nbdkit PATCH v2 1/5] python: Let zero's may_trim parameter be optional

In preparation for adding other optional flag arguments to the python bindings, start by making the existing 'may_trim' flag to 'zero' be optional. That is, the plugin need not define the parameter if it does not make any semantic difference (ie. if the plugin ignores the hint and never trims); while if the parameter exists, we now pass it as a keyword argument rather than as a positional argument. This requires the plugin to give the parameter a specific name, and works whether or not the plugin provides a default for the parameter (although we do now recommend that plugins provide a default value, as it makes plugins built against newer nbdkit more likely to run even on older nbdkit, although that's not the direction we typically guarantee). We can't see through a plugin that used '**kwargs', but that is less likely. If we are super-worried about back-compatibility to older plugins which hard-coded 4 required parameters, we could also tweak the introspection to assume that a 'zero' with 4 parameters and no defaults, where we do not recognize the fourth parameter name, is an old-style plugin, and call it with may_trim passed via a positional rather than a keyword argument. However, I suspect most plugins just copied from our examples, which means they used the right parameter naming to just keep working; furthermore, while we have been clear that backwards API compatibility is essential for C, we have not made the same guarantee for language plugins. The introspection framework here will also make it easy to probe for future flag additions (support for 'fua' being the obvious candidate in the short term). This patch also fixes failure to check for (unlikely) errors during creation of 'args' within py_zero(). Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- plugins/python/nbdkit-python-plugin.pod | 35 ++++++++--- plugins/python/python.c | 108 +++++++++++++++++++++++++++++--- plugins/python/example.py | 2 +- tests/test.py | 2 +- 4 files changed, 129 insertions(+), 18 deletions(-)