nbdkit-python-plugin man page
nbdkit-python-plugin — nbdkit python plugin
nbdkit python script=/path/to/plugin.py [arguments...]
"nbdkit-python-plugin" is an embedded Python interpreter for nbdkit(1), allowing you to write nbdkit plugins in Python.
Broadly speaking, Python nbdkit plugins work like C ones, so you should read nbdkit-plugin(3) first.
Python 2 and Python 3
The Python plugin has to be compiled for either Python 2 or Python 3 when building nbdkit. You can set the
"PYTHON" environment variable to the desired interpreter, otherwise nbdkit will use the interpreter called
"python" on the current
$PATH. For example:
Using a Python Nbdkit Plugin
Assuming you have a Python script which is an nbdkit plugin, you run it like this:
nbdkit python script=/path/to/plugin.py
You may have to add further
"key=value" arguments to the command line. Read the Python script to see if it requires any.
"script=..." must come first on the command line.
Writing a Python Nbdkit Plugin
There is an example Python nbdkit plugin called
"example.py" which ships with the nbdkit source.
To write a Python nbdkit plugin, you create a Python file which contains at least the following required functions (in the top level
def open(readonly): # see below def get_size(h): # see below def pread(h, count, offset): # see below
Note that the subroutines must have those literal names (like
"open"), because the C part looks up and calls those functions directly. You may want to include documentation and globals (eg. for storing global state). Any other top level statements are run when the script is loaded, just like ordinary Python.
The file does not need to include a
"#!" (hash-bang) at the top, and does not need to be executable. In fact it's a good idea not to do that, because running the plugin directly as a Python script won't work.
Python callbacks should throw exceptions to indicate errors.
This just documents the arguments to the callbacks in Python, and any way that they differ from the C callbacks. In all other respects they work the same way as the C callbacks, so you should go and read nbdkit-plugin(3).
def config(key, value): # no return value
There are no arguments or return value.
def open(readonly): # return handle
You can return any non-NULL Python value as the handle. It is passed back in subsequent calls.
def close(h): # no return value
"close"returns, the reference count of the handle is decremented in the C part, which usually means that the handle and its contents will be garbage collected.
def get_size(h): # return the size of the disk
def can_write(h): # return a boolean
def can_flush(h): # return a boolean
def is_rotational(h): # return a boolean
def can_trim(h): # return a boolean
def pread(h, count, offset): # construct a bytearray of length count bytes and return it
The body of your
"pread"function should construct a buffer of length (at least)
"count"bytes. You should read
"count"bytes from the disk starting at
NBD only supports whole reads, so your function should try to read the whole region (perhaps requiring a loop). If the read fails or is partial, your function should throw an exception.
def pwrite(h, buf, offset): length = len (buf) # no return value
The body of your
"pwrite"function should write the
"buf"string to the disk. You should write
"count"bytes to the disk starting at
NBD only supports whole writes, so your function should try to write the whole region (perhaps requiring a loop). If the write fails or is partial, your function should throw an exception.
def flush(h): # no return value
The body of your
"flush"function should do a sync(2) or fdatasync(2) or equivalent on the backing store.
def trim(h, count, offset): # no return value
The body of your
"trim"function should "punch a hole" in the backing store.
- Missing: "load" and "unload"
These are not needed because you can just use ordinary Python constructs.
- Missing: "name", "version", "longname", "description", "config_help"
These are not yet supported.
The thread model for Python callbacks currently cannot be set from Python. It is hard-coded in the C part to
"NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS". This may change or be settable in future.
nbdkit(1), nbdkit-plugin(3), python(1).
Richard W.M. Jones
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