NAME¶
nbdkit-python-plugin - nbdkit python plugin
SYNOPSIS¶
nbdkit python script=/path/to/plugin.py [arguments...]
DESCRIPTION¶
"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.
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 "__main__"
module):
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.
EXCEPTIONS¶
Python callbacks should throw exceptions to indicate errors.
PYTHON CALLBACKS¶
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).
- "config"
- (Optional)
def config(key, value):
# no return value
- "config_complete"
- (Optional)
There are no arguments or return value.
- "open"
- (Required)
def open(readonly):
# return handle
You can return any non-NULL Python value as the handle. It is passed back in
subsequent calls.
- "close"
- (Optional)
def close(h):
# no return value
After "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.
- "get_size"
- (Required)
def get_size(h):
# return the size of the disk
- "can_write"
- (Optional)
def can_write(h):
# return a boolean
- "can_flush"
- (Optional)
def can_flush(h):
# return a boolean
- "is_rotational"
- (Optional)
def is_rotational(h):
# return a boolean
- "can_trim"
- (Optional)
def can_trim(h):
# return a boolean
- "pread"
- (Required)
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 "offset".
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.
- "pwrite"
- (Optional)
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 "offset".
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.
- "flush"
- (Optional)
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.
- "trim"
- (Optional)
def trim(h, count, offset):
# no return value
The body of your "trim" function should "punch a hole"
in the backing store.
MISSING CALLBACKS¶
- 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.
THREADS¶
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.
SEE ALSO¶
nbdkit(1),
nbdkit-plugin(3),
python(1).
AUTHORS¶
Richard W.M. Jones
COPYRIGHT¶
Copyright (C) 2013-2014 Red Hat Inc.
LICENSE¶
Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:
- •
- Redistributions of source code must retain the above copyright notice,
this list of conditions and the following disclaimer.
- •
- Redistributions in binary form must reproduce the above copyright notice,
this list of conditions and the following disclaimer in the documentation
and/or other materials provided with the distribution.
- •
- Neither the name of Red Hat nor the names of its contributors may be used
to endorse or promote products derived from this software without specific
prior written permission.
THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS
OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
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NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
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