xenial (3) nbdkit-python-plugin.3.gz

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,
       INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
       ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
       DAMAGE.