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.

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