=head1 NAME

nbdkit-python-plugin - nbdkit python plugin

=head1 SYNOPSIS

 nbdkit python /path/to/plugin.py [arguments...]

=head1 DESCRIPTION

C<nbdkit-python-plugin> is an embedded Python interpreter for
L<nbdkit(1)>, allowing you to write nbdkit plugins in Python 3.

=head2 If you have been given an nbdkit Python plugin

Assuming you have a Python script which is an nbdkit plugin, you run it
like this:

 nbdkit python /path/to/plugin.py

You may have to add further C<key=value> arguments to the command
line.  Read the Python script to see if it requires any.

=head1 WRITING A PYTHON NBDKIT PLUGIN

For example plugins written in Python, see:
L<https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/python/examples>

Broadly speaking, Python nbdkit plugins work like C ones, so you should
read L<nbdkit-plugin(3)> first.

To write a Python nbdkit plugin, you create a Python file which
contains at least the following required functions (in the top level
C<__main__> module):

 API_VERSION = 2
 def open(readonly):
   # see below
 def get_size(h):
   # see below
 def pread(h, buf, offset, flags):
   # see below

Note that the subroutines must have those literal names (like C<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.

=head2 Python versions

Since nbdkit E<ge> 1.16 only Python 3 is supported.  If you wish to
continue using nbdkit plugins written in Python 2 then you must use
nbdkit E<le> 1.14, but we advise you to update your plugins.

The version of Python 3 is chosen when nbdkit is built.  This is
compiled in and can't be changed at runtime.  C<./configure> looks for
(in order):

=over 4

=item *

the C<PYTHON> variable
(eg C<./configure PYTHON=/usr/bin/python3.9>)

=item *

F<python3> on C<$PATH>

=item *

F<python> on C<$PATH>

=back

C<./configure> will fail if the first interpreter found is a Python 2
interpreter.

To find out which version of Python C<nbdkit-python-plugin> was
compiled for, use the I<--dump-plugin> option:

 $ nbdkit python --dump-plugin
 ...
 python_version=3.7.0
 python_pep_384_abi_version=3

=head2 API versions

The nbdkit API has evolved and new versions are released periodically.
To ensure backwards compatibility plugins have to opt in to the new
version.  From Python you do this by declaring a constant in your
module:

 API_VERSION = 2

(where 2 is the latest version at the time this documentation was
written).  All newly written Python modules must have this constant.

=head2 Executable script

If you want you can make the script executable and include a "shebang"
at the top:

 #!/usr/sbin/nbdkit python

See also L<nbdkit(1)/Shebang scripts>.

These scripts can also be installed in the C<$plugindir>.  See
L<nbdkit-plugin(3)/WRITING PLUGINS IN OTHER PROGRAMMING LANGUAGES>.

=head2 Module functions

Your script may use C<import nbdkit> to have access to the following
methods in the C<nbdkit> module:

=head3 C<nbdkit.debug(msg)>

Send a debug message to stderr or syslog if verbose messages are
enabled.

=head3 C<nbdkit.disconnect(force)>

Disconnect from the client.  If C<force> is C<True> then nbdkit will
disconnect the client immediately.

=head3 C<nbdkit.export_name()>

Return the export name negotiated with the client as a Unicode string.
Note this should not be trusted because the client can send whatever
it wants.

=head3 C<nbdkit.is_tls()>

Returns C<True> if the client completed TLS authentication, or
C<False> if the connection is plaintext.

=head3 C<nbdkit.nanosleep(secs, nsecs)>

Sleep for seconds and nanoseconds.

=head3 C<nbdkit.parse_delay(what, str)>

Parse a delay or sleep (such as "10ms") into a pair (sec, nsec).
Wraps the L<nbdkit_parse_delay(3)> function.

=head3 C<nbdkit.parse_size(str)>

Parse a string (such as "100M") into a size in bytes.  Wraps the
L<nbdkit_parse_size(3)> C function.

=head3 C<nbdkit.parse_probability(what, str)>

Parse a string (such as "100%") into a probability, returning a
floating point number.  Wraps the L<nbdkit_parse_probability(3)>
function.

=head3 C<nbdkit.peer_pid()>,
C<nbdkit.peer_uid()>,
C<nbdkit.peer_gid()>,
C<nbdkit.peer_security_context()>

Return the client process ID, user ID, group ID or security context.
The PID, UID and GID are only available when the client connects by
Unix domain socket, and then only on some operating systems.  The
security context is usually the SELinux label, IPSEC label or
NetLabel.

=head3 C<nbdkit.peer_tls_dn()>

Return the client TLS Distinguished Name.
See L<nbdkit_peer_tls_dn(3)>.

=head3 C<nbdkit.peer_tls_issuer_dn()>

Return the client certificate issuer's TLS Distinguished Name.
See L<nbdkit_peer_tls_issuer_dn(3)>.

=head3 C<nbdkit.read_password(value)>

Read a password from a config parameter.  This returns the password as
a Python C<bytes> object.  See L<nbdkit_read_password(3)> for more
information on the different ways that the C<value> parameter can be
parsed.

=head3 C<nbdkit.set_error(err)>

Throwing a Python exception from a callback causes an error message to
be sent back to the NBD client.  The NBD protocol allows an error code
(ie. errno) to be sent to the client, but by default the Python plugin
always sends C<EIO>.  To control what error code is sent call
C<nbdkit.set_error>:

 def pread(h, buf, offset):
   if access_denied:
     nbdkit.set_error(errno.EPERM)
     raise RuntimeError()

=head3 C<nbdkit.shutdown()>

Request asynchronous server shutdown.

=head3 C<nbdkit.stdio_safe()>

Returns C<True> if it is safe to interact with stdin and stdout
during the configuration phase.

=head2 Module constants

After C<import nbdkit> the following constants are available.  These
are used in the callbacks below.

=over 4

=item C<nbdkit.THREAD_MODEL_SERIALIZE_CONNECTIONS>

=item C<nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS>

=item C<nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS>

=item C<nbdkit.THREAD_MODEL_PARALLEL>

Possible return values from C<thread_model()>.

=item C<nbdkit.FLAG_MAY_TRIM>

=item C<nbdkit.FLAG_FUA>

=item C<nbdkit.FLAG_REQ_ONE>

=item C<nbdkit.FLAG_FAST_ZERO>

Flags bitmap passed to certain plugin callbacks.  Not all callbacks
with a flags parameter use all of these flags, consult the
documentation below and L<nbdkit-plugin(3)>.

=item C<nbdkit.FUA_NONE>

=item C<nbdkit.FUA_EMULATE>

=item C<nbdkit.FUA_NATIVE>

Possible return values from C<can_fua()>.

=item C<nbdkit.CACHE_NONE>

=item C<nbdkit.CACHE_EMULATE>

=item C<nbdkit.CACHE_NATIVE>

Possible return values from C<can_cache()>.

=item C<nbdkit.EXTENT_HOLE>

=item C<nbdkit.EXTENT_ZERO>

Used in the C<type> field returned by C<extents()>.

=back

=head2 Threads

The thread model for Python callbacks defaults to
C<nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS>.

Since S<nbdkit 1.22> it has been possible to set this by implementing
a C<thread_model()> function which returns one of the constants
C<nbdkit.THREAD_MODEL_*>.

The Python Global Interpreter Lock (GIL) is still used, so Python code
does not run in parallel.  However if a plugin callback calls a
library which blocks (eg. to make an HTTP request), then another
callback might be executed in parallel.  Plugins which use
C<nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS> or
C<nbdkit.THREAD_MODEL_PARALLEL> may need to use locks on shared data.

=head2 Exceptions

Python callbacks should throw exceptions to indicate errors.  Remember
to use C<nbdkit.set_error> if you need to control which error is sent
back to the client; if omitted, the client will see an error of C<EIO>.

=head2 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).

=over 4

=item C<dump_plugin>

(Optional)

There are no arguments or return value.

=item C<config>

(Optional)

 def config(key, value):
   # no return value

=item C<config_complete>

(Optional)

There are no arguments or return value.

=item C<thread_model>

(Optional, nbdkit E<ge> 1.22)

 def thread_model():
   return nbdkit.THEAD_MODEL_SERIALIZE_ALL_REQUESTS

See L</Threads> above.

=item C<get_ready>

(Optional)

There are no arguments or return value.

=item C<after_fork>

(Optional, nbdkit E<ge> 1.26)

There are no arguments or return value.

=item C<cleanup>

(Optional, nbdkit E<ge> 1.28)

There are no arguments or return value.

=item C<list_exports>

(Optional)

 def list_exports(readonly, is_tls):
   # return an iterable object (eg. list) of
   # (name, description) tuples or bare names:
   return [ (name1, desc1), name2, (name3, desc3), ... ]

=item C<default_export>

(Optional)

 def default_export(readonly, is_tls):
   # return a string
   return "name"

=item C<preconnect>

(Optional, nbdkit E<ge> 1.26)

 def preconnect(readonly):
   # no return value

=item C<open>

(Required)

 def open(readonly):
   # return handle

You can return any Python value (even C<None>) as the handle.  It is
passed back as the first arg C<'h'> in subsequent calls.  To return an
error from this method you must throw an exception.

=item C<close>

(Optional)

 def close(h):
   # no return value

After C<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.

=item C<export_description>

(Optional)

 def export_description(h):
   # return a string
   return "description"

=item C<get_size>

(Required)

 def get_size(h):
   # return the size of the disk

=item C<block_size>

(Option)

 def block_size(h):
   # return triple (minimum, preferred, maximum) block size

=item C<is_rotational>

(Optional)

 def is_rotational(h):
   # return a boolean

=item C<can_multi_conn>

(Optional)

 def can_multi_conn(h):
   # return a boolean

=item C<can_write>

(Optional)

 def can_write(h):
   # return a boolean

=item C<can_flush>

(Optional)

 def can_flush(h):
   # return a boolean

=item C<can_trim>

(Optional)

 def can_trim(h):
   # return a boolean

=item C<can_zero>

(Optional)

 def can_zero(h):
   # return a boolean

=item C<can_fast_zero>

(Optional)

 def can_fast_zero(h):
   # return a boolean

=item C<can_fua>

(Optional)

 def can_fua(h):
   # return nbdkit.FUA_NONE or nbdkit.FUA_EMULATE
   # or nbdkit.FUA_NATIVE

=item C<can_cache>

(Optional)

 def can_cache(h):
   # return nbdkit.CACHE_NONE or nbdkit.CACHE_EMULATE
   # or nbdkit.CACHE_NATIVE

=item C<can_extents>

(Optional)

 def can_extents(h):
   # return a boolean

=item C<pread>

(Required)

 def pread(h, buf, offset, flags):
   # read into the buffer

The body of your C<pread> function should read exactly C<len(buf)>
bytes of data starting at disk C<offset> and write it into the buffer
C<buf>.  C<flags> is always 0.

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, optionally using
C<nbdkit.set_error> first.

=item C<pwrite>

(Optional)

 def pwrite(h, buf, offset, flags):
   length = len(buf)
   # no return value

The body of your C<pwrite> function should write the buffer C<buf> to
the disk.  You should write C<count> bytes to the disk starting at
C<offset>.  C<flags> may contain C<nbdkit.FLAG_FUA>.

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,
 optionally using C<nbdkit.set_error> first.

=item C<flush>

(Optional)

 def flush(h, flags):
   # no return value

The body of your C<flush> function should do a L<sync(2)> or
L<fdatasync(2)> or equivalent on the backing store.
C<flags> is always 0.

If the flush fails, your function should throw an exception, optionally
using C<nbdkit.set_error> first.

=item C<trim>

(Optional)

 def trim(h, count, offset, flags):
   # no return value

The body of your C<trim> function should "punch a hole" in the backing
store.  C<flags> may contain C<nbdkit.FLAG_FUA>.  If the trim fails,
your function should throw an exception, optionally using
C<nbdkit.set_error> first.

=item C<zero>

(Optional)

 def zero(h, count, offset, flags):
   # no return value

The body of your C<zero> function should ensure that C<count> bytes of
the disk, starting at C<offset>, will read back as zero.  C<flags> is
a bitmask which may include C<nbdkit.FLAG_MAY_TRIM>,
C<nbdkit.FLAG_FUA>, C<nbdkit.FLAG_FAST_ZERO>.

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, optionally using C<nbdkit.set_error> first.  In particular,
if you would like to automatically fall back to C<pwrite> (perhaps
because there is nothing to optimize if
S<C<flags & nbdkit.FLAG_MAY_TRIM>> is false), use
S<C<nbdkit.set_error(errno.EOPNOTSUPP)>>.

=item C<cache>

(Optional)

 def cache(h, count, offset, flags):
   # no return value

The body of your C<cache> function should prefetch data in the
indicated range.

If the cache operation fails, your function should throw an exception,
optionally using C<nbdkit.set_error> first.

=item C<extents>

(Optional)

 def extents(h, count, offset, flags):
   # return an iterable object (eg. list) of
   # (offset, length, type) tuples:
   return [ (off1, len1, type1), (off2, len2, type2), ... ]

=back

=head2 Missing callbacks

=over 4

=item Missing: C<load>

This is not needed since you can use regular Python mechanisms like
top level statements to run code when the module is loaded.

=item Missing: C<unload>

This is missing, but in nbdkit E<ge> 1.28 you can put code in the
C<cleanup()> function to have it run when nbdkit exits.  In earlier
versions of nbdkit, using a Python
L<atexit|https://docs.python.org/3/library/atexit.html> handler is
recommended.

=item Missing:
C<name>,
C<version>,
C<longname>,
C<description>,
C<config_help>,
C<magic_config_key>.

These are not yet supported.

=back

=head1 FILES

=over 4

=item F<$plugindir/nbdkit-python-plugin.so>

The plugin.

Use C<nbdkit --dump-config> to find the location of C<$plugindir>.

=back

=head1 VERSION

C<nbdkit-python-plugin> first appeared in nbdkit 1.2.

=head1 SEE ALSO

L<nbdkit(1)>,
L<nbdkit-plugin(3)>,
L<python(1)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

Nir Soffer

=head1 COPYRIGHT

Copyright Red Hat
