1nbdkit-python-plugin(3)             NBDKIT             nbdkit-python-plugin(3)
2
3
4

NAME

6       nbdkit-python-plugin - nbdkit python plugin
7

SYNOPSIS

9        nbdkit python /path/to/plugin.py [arguments...]
10

DESCRIPTION

12       "nbdkit-python-plugin" is an embedded Python interpreter for nbdkit(1),
13       allowing you to write nbdkit plugins in Python 3.
14
15   If you have been given an nbdkit Python plugin
16       Assuming you have a Python script which is an nbdkit plugin, you run it
17       like this:
18
19        nbdkit python /path/to/plugin.py
20
21       You may have to add further "key=value" arguments to the command line.
22       Read the Python script to see if it requires any.
23

WRITING A PYTHON NBDKIT PLUGIN

25       For example plugins written in Python, see:
26       https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/python/examples
27
28       Broadly speaking, Python nbdkit plugins work like C ones, so you should
29       read nbdkit-plugin(3) first.
30
31       To write a Python nbdkit plugin, you create a Python file which
32       contains at least the following required functions (in the top level
33       "__main__" module):
34
35        API_VERSION = 2
36        def open(readonly):
37          # see below
38        def get_size(h):
39          # see below
40        def pread(h, buf, offset, flags):
41          # see below
42
43       Note that the subroutines must have those literal names (like "open"),
44       because the C part looks up and calls those functions directly.  You
45       may want to include documentation and globals (eg. for storing global
46       state).  Any other top level statements are run when the script is
47       loaded, just like ordinary Python.
48
49   Python versions
50       Since nbdkit ≥ 1.16 only Python 3 is supported.  If you wish to
51       continue using nbdkit plugins written in Python 2 then you must use
52       nbdkit ≤ 1.14, but we advise you to update your plugins.
53
54       The version of Python 3 is chosen when nbdkit is built.  This is
55       compiled in and can't be changed at runtime.  "./configure" looks for
56       (in order):
57
58       •   the "PYTHON" variable (eg "./configure PYTHON=/usr/bin/python3.9")
59
60python3 on $PATH
61
62python on $PATH
63
64       "./configure" will fail if the first interpreter found is a Python 2
65       interpreter.
66
67       To find out which version of Python "nbdkit-python-plugin" was compiled
68       for, use the --dump-plugin option:
69
70        $ nbdkit python --dump-plugin
71        ...
72        python_version=3.7.0
73        python_pep_384_abi_version=3
74
75   API versions
76       The nbdkit API has evolved and new versions are released periodically.
77       To ensure backwards compatibility plugins have to opt in to the new
78       version.  From Python you do this by declaring a constant in your
79       module:
80
81        API_VERSION = 2
82
83       (where 2 is the latest version at the time this documentation was
84       written).  All newly written Python modules must have this constant.
85
86   Executable script
87       If you want you can make the script executable and include a "shebang"
88       at the top:
89
90        #!/usr/sbin/nbdkit python
91
92       See also "Shebang scripts" in nbdkit(1).
93
94       These scripts can also be installed in the $plugindir.  See "WRITING
95       PLUGINS IN OTHER PROGRAMMING LANGUAGES" in nbdkit-plugin(3).
96
97   Module functions
98       Your script may use "import nbdkit" to have access to the following
99       methods in the "nbdkit" module:
100
101       "nbdkit.debug(msg)"
102
103       Send a debug message to stderr or syslog if verbose messages are
104       enabled.
105
106       "nbdkit.export_name()"
107
108       Return the export name negotiated with the client as a Unicode string.
109       Note this should not be trusted because the client can send whatever it
110       wants.
111
112       "nbdkit.set_error(err)"
113
114       Record "err" as the reason you are about to throw an exception. "err"
115       should correspond to usual errno values, where it may help to "import
116       errno".
117
118       "nbdkit.parse_size(str)"
119
120       Parse a string (such as "100M") into a size in bytes. Wraps the
121       nbdkit_parse_size() C function.
122
123       "nbdkit.shutdown()"
124
125       Request asynchronous server shutdown.
126
127   Module constants
128       After "import nbdkit" the following constants are available.  These are
129       used in the callbacks below.
130
131       "nbdkit.THREAD_MODEL_SERIALIZE_CONNECTIONS"
132       "nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS"
133       "nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS"
134       "nbdkit.THREAD_MODEL_PARALLEL"
135           Possible return values from thread_model().
136
137       "nbdkit.FLAG_MAY_TRIM"
138       "nbdkit.FLAG_FUA"
139       "nbdkit.FLAG_REQ_ONE"
140       "nbdkit.FLAG_FAST_ZERO"
141           Flags bitmap passed to certain plugin callbacks.  Not all callbacks
142           with a flags parameter use all of these flags, consult the
143           documentation below and nbdkit-plugin(3).
144
145       "nbdkit.FUA_NONE"
146       "nbdkit.FUA_EMULATE"
147       "nbdkit.FUA_NATIVE"
148           Possible return values from can_fua().
149
150       "nbdkit.CACHE_NONE"
151       "nbdkit.CACHE_EMULATE"
152       "nbdkit.CACHE_NATIVE"
153           Possible return values from can_cache().
154
155       "nbdkit.EXTENT_HOLE"
156       "nbdkit.EXTENT_ZERO"
157           Used in the "type" field returned by extents().
158
159   Threads
160       The thread model for Python callbacks defaults to
161       "nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS".
162
163       Since nbdkit 1.22 it has been possible to set this by implementing a
164       thread_model() function which returns one of the constants
165       "nbdkit.THREAD_MODEL_*".
166
167       The Python Global Interpreter Lock (GIL) is still used, so Python code
168       does not run in parallel.  However if a plugin callback calls a library
169       which blocks (eg. to make an HTTP request), then another callback might
170       be executed in parallel.  Plugins which use
171       "nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS" or
172       "nbdkit.THREAD_MODEL_PARALLEL" may need to use locks on shared data.
173
174   Exceptions
175       Python callbacks should throw exceptions to indicate errors.  Remember
176       to use "nbdkit.set_error" if you need to control which error is sent
177       back to the client; if omitted, the client will see an error of "EIO".
178
179   Python callbacks
180       This just documents the arguments to the callbacks in Python, and any
181       way that they differ from the C callbacks.  In all other respects they
182       work the same way as the C callbacks, so you should go and read
183       nbdkit-plugin(3).
184
185       "dump_plugin"
186           (Optional)
187
188           There are no arguments or return value.
189
190       "config"
191           (Optional)
192
193            def config(key, value):
194              # no return value
195
196       "config_complete"
197           (Optional)
198
199           There are no arguments or return value.
200
201       "thread_model"
202           (Optional, nbdkit ≥ 1.22)
203
204            def thread_model():
205              return nbdkit.THEAD_MODEL_SERIALIZE_ALL_REQUESTS
206
207           See "Threads" above.
208
209       "get_ready"
210           (Optional)
211
212           There are no arguments or return value.
213
214       "after_fork"
215           (Optional, nbdkit ≥ 1.26)
216
217           There are no arguments or return value.
218
219       "cleanup"
220           (Optional, nbdkit ≥ 1.28)
221
222           There are no arguments or return value.
223
224       "list_exports"
225           (Optional)
226
227            def list_exports(readonly, is_tls):
228              # return an iterable object (eg. list) of
229              # (name, description) tuples or bare names:
230              return [ (name1, desc1), name2, (name3, desc3), ... ]
231
232       "default_export"
233           (Optional)
234
235            def default_export(readonly, is_tls):
236              # return a string
237              return "name"
238
239       "preconnect"
240           (Optional, nbdkit ≥ 1.26)
241
242            def preconnect(readonly):
243              # no return value
244
245       "open"
246           (Required)
247
248            def open(readonly):
249              # return handle
250
251           You can return any non-NULL Python value as the handle.  It is
252           passed back in subsequent calls.
253
254       "close"
255           (Optional)
256
257            def close(h):
258              # no return value
259
260           After "close" returns, the reference count of the handle is
261           decremented in the C part, which usually means that the handle and
262           its contents will be garbage collected.
263
264       "export_description"
265           (Optional)
266
267            def export_description(h):
268              # return a string
269              return "description"
270
271       "get_size"
272           (Required)
273
274            def get_size(h):
275              # return the size of the disk
276
277       "block_size"
278           (Option)
279
280            def block_size(h):
281              # return triple (minimum, preferred, maximum) block size
282
283       "is_rotational"
284           (Optional)
285
286            def is_rotational(h):
287              # return a boolean
288
289       "can_multi_conn"
290           (Optional)
291
292            def can_multi_conn(h):
293              # return a boolean
294
295       "can_write"
296           (Optional)
297
298            def can_write(h):
299              # return a boolean
300
301       "can_flush"
302           (Optional)
303
304            def can_flush(h):
305              # return a boolean
306
307       "can_trim"
308           (Optional)
309
310            def can_trim(h):
311              # return a boolean
312
313       "can_zero"
314           (Optional)
315
316            def can_zero(h):
317              # return a boolean
318
319       "can_fast_zero"
320           (Optional)
321
322            def can_fast_zero(h):
323              # return a boolean
324
325       "can_fua"
326           (Optional)
327
328            def can_fua(h):
329              # return nbdkit.FUA_NONE or nbdkit.FUA_EMULATE
330              # or nbdkit.FUA_NATIVE
331
332       "can_cache"
333           (Optional)
334
335            def can_cache(h):
336              # return nbdkit.CACHE_NONE or nbdkit.CACHE_EMULATE
337              # or nbdkit.CACHE_NATIVE
338
339       "can_extents"
340           (Optional)
341
342            def can_extents(h):
343              # return a boolean
344
345       "pread"
346           (Required)
347
348            def pread(h, buf, offset, flags):
349              # read into the buffer
350
351           The body of your "pread" function should read exactly len(buf)
352           bytes of data starting at disk "offset" and write it into the
353           buffer "buf".  "flags" is always 0.
354
355           NBD only supports whole reads, so your function should try to read
356           the whole region (perhaps requiring a loop).  If the read fails or
357           is partial, your function should throw an exception, optionally
358           using "nbdkit.set_error" first.
359
360       "pwrite"
361           (Optional)
362
363            def pwrite(h, buf, offset, flags):
364              length = len(buf)
365              # no return value
366
367           The body of your "pwrite" function should write the buffer "buf" to
368           the disk.  You should write "count" bytes to the disk starting at
369           "offset".  "flags" may contain "nbdkit.FLAG_FUA".
370
371           NBD only supports whole writes, so your function should try to
372           write the whole region (perhaps requiring a loop).  If the write
373           fails or is partial, your function should throw an exception,
374            optionally using "nbdkit.set_error" first.
375
376       "flush"
377           (Optional)
378
379            def flush(h, flags):
380              # no return value
381
382           The body of your "flush" function should do a sync(2) or
383           fdatasync(2) or equivalent on the backing store.  "flags" is always
384           0.
385
386           If the flush fails, your function should throw an exception,
387           optionally using "nbdkit.set_error" first.
388
389       "trim"
390           (Optional)
391
392            def trim(h, count, offset, flags):
393              # no return value
394
395           The body of your "trim" function should "punch a hole" in the
396           backing store.  "flags" may contain "nbdkit.FLAG_FUA".  If the trim
397           fails, your function should throw an exception, optionally using
398           "nbdkit.set_error" first.
399
400       "zero"
401           (Optional)
402
403            def zero(h, count, offset, flags):
404              # no return value
405
406           The body of your "zero" function should ensure that "count" bytes
407           of the disk, starting at "offset", will read back as zero.  "flags"
408           is a bitmask which may include "nbdkit.FLAG_MAY_TRIM",
409           "nbdkit.FLAG_FUA", "nbdkit.FLAG_FAST_ZERO".
410
411           NBD only supports whole writes, so your function should try to
412           write the whole region (perhaps requiring a loop).
413
414           If the write fails or is partial, your function should throw an
415           exception, optionally using "nbdkit.set_error" first.  In
416           particular, if you would like to automatically fall back to
417           "pwrite" (perhaps because there is nothing to optimize if
418           "flags & nbdkit.FLAG_MAY_TRIM" is false), use
419           "nbdkit.set_error(errno.EOPNOTSUPP)".
420
421       "cache"
422           (Optional)
423
424            def cache(h, count, offset, flags):
425              # no return value
426
427           The body of your "cache" function should prefetch data in the
428           indicated range.
429
430           If the cache operation fails, your function should throw an
431           exception, optionally using "nbdkit.set_error" first.
432
433       "extents"
434           (Optional)
435
436            def extents(h, count, offset, flags):
437              # return an iterable object (eg. list) of
438              # (offset, length, type) tuples:
439              return [ (off1, len1, type1), (off2, len2, type2), ... ]
440
441   Missing callbacks
442       Missing: "load"
443           This is not needed since you can use regular Python mechanisms like
444           top level statements to run code when the module is loaded.
445
446       Missing: "unload"
447           This is missing, but in nbdkit ≥ 1.28 you can put code in the
448           cleanup() function to have it run when nbdkit exits.  In earlier
449           versions of nbdkit, using a Python atexit handler is recommended.
450
451       Missing: "name", "version", "longname", "description", "config_help",
452       "magic_config_key".
453           These are not yet supported.
454

FILES

456       $plugindir/nbdkit-python-plugin.so
457           The plugin.
458
459           Use "nbdkit --dump-config" to find the location of $plugindir.
460

VERSION

462       "nbdkit-python-plugin" first appeared in nbdkit 1.2.
463

SEE ALSO

465       nbdkit(1), nbdkit-plugin(3), python(1).
466

AUTHORS

468       Eric Blake
469
470       Richard W.M. Jones
471
472       Nir Soffer
473
475       Copyright Red Hat
476

LICENSE

478       Redistribution and use in source and binary forms, with or without
479       modification, are permitted provided that the following conditions are
480       met:
481
482       •   Redistributions of source code must retain the above copyright
483           notice, this list of conditions and the following disclaimer.
484
485       •   Redistributions in binary form must reproduce the above copyright
486           notice, this list of conditions and the following disclaimer in the
487           documentation and/or other materials provided with the
488           distribution.
489
490       •   Neither the name of Red Hat nor the names of its contributors may
491           be used to endorse or promote products derived from this software
492           without specific prior written permission.
493
494       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
495       EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
496       IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
497       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
498       LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
499       CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
500       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
501       BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
502       WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
503       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
504       ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
505
506
507
508nbdkit-1.34.4                     2023-09-26           nbdkit-python-plugin(3)
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