1nbdkit-filter(3)                    NBDKIT                    nbdkit-filter(3)
2
3
4

NAME

6       nbdkit-filter - how to write nbdkit filters
7

SYNOPSIS

9        #include <nbdkit-filter.h>
10
11        static int
12        myfilter_config (nbdkit_next_config *next, void *nxdata,
13                         const char *key, const char *value)
14        {
15          if (strcmp (key, "myparameter") == 0) {
16            // ...
17            return 0;
18          }
19          else {
20            // pass through to next filter or plugin
21            return next (nxdata, key, value);
22          }
23        }
24
25        static struct nbdkit_filter filter = {
26          .name              = "filter",
27          .config            = myfilter_config,
28          /* etc */
29        };
30
31        NBDKIT_REGISTER_FILTER(filter)
32
33       When this has been compiled to a shared library, do:
34
35        nbdkit [--args ...] --filter=./myfilter.so plugin [key=value ...]
36
37       When debugging, use the -fv options:
38
39        nbdkit -fv --filter=./myfilter.so plugin [key=value ...]
40

DESCRIPTION

42       One or more nbdkit filters can be placed in front of an nbdkit plugin
43       to modify the behaviour of the plugin.  This manual page describes how
44       to create an nbdkit filter.
45
46       Filters can be used for example to limit requests to an offset/limit,
47       add copy-on-write support, or inject delays or errors (for testing).
48
49       Different filters can be stacked:
50
51            NBD     ┌─────────┐    ┌─────────┐          ┌────────┐
52         client ───▶│ filter1 │───▶│ filter2 │── ─ ─ ──▶│ plugin │
53        request     └─────────┘    └─────────┘          └────────┘
54
55       Each filter intercepts plugin functions (see nbdkit-plugin(3)) and can
56       call the next filter or plugin in the chain, modifying parameters,
57       calling before the filter function, in the middle or after.  Filters
58       may even short-cut the chain.  As an example, to process its own
59       parameters the filter can intercept the ".config" method:
60
61        static int
62        myfilter_config (nbdkit_next_config *next, void *nxdata,
63                         const char *key, const char *value)
64        {
65          if (strcmp (key, "myparameter") == 0) {
66            // ...
67            // here you would handle this key, value
68            // ...
69            return 0;
70          }
71          else {
72            // pass through to next filter or plugin
73            return next (nxdata, key, value);
74          }
75        }
76
77        static struct nbdkit_filter filter = {
78          // ...
79          .config            = myfilter_config,
80          // ...
81        };
82
83       The call to "next (nxdata, ...)" calls the ".config" method of the next
84       filter or plugin in the chain.  In the example above any instances of
85       "myparameter=..." on the command line would not be seen by the plugin.
86
87       To see example filters:
88       https://gitlab.com/nbdkit/nbdkit/tree/master/filters
89
90       Filters must be written in C.
91
92       Unlike plugins, where we provide a stable ABI guarantee that permits
93       operation across version differences, filters can only be run with the
94       same version of nbdkit that they were compiled with.  The reason for
95       this is two-fold: the filter API includes access to struct
96       nbdkit_next_ops that is likely to change if new callbacks are added
97       (old nbdkit cannot safely run new filters that access new methods); and
98       if we added new methods then an old filter would not see them and so
99       they would be passed unmodified through the filter, and in some cases
100       that leads to data corruption (new nbdkit cannot safely run old filters
101       unaware of new methods).  Therefore, unlike plugins, you should not
102       expect to distribute filters separately from nbdkit.
103

"#include <nbdkit-filter.h>"

105       All filters should start by including this header file.
106

"struct nbdkit_filter"

108       All filters must define and register one "struct nbdkit_filter", which
109       contains the name of the filter and pointers to plugin methods that the
110       filter wants to intercept.
111
112        static struct nbdkit_filter filter = {
113          .name              = "filter",
114          .longname          = "My Filter",
115          .description       = "This is my great filter for nbdkit",
116          .config            = myfilter_config,
117          /* etc */
118        };
119
120        NBDKIT_REGISTER_FILTER(filter)
121
122       The ".name" field is the name of the filter.  This is the only field
123       which is required.
124

NEXT PLUGIN

126       nbdkit-filter.h defines some function types ("nbdkit_next_config",
127       "nbdkit_next_config_complete", "nbdkit_next_preconnect",
128       "nbdkit_next_list_exports", "nbdkit_next_default_export",
129       "nbdkit_next_open") and a structure called "struct nbdkit_next_ops".
130       These abstract the next plugin or filter in the chain.  There is also
131       an opaque pointer "backend", "context" or "nxdata" which must be passed
132       along when calling these functions.  The value of "backend" is stable
133       between ".after_fork", ".preconnect", ".list_exports", and
134       ".default_export", and can also be obtained by using
135       "nbdkit_context_get_backend" on the "context" parameter to ".open".
136
137       Meanwhile, if the filter does not use "nbdkit_context_set_next", the
138       value of "next" passed to ".prepare" has a stable lifetime that lasts
139       to the corresponding ".finalize", with all intermediate functions (such
140       as ".pread") receiving the same value for convenience.  Functions where
141       "nxdata" is not reused are ".config", ".config_complete", and
142       ".get_ready", which are all called during initialization outside any
143       connections.  The value of "backend" passed to ".after_fork" also
144       occurs without connections, but is shared with ".preconnect",
145       ".list_exports", and ".default_export", and can also be obtained from
146       the "context" passed to ".open", and has a lifetime that lasts to
147       ".cleanup" for use by "nbdkit_next_context_open".  In turn, the value
148       of "context" passed to ".open" has a lifetime that lasts until the
149       matching ".close" for use by "nbdkit_context_get_backend" and
150       "nbdkit_context_set_next".
151
152   Next config, open and close
153       The filter’s ".config", ".config_complete", ".get_ready",
154       ".after_fork", ".preconnect", ".list_exports", ".default_export" and
155       ".open" methods may only call the next ".config", ".config_complete",
156       ".get_ready", ".after_fork", ".preconnect", ".list_exports",
157       ".default_export" and ".open" method in the chain (optionally for
158       ".config" and ".open").
159
160       The filter’s ".close" method is called when an old connection closed,
161       and this has no "next" parameter because it cannot be short-circuited.
162
163   "nbdkit_next"
164       The filter generally needs to call into the underlying plugin, which is
165       done via a pointer to "struct nbdkit_next_ops", also available as the
166       typedef "nbdkit_next".  The most common behavior is to create a next
167       context per connection by calling the "next_open" parameter during
168       ".open", at which point the next context will be automatically provided
169       to the filter’s other methods like ".prepare", ".get_size", ".pread"
170       etc.  The "nbdkit_next" struct contains a comparable set of accessors
171       to plugin methods that can be called during a connection.  When using
172       automatic registration, the "next" parameter is stable between
173       ".prepare" and ".finalize", and nbdkit automatically prepares,
174       finalizes, and closes the next context at the right point in the filter
175       connection lifecycle.
176
177       Alternatively, the filter can manage plugin contexts manually, whether
178       to multiplex multiple client connections through a single context into
179       the plugin, or to open multiple plugin contexts to perform retries or
180       otherwise service a single client connection more efficiently.  In this
181       mode of operation, the filter uses "nbdkit_next_context_open" to open a
182       plugin context using the "backend" parameter passed to ".after_fork",
183       ".preconnect", ".list_exports", ".default_export", or obtained from
184       using "nbdkit_context_get_backend" on the "context" parameter to
185       ".open".  The resulting next context has a lifecycle under manual
186       control, where the filter must use "next->prepare (next)" before using
187       any other function pointers within the next context, and must reclaim
188       the memory using "next->finalize (next)" and
189       "nbdkit_next_context_close" when done.  A filter using manual lifecycle
190       management may use "nbdkit_context_set_next" to associate the next
191       context into the current connection, which lets nbdkit then pass that
192       context as the "next" parameter to future connection-related functions
193       like ".pread" and take over lifecycle responsibility.
194
195       "nbdkit_context_get_backend"
196
197       "nbdkit_next_context_open"
198
199       "nbdkit_next_context_close"
200
201       "nbdkit_context_set_next"
202
203        nbdkit_backend *nbdkit_context_get_backend (nbdkit_context *context);
204
205       Obtains the backend pointer from the "context" parameter to ".open",
206       matching the backend pointer available to ".after_fork", ".preconnect",
207       ".list_exports", and ".default_export".  This backend pointer has a
208       stable lifetime from the time of ".after_fork" until ".cleanup".
209
210        nbdkit_next *nbdkit_next_context_open (nbdkit_backend *backend,
211                                               int readonly, const char *exportname,
212                                               int shared);
213
214       This function attempts to open a new context into the plugin in
215       relation to the filter's current "backend".  The "readonly" and
216       "exportname" parameters behave the same as documented in ".open".  The
217       resulting context will be under the filter's manual lifecycle control
218       unless the filter associates it into the connection with
219       "nbdkit_context_set_next".  The filter should be careful to not violate
220       any threading model restrictions of the plugin if it opens more than
221       one context.
222
223       If "shared" is false, this function must be called while servicing an
224       existing client connection, and the new context will share the same
225       connection details (export name, tls status, and shorter interned
226       string lifetimes) as the current connection, and thus should not be
227       used after the client connection ends.  Conversely, if "shared" is
228       true, this function may be called outside of a current client
229       connection (such as during ".after_fork"), and the resulting context
230       may be freely shared among multiple client connections.  In shared
231       mode, it will not be possible for the plugin to differentiate content
232       based on the client export name, the result of the plugin calling
233       nbdkit_is_tls() will depend solely whether "--tls=require" was on the
234       command line, the lifetime of interned strings (via
235       "nbdkit_strdup_intern" and friends) lasts for the life of the filter,
236       and the filter must take care to not expose potentially-secure
237       information from the backend to an insecure client.
238
239        void nbdkit_next_context_close (nbdkit_next *next);
240
241       This function closes a context into the plugin.  If the context has
242       previously been prepared, it should first be finalized before using
243       this function.  This function does not need to be called for a plugin
244       context that has been associated with the filter connection via
245       "nbdkit_context_set_next" prior to the ".close" callback.
246
247        nbdkit_next *nbdkit_context_set_next (nbdkit_context *context,
248                                              nbdkit_next *next);
249
250       This function associates a plugin context with the filter's current
251       connection context, given by the "context" parameter to ".open".  Once
252       associated, this plugin context will be given as the "next" parameter
253       to all other connection-specific callbacks.  If associated during
254       ".open", nbdkit will take care of preparing the context prior to
255       ".prepare"; if still associated before ".finalize", nbdkit will take
256       care of finalizing the context, and also for closing it.  A filter may
257       also pass "NULL" for "next", to remove any association; if no plugin
258       context is associated with the connection, then filter callbacks such
259       as ".pread" will receive "NULL" for their "next" parameter.
260
261       This function returns the previous context that had been associated
262       with the connection prior to switching the association to "next"; this
263       result will be "NULL" if there was no previous association.  The filter
264       assumes manual responsibility for any remaining lifecycle functions
265       that must be called on the returned context.
266
267   Using "nbdkit_next"
268       Regardless of whether the plugin context is managed automatically or
269       manually, it is possible for a filter to issue (for example) extra
270       "next->pread" calls in response to a single ".pwrite" call.
271
272       The "next" parameter serves two purposes: it serves as the struct to
273       access the pointers to all the plugin connection functions, and it
274       serves as the opaque data that must be passed as the first parameter to
275       those functions.  For example, calling the plugin's can_flush
276       functionality would be done via
277
278        next-E<gt>can_flush (next)
279
280       Note that the semantics of the functions in "struct nbdkit_next_ops"
281       are slightly different from what a plugin implements: for example, when
282       a plugin's ".pread" returns -1 on error, the error value to advertise
283       to the client is implicit (via the plugin calling "nbdkit_set_error" or
284       setting "errno"), whereas "next->pread" exposes this via an explicit
285       parameter, allowing a filter to learn or modify this error if desired.
286
287       Use of "next->prepare" and "next->finalize" is only needed when
288       manually managing the plugin context lifetime.
289
290   Other considerations
291       You can modify parameters when you call the "next" function.  However
292       be careful when modifying strings because for some methods (eg.
293       ".config") the plugin may save the string pointer that you pass along.
294       So you may have to ensure that the string is not freed for the lifetime
295       of the server; you may find "nbdkit_strdup_intern" helpful for avoiding
296       a memory leak while still obeying lifecycle constraints.
297
298       Note that if your filter registers a callback but in that callback it
299       doesn't call the "next" function then the corresponding method in the
300       plugin will never be called.  In particular, your ".open" method, if
301       you have one, must call the "next" method if you want the underlying
302       plugin to be available to all further "nbdkit_next" use.
303

CALLBACKS

305       "struct nbdkit_filter" has some static fields describing the filter and
306       optional callback functions which can be used to intercept plugin
307       methods.
308
309   ".name"
310        const char *name;
311
312       This field (a string) is required, and must contain only ASCII
313       alphanumeric characters or non-leading dashes, and be unique amongst
314       all filters.
315
316   ".longname"
317        const char *longname;
318
319       An optional free text name of the filter.  This field is used in error
320       messages.
321
322   ".description"
323        const char *description;
324
325       An optional multi-line description of the filter.
326
327   ".load"
328        void load (void);
329
330       This is called once just after the filter is loaded into memory.  You
331       can use this to perform any global initialization needed by the filter.
332
333   ".unload"
334        void unload (void);
335
336       This may be called once just before the filter is unloaded from memory.
337       Note that it's not guaranteed that ".unload" will always be called (eg.
338       the server might be killed or segfault), so you should try to make the
339       filter as robust as possible by not requiring cleanup.  See also
340       "SHUTDOWN" in nbdkit-plugin(3).
341
342   ".config"
343        int (*config) (nbdkit_next_config *next, void *nxdata,
344                       const char *key, const char *value);
345
346       This intercepts the plugin ".config" method and can be used by the
347       filter to parse its own command line parameters.  You should try to
348       make sure that command line parameter keys that the filter uses do not
349       conflict with ones that could be used by a plugin.
350
351       If there is an error, ".config" should call "nbdkit_error" with an
352       error message and return "-1".
353
354   ".config_complete"
355        int (*config_complete) (nbdkit_next_config_complete *next, void *nxdata);
356
357       This intercepts the plugin ".config_complete" method and can be used to
358       ensure that all parameters needed by the filter were supplied on the
359       command line.
360
361       If there is an error, ".config_complete" should call "nbdkit_error"
362       with an error message and return "-1".
363
364   ".config_help"
365        const char *config_help;
366
367       This optional multi-line help message should summarize any "key=value"
368       parameters that it takes.  It does not need to repeat what already
369       appears in ".description".
370
371       If the filter doesn't take any config parameters you should probably
372       omit this.
373
374   ".thread_model"
375        int (*thread_model) (void);
376
377       Filters may tighten (but not relax) the thread model of the plugin, by
378       defining this callback.  Note that while plugins use a compile-time
379       definition of "THREAD_MODEL", filters do not need to declare a model at
380       compile time; instead, this callback is called after ".config_complete"
381       and before any connections are created.  See "THREADS" in
382       nbdkit-plugin(3) for a discussion of thread models.
383
384       The final thread model used by nbdkit is the smallest (ie. most
385       serialized) out of all the filters and the plugin, and applies for all
386       connections.  Requests for a model larger than permitted by the plugin
387       are silently ignored. It is acceptable for decisions made during
388       ".config" and ".config_complete" to determine which model to request.
389
390       This callback is optional; if it is not present, the filter must be
391       written to handle fully parallel requests, including when multiple
392       requests are issued in parallel on the same connection, similar to a
393       plugin requesting "NBDKIT_THREAD_MODEL_PARALLEL".  This ensures the
394       filter doesn't slow down other filters or plugins.
395
396       If there is an error, ".thread_model" should call "nbdkit_error" with
397       an error message and return "-1".
398
399   ".get_ready"
400        int (*get_ready) (int thread_model);
401
402       This optional callback is reached if the plugin ".get_ready" method
403       succeeded (if the plugin failed, nbdkit has already exited), and can be
404       used by the filter to get ready to serve requests.
405
406       The "thread_model" parameter informs the filter about the final thread
407       model chosen by nbdkit after considering the results of ".thread_model"
408       of all filters in the chain after ".config_complete".
409
410       If there is an error, ".get_ready" should call "nbdkit_error" with an
411       error message and return "-1".
412
413   ".after_fork"
414        int (*after_fork) (nbdkit_backend *backend);
415
416       This optional callback is reached after the plugin ".after_fork" method
417       has succeeded (if the plugin failed, nbdkit has already exited), and
418       can be used by the filter to start background threads.  The "backend"
419       parameter is valid until ".cleanup", for creating manual contexts into
420       the backend with "nbdkit_next_context_open".
421
422       If there is an error, ".after_fork" should call "nbdkit_error" with an
423       error message and return "-1".
424
425   ".cleanup"
426        int (cleanup) (nbdkit_backend *backend);
427
428       This optional callback is reached once after all client connections
429       have been closed, but before the underlying plugin ".cleanup" or any
430       ".unload" callbacks.  It can be used by the filter to gracefully close
431       any background threads created during ".after_fork", as well as close
432       any manual contexts into "backend" previously opened with
433       "nbdkit_next_context_open".
434
435       Note that it's not guaranteed that ".cleanup" will always be called
436       (eg. the server might be killed or segfault), so you should try to make
437       the filter as robust as possible by not requiring cleanup.  See also
438       "SHUTDOWN" in nbdkit-plugin(3).
439
440   ".preconnect"
441        int (*preconnect) (nbdkit_next_preconnect *next, nbdkit_backend *nxdata,
442                           int readonly);
443
444       This intercepts the plugin ".preconnect" method and can be used to
445       filter access to the server.
446
447       If there is an error, ".preconnect" should call "nbdkit_error" with an
448       error message and return "-1".
449
450   ".list_exports"
451        int (*list_exports) (nbdkit_next_list_exports *next, nbdkit_backend *nxdata,
452                             int readonly, int is_tls,
453                             struct nbdkit_exports *exports);
454
455       This intercepts the plugin ".list_exports" method and can be used to
456       filter which exports are advertised.
457
458       The "readonly" parameter matches what is passed to <.preconnect> and
459       ".open", and may be changed by the filter when calling into the plugin.
460       The "is_tls" parameter informs the filter whether TLS negotiation has
461       been completed by the client, but is not passed on to "next" because it
462       cannot be altered.
463
464       It is possible for filters to transform the exports list received back
465       from the layer below.  Without error checking it would look like this:
466
467        myfilter_list_exports (...)
468        {
469          size_t i;
470          struct nbdkit_exports *exports2;
471          struct nbdkit_export e;
472          char *name, *desc;
473
474          exports2 = nbdkit_exports_new ();
475          next_list_exports (nxdata, readonly, exports);
476          for (i = 0; i < nbdkit_exports_count (exports2); ++i) {
477            e = nbdkit_get_export (exports2, i);
478            name = adjust (e.name);
479            desc = adjust (e.desc);
480            nbdkit_add_export (exports, name, desc);
481            free (name);
482            free (desc);
483          }
484          nbdkit_exports_free (exports2);
485        }
486
487       If there is an error, ".list_exports" should call "nbdkit_error" with
488       an error message and return "-1".
489
490       Allocating and freeing nbdkit_exports list
491
492       Two functions are provided to filters only for allocating and freeing
493       the list:
494
495        struct nbdkit_exports *nbdkit_exports_new (void);
496
497       Allocates and returns a new, empty exports list.
498
499       On error this function can return "NULL".  In this case it calls
500       "nbdkit_error" as required.  "errno" will be set to a suitable value.
501
502        void nbdkit_exports_free (struct nbdkit_exports *);
503
504       Frees an existing exports list.
505
506       Iterating over nbdkit_exports list
507
508       Two functions are provided to filters only to iterate over the exports
509       in order:
510
511        size_t nbdkit_exports_count (const struct nbdkit_exports *);
512
513       Returns the number of exports in the list.
514
515        struct nbdkit_export {
516          char *name;
517          char *description;
518        };
519        const struct nbdkit_export nbdkit_get_export (const struct nbdkit_exports *,
520                                                      size_t i);
521
522       Returns a copy of the "i"'th export.
523
524   ".default_export"
525        const char *default_export (nbdkit_next_default_export *next,
526                                    nbdkit_backend *nxdata,
527                                    int readonly, int is_tls)
528
529       This intercepts the plugin ".default_export" method and can be used to
530       alter the canonical export name used in place of the default "".
531
532       The "readonly" parameter matches what is passed to <.preconnect> and
533       ".open", and may be changed by the filter when calling into the plugin.
534       The "is_tls" parameter informs the filter whether TLS negotiation has
535       been completed by the client, but is not passed on to "next" because it
536       cannot be altered.
537
538   ".open"
539        void * (*open) (nbdkit_next_open *next, nbdkit_context *context,
540                        int readonly, const char *exportname, int is_tls);
541
542       This is called when a new client connection is opened and can be used
543       to allocate any per-connection data structures needed by the filter.
544       The handle (which is not the same as the plugin handle) is passed back
545       to other filter callbacks and could be freed in the ".close" callback.
546
547       Note that the handle is completely opaque to nbdkit, but it must not be
548       NULL.  If you don't need to use a handle, return
549       "NBDKIT_HANDLE_NOT_NEEDED" which is a static non-NULL pointer.
550
551       If there is an error, ".open" should call "nbdkit_error" with an error
552       message and return "NULL".
553
554       This callback is optional, but if provided, it should call "next",
555       passing "readonly" and "exportname" possibly modified according to how
556       the filter plans to use the plugin ("is_tls" is not passed, because a
557       filter cannot modify it).  Typically, the filter passes the same values
558       as it received, or passes readonly=true to provide a writable layer on
559       top of a read-only backend.  However, it is also acceptable to attempt
560       write access to the plugin even if this filter is readonly, such as
561       when a file system mounted read-only still requires write access to the
562       underlying device in case a journal needs to be replayed for
563       consistency as part of the mounting process.
564
565       The "exportname" string is only guaranteed to be available during the
566       call (different than the lifetime for the return of
567       "nbdkit_export_name" used by plugins).  If the filter needs to use it
568       (other than immediately passing it down to the next layer) it must take
569       a copy, although "nbdkit_strdup_intern" is useful for this task.  The
570       "exportname" and "is_tls" parameters are provided so that filters do
571       not need to use the plugin-only interfaces of "nbdkit_export_name" and
572       "nbdkit_is_tls".
573
574       The filter should generally call "next" as its first step, to allocate
575       from the plugin outwards, so that ".close" running from the outer
576       filter to the plugin will be in reverse.  Skipping a call to "next" is
577       acceptable if the filter will not access "nbdkit_next" during any of
578       the remaining callbacks reached on the same connection.  The "next"
579       function is provided for convenience; the same functionality can be
580       obtained manually (other than error checking) by using the following:
581
582        nbdkit_context_set_next (context, nbdkit_next_context_open
583           (nbdkit_context_get_backend (context), readonly, exportname, false));
584
585       The value of "context" in this call has a lifetime that lasts until the
586       counterpart ".close", and it is this value that may be passed to
587       "nbdkit_context_get_backend" to obtain the "backend" parameter used to
588       open a plugin context with "nbdkit_next_context_open", as well as the
589       "context" parameter used to associate a plugin context into the current
590       connection with "nbdkit_context_set_next".
591
592   ".close"
593        void (*close) (void *handle);
594
595       This is called when the client closes the connection.  It should clean
596       up any per-connection resources used by the filter.  It is called
597       beginning with the outermost filter and ending with the plugin (the
598       opposite order of ".open" if all filters call "next" first), although
599       this order technically does not matter since the callback cannot report
600       failures or access the underlying plugin.
601
602   ".prepare"
603   ".finalize"
604         int (*prepare) (nbdkit_next *next, void *handle, int readonly);
605         int (*finalize) (nbdkit_next *next, void *handle);
606
607       These two methods can be used to perform any necessary operations just
608       after opening the connection (".prepare") or just before closing the
609       connection (".finalize").
610
611       For example if you need to scan the underlying disk to check for a
612       partition table, you could do it in your ".prepare" method (calling the
613       plugin's ".get_size" and ".pread" methods via "next").  Or if you need
614       to cleanly update superblock data in the image on close you can do it
615       in your ".finalize" method (calling the plugin's ".pwrite" method).
616       Doing these things in the filter's ".open" or ".close" method is not
617       possible without using manual context lifecycle management.
618
619       For ".prepare", the value of "readonly" is the same as was passed to
620       ".open", declaring how this filter will be used.
621
622       Note that nbdkit performs sanity checking on requests made to the
623       underlying plugin; for example, "next->pread" cannot be called on a
624       given connection unless "next->get_size" has first been called at least
625       once in the same connection (to ensure the read requests are in
626       bounds), and "next->pwrite" further requires an earlier successful call
627       to "next->can_write".  In many filters, these prerequisites will be
628       automatically called during the client negotiation phase, but there are
629       cases where a filter overrides query functions or makes I/O calls into
630       the plugin before handshaking is complete, where the filter needs to
631       make those prerequisite calls manually during ".prepare".
632
633       While there are "next->prepare" and "next->finalize" functions, these
634       are different from other filter methods, in that any plugin context
635       associated with the current connection (via the "next" parameter to
636       ".open", or via "nbdkit_context_set_next", is prepared and finalized
637       automatically by nbdkit, so they are only used during manual lifecycle
638       management.  Prepare methods are called starting with the filter
639       closest to the plugin and proceeding outwards (matching the order of
640       ".open" if all filters call "next" before doing anything locally), and
641       only when an outer filter did not skip the "next" call during ".open".
642       Finalize methods are called in the reverse order of prepare methods,
643       with the outermost filter first (and matching the order of ".close"),
644       and only if the prepare method succeeded.
645
646       If there is an error, both callbacks should call "nbdkit_error" with an
647       error message and return "-1".  An error in ".prepare" is reported to
648       the client, but leaves the connection open (a client may try again with
649       a different export name, for example); while an error in ".finalize"
650       forces the client to disconnect.
651
652   ".get_size"
653        int64_t (*get_size) (nbdkit_next *next, void *handle);
654
655       This intercepts the plugin ".get_size" method and can be used to read
656       or modify the apparent size of the block device that the NBD client
657       will see.
658
659       The returned size must be ≥ 0.  If there is an error, ".get_size"
660       should call "nbdkit_error" with an error message and return "-1".  This
661       function is only called once per connection and cached by nbdkit.
662       Similarly, repeated calls to "next->get_size" will return a cached
663       value.
664
665   ".export_description"
666        const char *export_description (nbdkit_next *next, void *handle);
667
668       This intercepts the plugin ".export_description" method and can be used
669       to read or modify the export description that the NBD client will see.
670
671   ".can_write"
672   ".can_flush"
673   ".is_rotational"
674   ".can_trim"
675   ".can_zero"
676   ".can_fast_zero"
677   ".can_extents"
678   ".can_fua"
679   ".can_multi_conn"
680   ".can_cache"
681        int (*can_write) (nbdkit_next *next, void *handle);
682        int (*can_flush) (nbdkit_next *next, void *handle);
683        int (*is_rotational) (nbdkit_next *next, void *handle);
684        int (*can_trim) (nbdkit_next *next, void *handle);
685        int (*can_zero) (nbdkit_next *next, void *handle);
686        int (*can_fast_zero) (nbdkit_next *next, void *handle);
687        int (*can_extents) (nbdkit_next *next, void *handle);
688        int (*can_fua) (nbdkit_next *next, void *handle);
689        int (*can_multi_conn) (nbdkit_next *next, void *handle);
690        int (*can_cache) (nbdkit_next *next, void *handle);
691
692       These intercept the corresponding plugin methods, and control feature
693       bits advertised to the client.
694
695       Of note, the semantics of ".can_zero" callback in the filter are
696       slightly different from the plugin, and must be one of three success
697       values visible only to filters:
698
699       "NBDKIT_ZERO_NONE"
700           Completely suppress advertisement of write zero support (this can
701           only be done from filters, not plugins).
702
703       "NBDKIT_ZERO_EMULATE"
704           Inform nbdkit that write zeroes should immediately fall back to
705           ".pwrite" emulation without trying ".zero" (this value is returned
706           by "next->can_zero" if the plugin returned false in its
707           ".can_zero").
708
709       "NBDKIT_ZERO_NATIVE"
710           Inform nbdkit that write zeroes should attempt to use ".zero",
711           although it may still fall back to ".pwrite" emulation for
712           "ENOTSUP" or "EOPNOTSUPP" failures (this value is returned by
713           "next->can_zero" if the plugin returned true in its ".can_zero").
714
715       Remember that most of the feature check functions return merely a
716       boolean success value, while ".can_zero", ".can_fua" and ".can_cache"
717       have three success values.
718
719       The difference between ".can_fua" values may affect choices made in the
720       filter: when splitting a write request that requested FUA from the
721       client, if "next->can_fua" returns "NBDKIT_FUA_NATIVE", then the filter
722       should pass the FUA flag on to each sub-request; while if it is known
723       that FUA is emulated by a flush because of a return of
724       "NBDKIT_FUA_EMULATE", it is more efficient to only flush once after all
725       sub-requests have completed (often by passing "NBDKIT_FLAG_FUA" on to
726       only the final sub-request, or by dropping the flag and ending with a
727       direct call to "next->flush").
728
729       If there is an error, the callback should call "nbdkit_error" with an
730       error message and return "-1".  These functions are called at most once
731       per connection and cached by nbdkit. Similarly, repeated calls to any
732       of the "nbdkit_next" counterparts will return a cached value; by
733       calling into the plugin during ".prepare", you can ensure that later
734       use of the cached values during data commands like <.pwrite> will not
735       fail.
736
737   ".pread"
738        int (*pread) (nbdkit_next *next,
739                      void *handle, void *buf, uint32_t count, uint64_t offset,
740                      uint32_t flags, int *err);
741
742       This intercepts the plugin ".pread" method and can be used to read or
743       modify data read by the plugin.
744
745       The parameter "flags" exists in case of future NBD protocol extensions;
746       at this time, it will be 0 on input, and the filter should not pass any
747       flags to "next->pread".
748
749       If there is an error (including a short read which couldn't be
750       recovered from), ".pread" should call "nbdkit_error" with an error
751       message and return -1 with "err" set to the positive errno value to
752       return to the client.
753
754   ".pwrite"
755        int (*pwrite) (nbdkit_next *next,
756                       void *handle,
757                       const void *buf, uint32_t count, uint64_t offset,
758                       uint32_t flags, int *err);
759
760       This intercepts the plugin ".pwrite" method and can be used to modify
761       data written by the plugin.
762
763       This function will not be called if ".can_write" returned false; in
764       turn, the filter should not call "next->pwrite" if "next->can_write"
765       did not return true.
766
767       The parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on
768       the result of ".can_fua".  In turn, the filter should only pass
769       "NBDKIT_FLAG_FUA" on to "next->pwrite" if "next->can_fua" returned a
770       positive value.
771
772       If there is an error (including a short write which couldn't be
773       recovered from), ".pwrite" should call "nbdkit_error" with an error
774       message and return -1 with "err" set to the positive errno value to
775       return to the client.
776
777   ".flush"
778        int (*flush) (nbdkit_next *next,
779                      void *handle, uint32_t flags, int *err);
780
781       This intercepts the plugin ".flush" method and can be used to modify
782       flush requests.
783
784       This function will not be called if ".can_flush" returned false; in
785       turn, the filter should not call "next->flush" if "next->can_flush" did
786       not return true.
787
788       The parameter "flags" exists in case of future NBD protocol extensions;
789       at this time, it will be 0 on input, and the filter should not pass any
790       flags to "next->flush".
791
792       If there is an error, ".flush" should call "nbdkit_error" with an error
793       message and return -1 with "err" set to the positive errno value to
794       return to the client.
795
796   ".trim"
797        int (*trim) (nbdkit_next *next,
798                     void *handle, uint32_t count, uint64_t offset,
799                     uint32_t flags, int *err);
800
801       This intercepts the plugin ".trim" method and can be used to modify
802       trim requests.
803
804       This function will not be called if ".can_trim" returned false; in
805       turn, the filter should not call "next->trim" if "next->can_trim" did
806       not return true.
807
808       The parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on
809       the result of ".can_fua".  In turn, the filter should only pass
810       "NBDKIT_FLAG_FUA" on to "next->trim" if "next->can_fua" returned a
811       positive value.
812
813       If there is an error, ".trim" should call "nbdkit_error" with an error
814       message and return -1 with "err" set to the positive errno value to
815       return to the client.
816
817   ".zero"
818        int (*zero) (nbdkit_next *next,
819                     void *handle, uint32_t count, uint64_t offset, uint32_t flags,
820                     int *err);
821
822       This intercepts the plugin ".zero" method and can be used to modify
823       zero requests.
824
825       This function will not be called if ".can_zero" returned
826       "NBDKIT_ZERO_NONE"; in turn, the filter should not call "next->zero" if
827       "next->can_zero" returned "NBDKIT_ZERO_NONE".
828
829       On input, the parameter "flags" may include "NBDKIT_FLAG_MAY_TRIM"
830       unconditionally, "NBDKIT_FLAG_FUA" based on the result of ".can_fua",
831       and "NBDKIT_FLAG_FAST_ZERO" based on the result of ".can_fast_zero".
832       In turn, the filter may pass "NBDKIT_FLAG_MAY_TRIM" unconditionally,
833       but should only pass "NBDKIT_FLAG_FUA" or "NBDKIT_FLAG_FAST_ZERO" on to
834       "next->zero" if the corresponding "next->can_fua" or
835       "next->can_fast_zero" returned a positive value.
836
837       Note that unlike the plugin ".zero" which is permitted to fail with
838       "ENOTSUP" or "EOPNOTSUPP" to force a fallback to ".pwrite", the
839       function "next->zero" will not fail with "err" set to "ENOTSUP" or
840       "EOPNOTSUPP" unless "NBDKIT_FLAG_FAST_ZERO" was used, because otherwise
841       the fallback has already taken place.
842
843       If there is an error, ".zero" should call "nbdkit_error" with an error
844       message and return -1 with "err" set to the positive errno value to
845       return to the client.  The filter should not fail with "ENOTSUP" or
846       "EOPNOTSUPP" unless "flags" includes "NBDKIT_FLAG_FAST_ZERO" (while
847       plugins have automatic fallback to ".pwrite", filters do not).
848
849   ".extents"
850        int (*extents) (nbdkit_next *next,
851                        void *handle, uint32_t count, uint64_t offset, uint32_t flags,
852                        struct nbdkit_extents *extents,
853                        int *err);
854
855       This intercepts the plugin ".extents" method and can be used to modify
856       extent requests.
857
858       This function will not be called if ".can_extents" returned false; in
859       turn, the filter should not call "next->extents" if "next->can_extents"
860       did not return true.
861
862       It is possible for filters to transform the extents list received back
863       from the layer below.  Without error checking it would look like this:
864
865        myfilter_extents (..., uint32_t count, uint64_t offset, ...)
866        {
867          size_t i;
868          struct nbdkit_extents *extents2;
869          struct nbdkit_extent e;
870          int64_t size;
871
872          size = next->get_size (next);
873          extents2 = nbdkit_extents_new (offset + shift, size);
874          next->extents (next, count, offset + shift, flags, extents2, err);
875          for (i = 0; i < nbdkit_extents_count (extents2); ++i) {
876            e = nbdkit_get_extent (extents2, i);
877            e.offset -= shift;
878            nbdkit_add_extent (extents, e.offset, e.length, e.type);
879          }
880          nbdkit_extents_free (extents2);
881        }
882
883       If there is an error, ".extents" should call "nbdkit_error" with an
884       error message and return -1 with "err" set to the positive errno value
885       to return to the client.
886
887       Allocating and freeing nbdkit_extents list
888
889       Two functions are provided to filters only for allocating and freeing
890       the map:
891
892        struct nbdkit_extents *nbdkit_extents_new (uint64_t start, uint64_t end);
893
894       Allocates and returns a new, empty extents list.  The "start" parameter
895       is the start of the range described in the list, and the "end"
896       parameter is the offset of the byte beyond the end.  Normally you would
897       pass in "offset" as the start and the size of the plugin as the end,
898       but for filters which adjust offsets, they should pass in the adjusted
899       offset.
900
901       On error this function can return "NULL".  In this case it calls
902       "nbdkit_error" and/or "nbdkit_set_error" as required.  "errno" will be
903       set to a suitable value.
904
905        void nbdkit_extents_free (struct nbdkit_extents *);
906
907       Frees an existing extents list.
908
909       Iterating over nbdkit_extents list
910
911       Two functions are provided to filters only to iterate over the extents
912       in order:
913
914        size_t nbdkit_extents_count (const struct nbdkit_extents *);
915
916       Returns the number of extents in the list.
917
918        struct nbdkit_extent {
919          uint64_t offset;
920          uint64_t length;
921          uint32_t type;
922        };
923        struct nbdkit_extent nbdkit_get_extent (const struct nbdkit_extents *,
924                                                size_t i);
925
926       Returns a copy of the "i"'th extent.
927
928       Reading the full extents from the plugin
929
930       A convenience function is provided to filters only which makes one or
931       more requests to the underlying plugin until we have a full set of
932       extents covering the region "[offset..offset+count-1]".
933
934        struct nbdkit_extents *nbdkit_extents_full (
935                                    nbdkit_next *next,
936                                    uint32_t count, uint64_t offset,
937                                    uint32_t flags, int *err);
938
939       Note this allocates a new "struct nbdkit_extents" which the caller must
940       free.  "flags" is passed through to the underlying plugin, but
941       "NBDKIT_FLAG_REQ_ONE" is removed from the set of flags so that the
942       plugin returns as much information as possible (this is usually what
943       you want).
944
945       On error this function can return "NULL".  In this case it calls
946       "nbdkit_error" and/or "nbdkit_set_error" as required.  *err will be set
947       to a suitable value.
948
949       Enforcing alignment of an nbdkit_extents list
950
951       A convenience function is provided to filters only which makes it
952       easier to ensure that the client only encounters aligned extents.
953
954        int nbdkit_extents_aligned (nbdkit_next *next,
955                                    uint32_t count, uint64_t offset,
956                                    uint32_t flags, uint32_t align,
957                                    struct nbdkit_extents *extents, int *err);
958
959       Calls "next->extents" as needed until at least "align" bytes are
960       obtained, where "align" is a power of 2.  Anywhere the underlying
961       plugin returns differing extents within "align" bytes, this function
962       treats that portion of the disk as a single extent with zero and sparse
963       status bits determined by the intersection of all underlying extents.
964       It is an error to call this function with "count" or "offset" that is
965       not already aligned.
966
967   ".cache"
968        int (*cache) (nbdkit_next *next,
969                      void *handle, uint32_t count, uint64_t offset,
970                      uint32_t flags, int *err);
971
972       This intercepts the plugin ".cache" method and can be used to modify
973       cache requests.
974
975       This function will not be called if ".can_cache" returned
976       "NBDKIT_CACHE_NONE" or "NBDKIT_CACHE_EMULATE"; in turn, the filter
977       should not call "next->cache" unless "next->can_cache" returned
978       "NBDKIT_CACHE_NATIVE".
979
980       The parameter "flags" exists in case of future NBD protocol extensions;
981       at this time, it will be 0 on input, and the filter should not pass any
982       flags to "next->cache".
983
984       If there is an error, ".cache" should call "nbdkit_error" with an error
985       message and return -1 with "err" set to the positive errno value to
986       return to the client.
987

ERROR HANDLING

989       If there is an error in the filter itself, the filter should call
990       "nbdkit_error" to report an error message.  If the callback is involved
991       in serving data, the explicit "err" parameter determines the error code
992       that will be sent to the client; other callbacks should return the
993       appropriate error indication, eg. "NULL" or "-1".
994
995       "nbdkit_error" has the following prototype and works like printf(3):
996
997        void nbdkit_error (const char *fs, ...);
998        void nbdkit_verror (const char *fs, va_list args);
999
1000       For convenience, "nbdkit_error" preserves the value of "errno", and
1001       also supports the glibc extension of a single %m in a format string
1002       expanding to "strerror(errno)", even on platforms that don't support
1003       that natively.
1004

DEBUGGING

1006       Run the server with -f and -v options so it doesn't fork and you can
1007       see debugging information:
1008
1009        nbdkit -fv --filter=./myfilter.so plugin [key=value [key=value [...]]]
1010
1011       To print debugging information from within the filter, call
1012       "nbdkit_debug", which has the following prototype and works like
1013       printf(3):
1014
1015        void nbdkit_debug (const char *fs, ...);
1016        void nbdkit_vdebug (const char *fs, va_list args);
1017
1018       For convenience, "nbdkit_debug" preserves the value of "errno", and
1019       also supports the glibc extension of a single %m in a format string
1020       expanding to "strerror(errno)", even on platforms that don't support
1021       that natively.  Note that "nbdkit_debug" only prints things when the
1022       server is in verbose mode (-v option).
1023
1024   Debug Flags
1025       Debug Flags in filters work exactly the same way as plugins.  See
1026       "Debug Flags" in nbdkit-plugin(3).
1027

INSTALLING THE FILTER

1029       The filter is a "*.so" file and possibly a manual page.  You can of
1030       course install the filter "*.so" file wherever you want, and users will
1031       be able to use it by running:
1032
1033        nbdkit --filter=/path/to/filter.so plugin [args]
1034
1035       However if the shared library has a name of the form
1036       "nbdkit-name-filter.so" and if the library is installed in the
1037       $filterdir directory, then users can be run it by only typing:
1038
1039        nbdkit --filter=name plugin [args]
1040
1041       The location of the $filterdir directory is set when nbdkit is compiled
1042       and can be found by doing:
1043
1044        nbdkit --dump-config
1045
1046       If using the pkg-config/pkgconf system then you can also find the
1047       filter directory at compile time by doing:
1048
1049        pkg-config nbdkit --variable=filterdir
1050

PKG-CONFIG/PKGCONF

1052       nbdkit provides a pkg-config/pkgconf file called "nbdkit.pc" which
1053       should be installed on the correct path when the nbdkit development
1054       environment is installed.  You can use this in autoconf configure.ac
1055       scripts to test for the development environment:
1056
1057        PKG_CHECK_MODULES([NBDKIT], [nbdkit >= 1.2.3])
1058
1059       The above will fail unless nbdkit ≥ 1.2.3 and the header file is
1060       installed, and will set "NBDKIT_CFLAGS" and "NBDKIT_LIBS" appropriately
1061       for compiling filters.
1062
1063       You can also run pkg-config/pkgconf directly, for example:
1064
1065        if ! pkg-config nbdkit --exists; then
1066          echo "you must install the nbdkit development environment"
1067          exit 1
1068        fi
1069
1070       You can also substitute the filterdir variable by doing:
1071
1072        PKG_CHECK_VAR([NBDKIT_FILTERDIR], [nbdkit], [filterdir])
1073
1074       which defines "$(NBDKIT_FILTERDIR)" in automake-generated Makefiles.
1075

SEE ALSO

1077       nbdkit(1), nbdkit-plugin(3).
1078
1079       Standard filters provided by nbdkit:
1080
1081       nbdkit-blocksize-filter(1), nbdkit-cache-filter(1),
1082       nbdkit-cacheextents-filter(1), nbdkit-checkwrite-filter(1),
1083       nbdkit-cow-filter(1), nbdkit-ddrescue-filter(1),
1084       nbdkit-delay-filter(1), nbdkit-error-filter(1),
1085       nbdkit-exitlast-filter(1), nbdkit-exitwhen-filter(1),
1086       nbdkit-exportname-filter(1), nbdkit-ext2-filter(1),
1087       nbdkit-extentlist-filter(1), nbdkit-fua-filter(1),
1088       nbdkit-gzip-filter(1), nbdkit-ip-filter(1), nbdkit-limit-filter(1),
1089       nbdkit-log-filter(1), nbdkit-multi-conn-filter(1),
1090       nbdkit-nocache-filter(1), nbdkit-noextents-filter(1),
1091       nbdkit-nofilter-filter(1), nbdkit-noparallel-filter(1),
1092       nbdkit-nozero-filter(1), nbdkit-offset-filter(1),
1093       nbdkit-partition-filter(1), nbdkit-pause-filter(1),
1094       nbdkit-rate-filter(1), nbdkit-readahead-filter(1),
1095       nbdkit-retry-filter(1), nbdkit-stats-filter(1), nbdkit-swab-filter(1),
1096       nbdkit-tar-filter(1), nbdkit-tls-fallback-filter(1),
1097       nbdkit-truncate-filter(1), nbdkit-xz-filter(1) .
1098

AUTHORS

1100       Eric Blake
1101
1102       Richard W.M. Jones
1103
1105       Copyright (C) 2013-2020 Red Hat Inc.
1106

LICENSE

1108       Redistribution and use in source and binary forms, with or without
1109       modification, are permitted provided that the following conditions are
1110       met:
1111
1112       •   Redistributions of source code must retain the above copyright
1113           notice, this list of conditions and the following disclaimer.
1114
1115       •   Redistributions in binary form must reproduce the above copyright
1116           notice, this list of conditions and the following disclaimer in the
1117           documentation and/or other materials provided with the
1118           distribution.
1119
1120       •   Neither the name of Red Hat nor the names of its contributors may
1121           be used to endorse or promote products derived from this software
1122           without specific prior written permission.
1123
1124       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
1125       EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
1126       IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
1127       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
1128       LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
1129       CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
1130       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
1131       BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
1132       WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
1133       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
1134       ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1135
1136
1137
1138nbdkit-1.25.8                     2021-05-25                  nbdkit-filter(3)
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