nbdkit-captive(1)

1nbdkit-captive(1)                   NBDKIT                   nbdkit-captive(1)
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NAME

6       nbdkit-captive - run nbdkit under another process and have it reliably
7       cleaned up
8

SYNOPSIS

10        nbdkit PLUGIN [...] [-e|--exportname EXPORTNAME] --run "CMD ARGS ..."
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12        nbdkit --exit-with-parent PLUGIN [...]
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DESCRIPTION

15       You can run nbdkit under another process and have nbdkit reliably clean
16       up.  There are two techniques depending on whether you want nbdkit to
17       start the other process ("CAPTIVE NBDKIT"), or if you want the other
18       process to start nbdkit ("EXIT WITH PARENT").  Another way is to have
19       nbdkit exit after the last client connection
20       (nbdkit-exitlast-filter(1)) or after an event
21       (nbdkit-exitwhen-filter(1)).
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CAPTIVE NBDKIT

24       You can run nbdkit as a "captive process", using the --run option.
25       This means that nbdkit runs as long as (for example) qemu(1) or
26       guestfish(1) is running.  When those exit, nbdkit is killed.
27
28       Some examples should make this clear.
29
30       To run nbdkit captive under qemu:
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32        nbdkit file disk.img --run 'qemu -drive file=$nbd,if=virtio'
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34       On the qemu command line, $nbd is substituted automatically with the
35       right NBD path so it can connect to nbdkit.  When qemu exits, nbdkit is
36       killed and cleaned up automatically.
37
38       Running nbdkit captive under guestfish:
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40        nbdkit file disk.img --run 'guestfish --format=raw -a $nbd -i'
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42       When guestfish exits, nbdkit is killed.
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44       Running nbdkit captive under nbdsh for unit testing:
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46        nbdkit -U - memory 1 --run 'nbdsh -u "$uri" -c "print(h.pread(1, 0))"'
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48       The following shell variables are available in the --run argument:
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50       $nbd
51       $uri
52           A URI that refers to the nbdkit port or socket in the preferred
53           form documented by the NBD project.
54
55           As this variable may contain a bare "?" for Unix sockets, it is
56           safest to use $uri within double quotes to avoid unintentional
57           globbing.  For plugins that support distinct data based on export
58           names, the -e option to nbdkit controls which export name will be
59           set in the URI.
60
61           In nbdkit ≤ 1.22 $nbd tried to guess if you were using qemu or
62           guestfish and expanded differently.  Since NBD URIs are now widely
63           supported this magic is no longer necessary.  In nbdkit ≥ 1.24 both
64           variables expand to the same URI.
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66       $port
67           If ≠ "", the port number that nbdkit is listening on.
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69       $unixsocket
70           If ≠ "", the Unix domain socket that nbdkit is listening on.
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72       $exportname
73           The export name (which may be "") that the process should use when
74           connecting to nbdkit, as set by the -e (--exportname) command line
75           option of nbdkit.  This only matters to plugins that differentiate
76           what they serve based on the export name requested by the client.
77
78       --run implies --foreground.  It is not possible, and probably not
79       desirable, to have nbdkit fork into the background when using --run.
80
81       Even when running captive, nbdkit still listens on the regular TCP/IP
82       port, unless you specify the -p/-U options.  If you want a truly
83       private captive nbdkit, then you should create a private random Unix
84       socket, like this:
85
86        nbdkit -U - plugin [args] --run '...'
87
88   Copying data in and out of plugins with captive nbdkit
89       Captive nbdkit + qemu-img(1) can be used to copy data into and out of
90       nbdkit plugins.  For example nbdkit-example1-plugin(1) contains an
91       embedded disk image.  To copy it out:
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93        nbdkit -U - example1 --run 'qemu-img convert $nbd disk.img'
94
95       If plugin requests have a high overhead (for example making HTTP
96       requests to a remote server), adding nbdkit-readahead-filter(1) may
97       help performance:
98
99        nbdkit -U - --filter=readahead curl https://example.com/disk.img \
100               --run 'qemu-img convert $nbd disk.img'
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102       If the source suffers from temporary network failures
103       nbdkit-retry-filter(1) may help.
104
105       To overwrite a file inside an uncompressed tar file (the file being
106       overwritten must be the same size), use nbdkit-tar-filter(1) like this:
107
108        nbdkit -U - file data.tar --filter=tar tar-entry=disk.img \
109          --run 'qemu-img convert -n disk.img $nbd'
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EXIT WITH PARENT

112       The --exit-with-parent option is almost the opposite of "CAPTIVE
113       NBDKIT" described in the previous section.
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115       Running nbdkit with this option, for example from a script:
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117        nbdkit --exit-with-parent plugin ... &
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119       means that nbdkit will exit automatically if the parent program exits
120       for any reason.  This can be used to avoid complicated cleanups or
121       orphaned nbdkit processes.
122
123       --exit-with-parent is incompatible with forking into the background
124       (because when we fork into the background we lose track of the parent
125       process).  Therefore -f / --foreground is implied.
126
127       This is currently implemented using a non-POSIX feature available in
128       Linux ≥ 2.1.57 and FreeBSD ≥ 11.2, so it won't work on other operating
129       systems (patches welcome to make it work).
130
131       If the parent application is multithreaded, then (in the Linux
132       implementation) if the parent thread exits, that will cause nbdkit to
133       exit.  Thus in multithreaded applications you usually want to run
134       "nbdkit --exit-with-parent" only from the main thread (unless you
135       actually want nbdkit to exit with the thread, but that may not work
136       reliably on all operating systems).
137
138       To exit when an unrelated process exits, use nbdkit-exitwhen-filter(1)
139       "exit-when-process-exits" feature.
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AUTHORS

146       Eric Blake
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148       Richard W.M. Jones
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150       Pino Toscano
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COPYRIGHT

153       Copyright (C) 2013-2020 Red Hat Inc.
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LICENSE

156       Redistribution and use in source and binary forms, with or without
157       modification, are permitted provided that the following conditions are
158       met:
159
160       ·   Redistributions of source code must retain the above copyright
161           notice, this list of conditions and the following disclaimer.
162
163       ·   Redistributions in binary form must reproduce the above copyright
164           notice, this list of conditions and the following disclaimer in the
165           documentation and/or other materials provided with the
166           distribution.
167
168       ·   Neither the name of Red Hat nor the names of its contributors may
169           be used to endorse or promote products derived from this software
170           without specific prior written permission.
171
172       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
173       EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
174       IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
175       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
176       LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
177       CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
178       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
179       BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
180       WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
181       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
182       ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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186nbdkit-1.24.2                     2021-03-02                 nbdkit-captive(1)