Ubuntu Manpage: nbdkit-captive - run nbdkit under another process and have it reliably cleaned up

Provided by: nbdkit_1.40.4-5ubuntu1_amd64

NAME

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

SYNOPSIS

        nbdkit PLUGIN [...] [-e|--exportname EXPORTNAME] \
                            --run 'COMMAND ARGS ...'

        nbdkit --exit-with-parent PLUGIN [...]

DESCRIPTION

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

CAPTIVE NBDKIT

You can run nbdkit as a "captive process", using the --run option. This means that nbdkit
runs as long as (for example) qemu(1) or guestfish(1) is running. When those exit, nbdkit
is killed.

Some examples should make this clear.

To run nbdkit captive under qemu:

nbdkit file disk.img --run 'qemu -drive file="$uri",if=virtio'

On the qemu command line, $uri is substituted automatically with the right NBD path so it
can connect to nbdkit. When qemu exits, nbdkit is killed and cleaned up automatically.

Running nbdkit captive under guestfish:

nbdkit file disk.img --run 'guestfish --format=raw -a "$uri" -i'

When guestfish exits, nbdkit is killed.

Running nbdkit captive under nbdsh for unit testing:

nbdkit memory 1 --run 'nbdsh -u "$uri" -c "print(h.pread(1, 0))"'

Running an fio benchmark:

nbdkit -f null 1G --run 'export uri; fio fio.git/examples/nbd.fio'

The following shell variables are available in the --run argument (local to the run script
unless you use "export"):

$uri
$nbd
A URI that refers to the nbdkit port or socket in the preferred form documented by the
NBD project.

As this variable may contain a bare "?" for Unix sockets, it is safest to use $uri
within double quotes to avoid unintentional globbing. For plugins that support
distinct data based on export names, the -e option to nbdkit controls which export
name will be set in the URI.

In nbdkit ≤ 1.22 $nbd tried to guess if you were using qemu or guestfish and expanded
differently. Since NBD URIs are now widely supported this magic is no longer
necessary. In nbdkit ≥ 1.24 both variables expand to the same URI.

See also "NBD URIs and endpoints" in nbdkit(1).

$port
If ≠ "", the port number that nbdkit is listening on.

$unixsocket
If ≠ "", the Unix domain socket that nbdkit is listening on.

$exportname
The export name (which may be "") that the process should use when connecting to
nbdkit, as set by the -e (--exportname) command line option of nbdkit. This only
matters to plugins that differentiate what they serve based on the export name
requested by the client.

$tls
Corresponds to the --tls option passed to nbdkit. If --tls=off this is not set. If
--tls=on this is set to "1". If --tls=require this is set to "2".

$tls_certificates
If --tls-certificates was passed to nbdkit, the value is copied here. It is usually
the directory containing PKI certificates. Note that the path might not be an
absolute path, or even valid.

$tls_psk
If --tls-psk was passed to nbdkit, the value is copied here. It is usually the
filename of a TLS Pre-Shared Keys (PSK) file. Note that the filename might not be an
absolute path, or even valid.

--run implies --foreground. It is not possible, and probably not desirable, to have
nbdkit fork into the background when using --run.

Copying data in and out of plugins with captive nbdkit
Captive nbdkit + qemu-img(1) can be used to copy data into and out of nbdkit plugins. For
example nbdkit-example1-plugin(1) contains an embedded disk image. To copy it out:

nbdkit example1 --run 'qemu-img convert "$uri" disk.img'

If the source suffers from temporary network failures nbdkit-retry-filter(1) or
nbdkit-retry-request-filter(1) may help.

To overwrite a file inside an uncompressed tar file (the file being overwritten must be
the same size), use nbdkit-tar-filter(1) like this:

nbdkit file data.tar --filter=tar tar-entry=disk.img \
--run 'qemu-img convert -n disk.img "$uri"'

EXIT WITH PARENT

The --exit-with-parent option is almost the opposite of "CAPTIVE NBDKIT" described in the
previous section.

Running nbdkit with this option, for example from a script:

nbdkit --exit-with-parent plugin ... &

means that nbdkit will exit automatically if the parent program exits for any reason.
This can be used to avoid complicated cleanups or orphaned nbdkit processes.

--exit-with-parent is incompatible with forking into the background (because when we fork
into the background we lose track of the parent process). Therefore -f / --foreground is
implied.

If the parent application is multithreaded, then (in the Linux implementation) if the
parent thread exits, that will cause nbdkit to exit. Thus in multithreaded applications
you usually want to run "nbdkit --exit-with-parent" only from the main thread (unless you
actually want nbdkit to exit with the thread, but that may not work reliably on all
operating systems).

To exit when an unrelated process exits, use nbdkit-exitwhen-filter(1)
"exit-when-process-exits" feature.

Support for --exit-with-parent
This is currently implemented using a non-POSIX feature available in Linux ≥ 2.1.57,
FreeBSD ≥ 11.2 and macOS. It won't work on other operating systems (patches welcome to
make it work).

To test if the current binary supports this feature the most backwards compatible way is:

nbdkit --exit-with-parent --version && echo "supported"

In nbdkit ≥ 1.34, "nbdkit --dump-config" prints either "exit_with_parent=yes" or
"exit_with_parent=no" but earlier versions did not have this.

AUTHORS

       Eric Blake

       Richard W.M. Jones

       Pino Toscano

COPYRIGHT

       Copyright Red Hat

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