Ubuntu Manpage: nbdkit-exitwhen-filter - exit gracefully when an event occurs

Provided by: nbdkit_1.36.3-1ubuntu10_amd64

NAME

       nbdkit-exitwhen-filter - exit gracefully when an event occurs

SYNOPSIS

        nbdkit --filter=exitwhen PLUGIN
                                 [exit-when-file-created=FILENAME]
                                 [exit-when-file-deleted=FILENAME]
                                 [exit-when-pipe-closed=FD]
                                 [exit-when-process-exits=PID]
                                 [exit-when-script=SCRIPT]
                                 [exit-when-poll=SECS]

DESCRIPTION

       "nbdkit-exitwhen-filter" is an nbdkit filter that causes nbdkit to exit gracefully when some external
       event occurs.  Built-in events are: a file being created or deleted, a pipe being closed, or a process
       exiting.  You can also define custom events using an external script or command.

       After the event occurs nbdkit refuses new connections, waits for all current clients to disconnect, and
       then exits.

       A similar filter is nbdkit-exitlast-filter(1).  For other ways to ensure that nbdkit exits when you want
       see nbdkit-captive(1) and nbdkit-service(1).

EXAMPLES

        nbdkit --filter=exitwhen memory 1G exit-when-file-created=/tmp/stop

       nbdkit will run normally until something creates /tmp/stop, whereupon nbdkit will refuse new connections
       and exit as soon as the last client has disconnected.  If /tmp/stop exists before nbdkit starts, it will
       exit immediately.

        nbdkit --filter=exitwhen memory 1G exit-when-process-exits=1234

       nbdkit will exit gracefully when PID 1234 exits and all connections close.  If you want to exit when the
       parent process of nbdkit exits, consider using the --exit-with-parent flag instead.

PARAMETERS

You can define multiple "exit-when-*" events on the command line: nbdkit will exit if any of the events
happens. If there are no "exit-when-*" events then the filter does nothing.

exit-when-file-created=FILENAME
exit-when-file-deleted=FILENAME
Exit when the named file is created or deleted.

exit-when-pipe-closed=FD
The read end of a pipe(2) is passed to nbdkit in the given file descriptor number. Exit when the
pipe is closed. The filter does not read any data from the pipe.

For an example of how to use this parameter, see:
https://gitlab.com/nbdkit/nbdkit/-/blob/master/tests/test-exitwhen-pipe-closed.c

exit-when-process-exits=PID
Exit when process ID "PID" exits.

Note there is a small race between passing the process ID to the filter and the filter checking it
for the first time. During this window the original PID might exit and an unrelated program might
get the same PID, thus holding nbdkit open for longer than wanted. The pipe method above is more
reliable if you are able to modify the other process.

exit-when-script="SCRIPT"
Create a custom event using the script or command "SCRIPT". The "SCRIPT" can be a program, shell
script or a command with optional parameters. Note if using a filename here: you may need to use the
absolute path because nbdkit changes directory when it daemonizes.

The script should exit with code 88 if the event is detected. The filter does different things
depending on the exit code of the script:

0 The event has not been triggered, so nbdkit continues to process requests as normal.

"1-87"
An error is logged, but the event is not triggered and nbdkit continues to process requests as
normal.

88 The event has been triggered. nbdkit will refuse new connections and exit gracefully as soon as
all current clients disconnect.

"89-"
Exit codes 89 and above are reserved for future use. The behaviour may change in future if
scripts return any of these exit codes.

exit-when-poll=SECS
When nbdkit is serving clients this filter does not need to poll because it can check for events when
a client connects or disconnects. However when nbdkit is idle the filter needs to poll for events
every "SECS" seconds and if any event happens exit immediately.

The default is 60 seconds.

NOTES

Compared to --exit-with-parent
The nbdkit server option --exit-with-parent causes nbdkit to exit when the parent process exits. It
seems similar to:

nbdkit --filter=exitwhen ... exit-when-process-exits=$$

($$ is the parent PID of nbdkit).

But there are significant differences caused by the implementation. --exit-with-parent is implemented
using the Linux feature "PR_SET_PDEATHSIG" ("PROC_PDEATHSIG_CTL" on FreeBSD). This causes a signal to be
sent to the server when the parent process dies. On receiving the signal nbdkit closes client
connections and terminates at once.

On the other hand "exit-when-process-exits" monitors the other process (which does not need to be the
parent) and shuts down the server in a different way: currently open connections are allowed to continue
until they close.

Neither of these methods is completely reliable in all cases: signals can be lost and there is a possible
(albeit very small) race when passing the PID to "exit-when-process-exits". More reliable methods of
clean up are "exit-when-pipe-closed" or putting all of the processes into a cgroup. (See
nbdkit-captive(1) and nbdkit-service(1)).

Query --dump-plugin output
Not all events are supported on all platforms. To query which events the filter supports use:

$ nbdkit null --filter=exitwhen --dump-plugin
[...]
exitwhen_file_created=yes
exitwhen_file_deleted=yes
exitwhen_process_exits=yes
exitwhen_pipe_closed=yes
exitwhen_script=yes

FILES

       $filterdir/nbdkit-exitwhen-filter.so
           The filter.

           Use "nbdkit --dump-config" to find the location of $filterdir.

VERSION

       "nbdkit-exitwhen-filter" first appeared in nbdkit 1.24.

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

LICENSE

       Redistribution  and  use in source and binary forms, with or without modification, are permitted provided
       that the following conditions are met:

       •   Redistributions of source code must retain the above copyright notice, this list  of  conditions  and
           the following disclaimer.

       •   Redistributions in binary form must reproduce the above copyright notice, this list of conditions and
           the following disclaimer in the documentation and/or other materials provided with the distribution.

       •   Neither  the  name  of  Red  Hat  nor the names of its contributors may be used to endorse or promote
           products derived from this software without specific prior written permission.

       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS  OR  IMPLIED  WARRANTIES,
       INCLUDING,  BUT  NOT  LIMITED  TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE  FOR  ANY  DIRECT,  INDIRECT,
       INCIDENTAL,  SPECIAL,  EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
       ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT  LIABILITY,  OR  TORT  (INCLUDING  NEGLIGENCE  OR
       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
       DAMAGE.

nbdkit-1.36.3                                      2024-03-31                          nbdkit-exitwhen-filter(1)