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.

       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)).

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.

SEE ALSO

       nbdkit(1), nbdkit-exitlast-filter(1), nbdkit-ip-filter(1), nbdkit-limit-filter(1), nbdkit-rate-filter(1),
       nbdkit-captive(1), nbdkit-service(1), nbdkit-filter(3), nbdkit-plugin(3).

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2020 Red Hat Inc.

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.

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nbdkit-1.24.1                                      2021-01-20                          nbdkit-exitwhen-filter(1)