=head1 NAME

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

=head1 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]

=head1 DESCRIPTION

C<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 L<nbdkit-exitlast-filter(1)>.  For other ways to
ensure that nbdkit exits when you want see L<nbdkit-captive(1)> and
L<nbdkit-service(1)>.

=head1 EXAMPLES

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

nbdkit will run normally until something creates F</tmp/stop>,
whereupon nbdkit will refuse new connections and exit as soon as the
last client has disconnected.  If F</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 I<--exit-with-parent> flag instead.

=head1 PARAMETERS

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

=over 4

=item B<exit-when-file-created=>FILENAME

=item B<exit-when-file-deleted=>FILENAME

Exit when the named file is created or deleted.

=item B<exit-when-pipe-closed=>FD

The read end of a L<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:
L<https://gitlab.com/nbdkit/nbdkit/-/blob/master/tests/test-exitwhen-pipe-closed.c>

=item B<exit-when-process-exits=>PID

Exit when process ID C<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.

=item B<exit-when-script=">SCRIPTB<">

Create a custom event using the script or command C<SCRIPT>.  The
C<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:

=over 4

=item C<0>

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

=item C<1-87>

An error is logged, but the event is I<not> triggered and nbdkit
continues to process requests as normal.

=item C<88>

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

=item C<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.

=back

=item B<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
C<SECS> seconds and if any event happens exit immediately.

The default is 60 seconds.

=back

=head1 NOTES

=head2 Compared to I<--exit-with-parent>

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

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

(C<$$> is the parent PID of nbdkit).

But there are significant differences caused by the implementation.
I<--exit-with-parent> is implemented using the Linux feature
C<PR_SET_PDEATHSIG> (C<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 C<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 C<exit-when-process-exits>.  More reliable methods
of clean up are C<exit-when-pipe-closed> or putting all of the
processes into a cgroup.  (See L<nbdkit-captive(1)> and
L<nbdkit-service(1)>).

=head2 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

=head1 FILES

=over 4

=item F<$filterdir/nbdkit-exitwhen-filter.so>

The filter.

Use C<nbdkit --dump-config> to find the location of C<$filterdir>.

=back

=head1 VERSION

C<nbdkit-exitwhen-filter> first appeared in nbdkit 1.24.

=head1 SEE ALSO

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

=head1 AUTHORS

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
