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<title>s6: the s6-notifyoncheck program</title>
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<p>
<a href="index.html">s6</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
</p>
<h1> The s6-notifyoncheck program </h1>
<p>
<tt>s6-notifyoncheck</tt> is a chain-loading program meant to be used
in run scripts, in a service that has been declared to honor
readiness notification. It implements a policy of running a user-provided
executable in the background that polls the service currently being
launched, in order to check when it becomes ready. It feeds the
result of this check into the s6 notification mechanism.
</p>
<p>
s6-notifyoncheck should <strong>only</strong> be used with daemons
that can be polled from the outside to check readiness, and that
<strong>do not implement readiness notification themselves</strong>.
</p>
<h2> Interface </h2>
<pre>
s6-notifyoncheck [ -d ] [ -3 <em>notiffd</em> ] [ -s <em>initialsleep</em> ] [ -T <em>globaltimeout</em> ] [ -t <em>localtimeout</em> ] [ -w <em>waitingtime</em> ] [ -n <em>n</em> ] [ -c <em>checkprog</em> ] <em>prog...</em>
</pre>
<p>
s6-notifyoncheck forks and runs as the child; the parent immediately execs into
<em>prog...</em>, the daemon that must be checked for readiness.
</p>
<p>
s6-notifyoncheck first waits for a little time, then it spawns the
<tt>./data/check</tt> executable and waits for it to exit. If <tt>./data/check</tt>
exits 0, then s6-notifyoncheck reports that the service is ready, then
exits. If <tt>./data/check</tt> exits anything else, s6-notifyoncheck sleeps
for a little time, then spawns <tt>./data/check</tt> again. It loops until
<tt>./data/check</tt> succeeds, or 7 attempts fail, or a certain amount of
time elapses.
</p>
<h2> Exit codes </h2>
<p>
s6-notifyoncheck can exit before executing into <em>prog</em>:
</p>
<ul>
<li> 100: wrong usage </li>
<li> 111: system call failed </li>
</ul>
<p>
After forking, s6-notifyoncheck (running as the child) can
exit with the following exit codes, but those are meaningless
because no process will, or should, check them. They are only
differentiated for clarity in the code:
</p>
<ul>
<li> 0: service readiness achieved and notification sent </li>
<li> 1: maximum number of attempts reached, all unsuccessful </li>
<li> 2: <em>prog</em> died, so s6-notifyoncheck exited early </li>
<li> 3: timed out before readiness was achieved </li>
<li> 111: system call failed </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-d</tt> : doublefork. s6-notifyoncheck will run as the
grandchild of <em>prog...</em> instead of its direct child. This is useful
if <em>prog...</em> never reaps zombies it does not know it has. </li>
<li> <tt>-3 <em>notiffd</em></tt> : use <em>notiffd</em> as the
file descriptor to send a readiness notification to. By default, this
number is automatically read from the <tt>./notification-fd</tt> file. </li>
<li> <tt>-s <em>initialsleep</em></tt> : sleep for
<em>initialsleep</em> milliseconds before starting to poll the service
for readiness. Default is 10 milliseconds. </li>
<li> <tt>-T <em>globaltimeout</em></tt> : give up (and leave
the service <em>up</em> but not <em>ready</em>) if service readiness still
has not been detected after <em>globaltimeout</em> milliseconds. Default
is 0, meaning infinite: s6-notifyoncheck will keep polling until it succeeds. </li>
<li> <tt>-t <em>localtimeout</em></tt> : on every attempt, if
<tt>./data/check</tt> still has not exited after <em>localtimeout</em> milliseconds,
kill it and declare that attempt failed. Default is 0, meaning infinite:
s6-notifyoncheck will wait forever for <tt>./data/check</tt> to exit. </li>
<li> <tt>-w <em>waitingtime</em></tt> : sleep for
<em>waitingtime</em> milliseconds between two invocations of <tt>./data/check</tt>.
This is basically the polling period. Default is 1000: the service will
be polled every second. </li>
<li> <tt>-n <em>n</em></tt> : give up after <em>n</em>
unsuccessful invocations of <tt>./data/check</tt>. 0 means infinite, i.e. keep
polling until it succeeds, or times out, or the service dies first.
Default is 7. </li>
<li> <tt>-c <em>checkprog...</em></tt> : invoke <em>checkprog...</em>
instead of <tt>./data/check</tt>. The <em>checkprog</em> string will be parsed by
<a href="//skarnet.org/software/execline/execlineb.html">execlineb</a>, so it
can contain a full command line. This option is mainly useful is the program
used to poll the service is very simple and can be inlined as a simple
command line, to avoid needing to manage a whole script and a <tt>./data/check</tt>
file. This option is only supported if the
<a href="//skarnet.org/software/execline/">execline</a> package is installed;
if it is not, the <tt>-c</tt> option cannot be used and the checking
script must always be placed in <tt>./data/check</tt>. </li>
</ul>
<h2> Usage </h2>
<p>
s6-notifyoncheck is designed to make it possible for services to use the
<a href="notifywhenup.html">s6 notification mechanism</a> even with daemons
that do not natively implement the mechanism of writing a newline to a file
descriptor of their choice when they're ready.
</p>
<p>
<a href="//skarnet.org/cgi-bin/archive.cgi?2:mss:1607:dfblejammjllfkggpcph">Polling</a>
is evil. Please make sure you really have no other choice before writing a
<tt>./data/check</tt> program and using s6-notifyoncheck in your run script.
If you have access to the source code of the daemon you want to check for
readiness, consider patching it to add readiness notification support, which
is extremely simple and does not require linking against any s6 library.
</p>
<p>
If using a <tt>./data/check</tt> program is your only option:
</p>
<ul>
<li> Make sure the <tt>./data</tt> subdirectory is readable and that
<tt>./data/check</tt> is executable, exits 0 if the daemon
it checks is ready, and exits nonzero if the daemon is not ready. </li>
<li> Add a <tt>./notification-fd</tt> file to your service directory,
which can contain any number that is not 0, 1 or 2, or anything else
explicitly used in your run script. The daemon does not need to care
about that file descriptor; from the daemon's point of view, nothing
changes. </li>
<li> In your run script, insert <tt>s6-notifyoncheck</tt> in the
command line that will execute into your daemon. </li>
<li> <tt>./data/check</tt> will run as the same user as s6-notifyoncheck.
If s6-notifyoncheck runs after the run script's process has lost its
root privileges, make sure that <tt>./data/check</tt> is accessible
and runnable as that user. </li>
<li> Note that s6-notifyoncheck should be used to check if <em>the
current service itself</em> is running. You cannot use it to
poll for the readiness of another service. </li>
</ul>
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