s6: the s6-notifyoncheck program
Keywords: s6 command s6-notifyoncheck notification service check polling

s6
Software
skarnet.org

# The s6-notifyoncheck program

`s6-notifyoncheck` 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.

s6-notifyoncheck should only be used with daemons that can be polled from the outside to check readiness, and that do not implement readiness notification themselves.

## Interface

         s6-notifyoncheck [ -d ] [ -3 notiffd ] [ -s initialsleep ] [ -T globaltimeout ] [ -t localtimeout ] [ -w waitingtime ] [ -n n ] [ -c checkprog ] prog...

s6-notifyoncheck forks and runs as the child; the parent immediately execs into prog..., the daemon that must be checked for readiness.

s6-notifyoncheck first waits for a little time, then it spawns the `./data/check` executable and waits for it to exit. If `./data/check` exits 0, then s6-notifyoncheck reports that the service is ready, then exits. If `./data/check` exits anything else, s6-notifyoncheck sleeps for a little time, then spawns `./data/check` again. It loops until `./data/check` succeeds, or 7 attempts fail, or a certain amount of time elapses.

## Exit codes

s6-notifyoncheck can exit before executing into prog:

- 100: wrong usage
- 111: system call failed

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:

- 0: service readiness achieved and notification sent
- 1: maximum number of attempts reached, all unsuccessful
- 2: prog died, so s6-notifyoncheck exited early
- 3: timed out before readiness was achieved
- 111: system call failed

## Options

- `-d` : doublefork. s6-notifyoncheck will run as the grandchild of prog... instead of its direct child. This is useful if prog... never reaps zombies it does not know it has.
- `-3 notiffd` : use notiffd as the file descriptor to send a readiness notification to. By default, this number is automatically read from the `./notification-fd` file.
- `-s initialsleep` : sleep for initialsleep milliseconds before starting to poll the service for readiness. Default is 10 milliseconds.
- `-T globaltimeout` : give up (and leave the service up but not ready) if service readiness still has not been detected after globaltimeout milliseconds. Default is 0, meaning infinite: s6-notifyoncheck will keep polling until it succeeds.
- `-t localtimeout` : on every attempt, if `./data/check` still has not exited after localtimeout milliseconds, kill it and declare that attempt failed. Default is 0, meaning infinite: s6-notifyoncheck will wait forever for `./data/check` to exit.
- `-w waitingtime` : sleep for waitingtime milliseconds between two invocations of `./data/check`. This is basically the polling period. Default is 1000: the service will be polled every second.
- `-n n` : give up after n unsuccessful invocations of `./data/check`. 0 means infinite, i.e. keep polling until it succeeds, or times out, or the service dies first. Default is 7.
- `-c checkprog...` : invoke checkprog... instead of `./data/check`. The checkprog string will be parsed by execlineb, 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 `./data/check` file. This option is only supported if the execline package is installed; if it is not, the `-c` option cannot be used and the checking script must always be placed in `./data/check`.

## Usage

s6-notifyoncheck is designed to make it possible for services to use the s6 notification mechanism 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.

Polling is evil. Please make sure you really have no other choice before writing a `./data/check` 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.

If using a `./data/check` program is your only option:

- Make sure the `./data` subdirectory is readable and that `./data/check` is executable, exits 0 if the daemon it checks is ready, and exits nonzero if the daemon is not ready.
- Add a `./notification-fd` 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.
- In your run script, insert `s6-notifyoncheck` in the command line that will execute into your daemon.
- `./data/check` 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 `./data/check` is accessible and runnable as that user.
- Note that s6-notifyoncheck should be used to check if the current service itself is running. You cannot use it to poll for the readiness of another service.