finit/doc/conditions.md

Finit Conditions

• Introduction
• Triggering
• Built-in Conditions
• Debugging
• Internals

Introduction

Conditions are a new addition to Finit, introduced in v3, with the intention of providing a mechanism for common synchronization problems. For example:

• "wait for service A to start before starting service B", or
• "wait for basic network access to be available"

Conditions are similar in syntax to declaring runlevels per service. They are specified within angle brackets <> and can be applied to any of the service, task, or run stanza. Multiple conditions may be specified separated by comma. Multiple conditions are logically AND'ed during evaluation, i.e. all conditions must be satisfied in order for a service to run.

Example

service [2345] <pid/setupd,pid/zebra> /sbin/netd -- Network monitor

In this example the Network monitor daemon netd is not started until both the pid/setupd and pid/zebra conditions are satisfied. A pid/ condition is satisfied by the corresponding service's PID file being created.

NOTE: Conditions also stop services when a condition is no longer asserted. I.e., if the Zebra process above stops or restarts, netd will also stop or restart.

Triggering

Conditions are mainly triggered (asserted) by built-in plugins, e.g., netlink.so and pidfile.so, see below for built-in conditions. Finit also supports user-defined conditions, sometimes referred to as static or one-shot conditions. They live in the <usr/... namespace and are constrained to a flat hierarchy without any sub-directories, unlike the pidfile plugin, for instance.

User-defined conditions are controlled using the initctl cond set and initctl cond clear commands:

initctl cond set foo
initctl cond clear foo

The purpose of user-defined conditions is to be able to start or stop services, or run/task jobs, on external site-dependent stimuli. Example:

service [2345] <usr/foo> alarm --arg foo -- Foo alarm

For convenience, prefixing with usr/ is allowed, but any other slashes or period characters are disallowed. E.g., to trigger the Foo alarm, the same as above, can also be achieved like this:

initctl cond set usr/foo
initctl cond clear usr/foo

Conditions retain their current state until the next reconfiguration or runlevel change. At that point all set conditions transition into the flux state, meaning the condition's state is unknown. (For more info on this, see Internals.) Thus, after a reconfiguration it is up to the "owner" of the condition to convey the new (or possibly unchanged) state of it.

Note: For pid/ conditions it is expected that services "touch" or recreate their PID file on SIGHUP.

Static (one-shot) conditions, like usr/, never enter the flux state.

Built-in Conditions

Finit comes with a set of plugins for conditions: pidfile, netlink, sys, and usr. The pidfile plugin (recursively) watches /run/ (recursively) for PID files created by the monitored services, and sets a corresponding condition in the pid/ namespace. Similarly, the netlink plugin provides basic conditions for when an interface is brought up/down and when a default route (gateway) is set, in the net/ namespace. The sys and usr plugins monitor are passive condition monitors where the action is provided by keventd, signal handlers, and in the case of usr, the end-user via the initctl tool

With the example listed above, finit does not start the /sbin/netd daemon until setupd and zebra has started and created their PID files. Which they do when they have completed their initial set up and are ready to receive signals.

Finit expects monitored services to touch their PID files, i.e. update the mtime, when they reload their configuration files after a SIGHUP. Some services do not support SIGHUP and are instead restarted, which is a crude but effective way to have the PID file touched (re-created).

Built-in conditions:

• pid/<SERVICE>
• net/route/default
• net/<IFNAME>/exist
• net/<IFNAME>/up
• net/<IFNAME>/running
• sys/pwr/ac
• sys/pwr/fail
• sys/key/ctrlaltdel
• usr/foo

Note: up means administratively up, the interface flag IFF_UP. running is the IFF_RUNNING flag, meaning operatively up. The difference is that running tells if the NIC has link.

Composition

The pid/ conditions are generated by the Finit pidfile.so plugin and composed from a service's name: and :id. By default the basename of the daemon and the empty string.

service	condition
/sbin/foo	pid/foo
/sbin/bar -p /run/baz.pid	pid/bar
name:lxc :foo lxc-start -n foo -p /run/lxc/foo.pid	pid/lxc:foo
/usr/bin/dbus-daemon	pid/dbus-daemon
:222 dropbear -p 222	pid/dropbear:222

The condition is asserted when pidfile.so receives an inotify event for a file matching /run/*.pid, /run/**/*.pid, or /run/**/pid, which contains the PID of the service Finit has started.

When Finit configuration files are changed and the initctl reload command is called, it is expected of services to touch their PID files for Finit to reassert their conditions.

Daemons that don't create PID files, or fail to touch them on reload, can be worked around by using the pid:/path/to/file.pid syntax in the service stanza for the daemon. It is far from optimal since any synchronization of depending services may fail due to the daemon not having reinitialized/created their IPC sockets, or similar.

Note: in versions of Finit prior to v4, the PID conditions were called 'svc' conditions, and they were far more complex.

Debugging

If a service is not being started as it should, the problem might be that one of its conditions is not in the expected state. Use the command initctl status to inspect service status. Services in the ready state are pending a condition.

In that situation, running initctl cond show reveals which of the conditions that are not satisfied. Listed as off below.

Example:

~ # initctl cond show
PID     IDENT         STATUS  CONDITION (+ ON, ~ FLUX, - OFF)
=======================================================================
1419    /sbin/netd    on      <+pid/setupd,+pid/zebra>
0       /sbin/udhcpc  off     <-net/vlan1/exist>

Here we can see that netd is allowed to run since both its conditions are in the on state, as indicated by the +-prefix. udhcpc however is not allowed to run since net/vlan1/exist condition is not satisfied. As indicated by the --prefix.

To fake interface vlan1 suddenly appearing, and test what happens to udhcpc we can enable debug mode and assert the condition, like this:

~ # initctl debug
~ # mkdir -p /var/run/finit/cond/net/vlan1
~ # cp /var/run/finit/cond /var/run/finit/cond/net/vlan1/exist

Then watch the console for the debug messages and then check the output from initctl cond show again. The client will likely have failed to start, but at least the condition is now satisfied.

There is also the initctl cond dump command, which dumps all known conditions, their current status, and their origin.

Internals

As shown previously, conditions are implemented as simple files in the file system, in the /var/run/finit/cond/ sub-directory. The files are created, updated, and removed by condition plugins. To debug them, see the previous section.

A condition is always in one of three states:

• on (+): The condition is asserted.
• off (-): The condition is deasserted.
• flux (~): The conditions state is unknown.

All conditions that have not explicitly been set are interpreted as being in the off state.

When a reconfiguration is requested, Finit transitions all conditions to the flux state. As a result, services that depend on a condition are sent SIGSTOP. Once the new state of the condition is asserted, the service receives SIGCONT. If the condition is no longer satisfied the service will then be stopped, otherwise no further action is taken.

This STOP/CONT handling minimizes the number of unnecessary service restarts that would otherwise occur because a depending service was sent SIGHUP for example.

Therefore, any plugin that supplies Finit with conditions must ensure that their state is updated after each reconfiguration. This can be done by binding to the HOOK_SVC_RECONF hook. For an example of how to do this, see plugins/pidfile.c.