Update docs with new readiness notification support

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

README.md

@@ -37,6 +37,8 @@ Features include:
   * Process supervision similar to [systemd][]
   * Sourcing environment files
   * Conditions for network/process/custom dependencies
+  * Readiness notification; PID files (native) for synchronizing system
+    startup, support for systemd [sd_notify()][], or [s6 style][] too
   * Pre/Post script actions
   * Tooling to enable/disable services
   * Built-in getty
@@ -620,6 +622,8 @@ and proposed extensions.
 [upstart]:          https://upstart.ubuntu.com/
 [systemd]:          https://www.freedesktop.org/wiki/Software/systemd/
 [openrc]:           https://www.gentoo.org/proj/en/base/openrc/
+[sd_notify()]:      https://www.freedesktop.org/software/systemd/man/sd_notify.html
+[s6 style]:         https://skarnet.org/software/s6/notifywhenup.html
 [run-parts(8)]:     https://manpages.debian.org/cgi-bin/man.cgi?query=run-parts
 [original finit]:   http://helllabs.org/finit/
 [EeePC fastinit]:   https://web.archive.org/web/20071208212450/http://wiki.eeeuser.com/boot_process:the_boot_process

doc/config.md

@@ -505,8 +505,8 @@ option:
 
         service notify:s6 mdevd -C -O 4 -D %n
 
-[sd_notify)]: https://www.freedesktop.org/software/systemd/man/sd_notify.html
-[s6 expect]:  https://skarnet.org/software/s6/notifywhenup.html
+[sd_notify()]: https://www.freedesktop.org/software/systemd/man/sd_notify.html
+[s6 expect]:   https://skarnet.org/software/s6/notifywhenup.html
 
 When a service is ready, either by Finit detecting its PID file, or
 their respective readiness mechanism has been triggered, Finit creates

man/finit.8

@@ -45,6 +45,11 @@ Sourcing environment files
 .It
 Conditions for network/process/custom dependencies
 .It
+Process readiness notification for synchronizing system startup as well
+as reconfiguration at runtime.  Natively PID files are used, but systemd
+.Cm sd_notify()
+and s6 notification is also supported.
+.It
 Pre/Post script actions
 .It
 Tooling to enable/disable services
@@ -192,7 +197,7 @@ file removed, service B is also stopped.
 .Pp
 The following condition families are available today:
 .Pp
-.Bl -tag -width pid -offset indent
+.Bl -tag -width service -offset indent
 .It Cm net
 Linux netlink events, e.g. net/route/default, net/eth0/up, and
 net/et0/running
@@ -200,6 +205,14 @@ net/et0/running
 PID files basd on the service declaration
 .Cm name:id ,
 gives the condition pid/name:id
+.It Cm service
+Tracks run/task/service state stansitions, including readiness.  E.g.,
+.Cm service/foo/ready
+can be used as a condition for service
+.Cm bar ,
+provided
+.Cm foo
+properly signals its readiness to Finit.
 .It Cm sys
 System conditions, e.g. sys/key/ctrlaltdel and sys/pwr/fail
 .It Cm usr

man/finit.conf.5

@@ -275,6 +275,50 @@ can depend on the
 .Cm <pid/bar>
 condition.
 .Pp
+As an alternative "readiness" notification, Finit supports both systemd
+and s6 style notification.  This can be enabled by using the `notify`
+option:
+.Bl -tag -width 1n
+.It Cm notify:systemd
+tells Finit the service uses the
+.Cm sd_notify()
+API to signal PID 1 when it has completed its startup and is ready
+to service events.  This API expects
+the environment variable
+.Cm NOTIFY_SOCKET
+to be set to the socket where the application can send
+.Cm "READY=1\n"
+when it is starting up or has processed a
+.Cm SIGHUP .
+For details, see:
+.Pp
+.Lk https://www.freedesktop.org/software/systemd/man/sd_notify.html
+.It Cm notify:s6
+puts Finit in s6 compatibility mode.  Compared to the systemd
+notification, s6 expect compliant daemons to send
+.Cm "\\n"
+and then close their socket.  For details, see:
+.Pp
+.Lk https://skarnet.org/software/s6/notifywhenup.html
+.Pp
+Finit takes care of "hard-wiring" the READY state as long as the
+application is running, events across any `SIGHUP`.  Since s6 can give
+its applications the descriptor number (must be >3) on then command
+line, Finit provides the following syntax (
+.Cm %n
+is replaced by Finit with then descriptor number):
+.Bd -unfilled -offset indent
+service notify:s6 mdevd -C -O 4 -D %n
+.Ed
+.Pp
+When a service is ready, either by Finit detecting its PID file, or
+their respective readiness mechanism has been triggered, Finit creates
+then service's ready condition which other services can depend on:
+
+    $ initctl -v cond get service/mdevd/ready
+    on
+.El
+.Pp
 If a service should not be automatically started, it can be configured
 as manual with the
 .Cm manual:yes