The s6-notifywhenup program

Starting with s6-2.1.4.0, the s6-notifywhenup program has been deprecated, because there was still a case (albeit extremely rare) where a race condition would occur and readiness would be improperly advertised. Readiness notification for a service can now be achieved via a notification-fd file in the service directory, containing the number of the descriptor the service will write its readiness notification newline to. The notification will directly be picked by s6-supervise.

Quick upgrade recipe: for every service using s6-notifywhenup, replace the s6-notifywhenup invocation in your run script with fdmove 1 3, then perform echo 3 > notification-fd. Done!

s6-notifywhenup launches a daemon while listening to a file descriptor, and sends a 'U' event to a fifodir when it receives something on that file descriptor.

This page explains why this program is needed.

Interface

     s6-notifywhenup [ -d fd ] [ -e fifodir ] [ -f ] [ -X ] [ -t timeout ] prog...

s6-notifywhenup forks and executes prog... as the parent, with a pipe from prog...'s stdout to the child.
The child waits for a newline (\n) to be written on the pipe. When it gets it, it creates a ./supervise/ready file then sends a 'U' event to the ./event fifodir.
The child exits 0.

Options

-d fd : listen to prog's output on descriptor fd. The default is 1.
-e fifodir : send a 'U' event to fifodir fifodir. Default is ./event.
-f : simple fork. Normally, s6-notifywhenup doubleforks, in order to accommodate for a prog that does not expect to have a child and avoid a pending zombie. This option avoids the doublefork, but it should only be set if prog reaps even children it doesn't know it has.
-t timeout : if no EOF has been received after timeout milliseconds, exit without sending the event. Default is 0, meaning infinite.
-X : fake readiness. s6-notifywhenup will actually send the newline itself before executing prog. This option should in principle never be used.

Notes

s6-notifywhenup executes prog... as the parent in order for prog to keep the same pid, which is vital for supervised processes.
s6-notifywhenup can be used, for instance, with s6-ipcserver and its -1 option, so that reliable startup notification is achieved. s6-notifywhenup -f s6-ipcserver -1 args will send a 'U' event to ./event when s6-ipcserver is actually listening to its socket.
The s6-svwait program can be used to wait for 'U' events, as well as the s6-svlisten1 and s6-svlisten programs.
The supervise/ready file, when created, contains at least the absolute time when s6-notifywhenup detected service readiness. The format and contents of this file are subject to change.