The s6-notifywhenup program

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 ] [ -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 an empty ./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.

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-tcpserver and its -1 option, so that reliable startup notification is achieved. s6-notifywhenup -f s6-tcpserver -1 args will send a 'U' event to ./event when s6-tcpserver is actually listening to its network socket.
The s6-svwait program can be used to wait for 'U' events.