Why does my Erlang boot script work from console but does not work when run from init system (sysvinit, upstart, systemd)?

Question

I have an Erlang boot script that I can launch like this:

/usr/bin/erl -boot /path/to/my-boot-script

It works when run from console, but fails without any error messages when I run it from systemd. The systemd unit file looks like this:

[Unit]
Description=My daemon written in Erlang

[Service]
Type=simple
ExecStart=/usr/bin/erl -boot /path/to/my-boot-script
Restart=always

[Install]
WantedBy=multi-user.target

The log shows that the system boots properly and then terminates abruptly without any kind of error message. What the hell is going on?

Answer 1

Turns out that you have to pass -noinput parameter to erl. Otherwise it will try to open stdin for reading, fail because there's nothing there and terminate without any kind of error message.

This works:

[Unit]
Description=My daemon written in Erlang

[Service]
Type=simple
ExecStart=/usr/bin/erl -noinput -boot /path/to/my-boot-script
Restart=always

[Install]
WantedBy=multi-user.target

Answer 2

I have just managed to use the erlang install system with systemd to install a code repository. This procedure is compliant with the erlang documenation about setting up embedded systems. Here is what I learnt:

1) Need to add a path to get to the install tools in erts/examples. Here is an excerpt from my .erlang file for this:

code:add_path(filename:join([os:getenv("ROOTDIR"),"lib/sasl-2.6/examples/ebin"])).

2) do the build as in erlang/doc/system_principles/create_target.html

target_system:create("mysystem").

3) extract the build into a temporary location. For my application it done from erlang with:

target_system:install("code_repository", "/home/tony/Projects/code_repository/release_test").

4) edit the last line of start where it starts run erl and remove the -daemon option and add any erlang runtime parameters required like -sname or -set_cookie. This is outlined in erlang/doc/embedded/embedded_solaris.html#idm45326372314928. Here is the edited line from my system:

$ROOTDIR/bin/run_erl  /tmp/ $ROOTDIR/log "exec $ROOTDIR/bin/start_erl $ROOTDIR $RELDIR $START_ERL_DATA -sname code_repository" 

5) Install to runtime location (don't forget about sys.config)
6) Set up .services file as follows. This file is stored in /etc/systemd/system/code_repository.service:

[Unit]
Description=Tonys Code Repository Daemon

[Service]
Type=simple
WorkingDirectory=/var/opt/code_repository
Environment=HOME=/var/opt/code_repository
ExecStart=/var/opt/code_repository/bin/start
Restart=yes

[Install]
WantedBy=multi-user.target

The removal of the -daemon flag is essential to operate as simple service, as simple services run and do not return unless they fail. The use of run_erl as in this procedure allows erlang tools to attach to the daemon for maintenance..