Best practise for creating sockets without a corresponding service

[Simon Mullis <simon@xxxxxxxxxxxx>]

Hello systemd fans.

I'm creating a pipeline of services and would like systemd to manage
all aspects possible.

The first service in the chain creates an arbitrary number of FIFO
outputs (as a demultiplexer), which I use as inputs to the rest of the
chain. These outputs are internal to the service itself and cannot be
managed by systemd.

Step 0
- service_data_gen => creates N outputs

Step 1
- service1@.service => N instances are created but don't actually need
to do anything.
- service1@.socket => N sockets are created which are the target FIFOs
for the output of - service_data_gen above.

Step P (where P is 2..whatever)
- serviceP@.service and serviceP@.socket => Processed data flows via
STDIN and STDOUT (using StandardInput=fd:socket_name and
StandardOutput=fd:socket_name and also
FileDescriptorName=serviceP.%i.socket for example) all managed by
systemd. Here there are indeed additional services which are launched
and further process the data and it flows through each link. All works
well here. Lovely...

So, as there are a variable number of inputs to the first link in the
chain, I use templates for services and sockets for the subsequent
links in the chain. In this case, the variability is based on the
number of CPU cores so I use a oneshot service to start them all with
the following fragment in the ExecStart script:

cpu_cores=$(nproc --all)
eval systemctl start step1@{1..${cpu_cores}}.service

It's all working really nicely, with STDIN and STDOUT flowing from
start to finish and dependencies and socket creation all good for the
rest of the chain. The only outstanding issue is the second service
instance templates for socket and services.

As far as I understand:
- Socket template instances need a corresponding service instance template.
- Services need an ExecStart= or ExecStop= to be valid.

I don't need any service to actually run in step1, I would like
systemd to manage the sockets and the dependencies (as it is for the
rest of the chain).

Now to the question:

What is the best practise for an ExecStart= entry to act as a dummy,
where no service is actually required?  At the moment I am using:

ExecStart=/usr/bin/sh -c "sleep infininty" in the service template for
service1@.service

But this does not feel like the right approach.

I think the crux of this is entirely related to the use of instance
templates and linking one unconnected single parent service to many
child services (and sockets).

Thank you very much in advance for any insight. Is there a
systemd-users list I should use instead?