Procd equivalent behaviour of systemd's restart=on-failure

[JackHack96]

Hi everyone!
I'm currently developing a custom image on a Mediatek Ralink MIPS MT76x8 (with LinkIt Smart 7688 target profile).
The system hosts two services, one enabled by default and the other disabled by default (let's call them A and B respectively). A is started at every boot and do some checks. If a certain condition is met, then A launches B and just exits, otherwise it does other things and reboot the system, repeating the loop.

Previously, this setup was tested with the Yocto framework using systemd, and the two services were the following.
A:

[Unit]
Description=Process A
Conflicts=B.service

[Service]
Type=simple
ExecStart=/usr/bin/process-a.sh
Restart=on-failure
RestartSec=2min

[Install]
WantedBy=multi-user.target

B:

[Unit]
Description=Process B
After=network.target

[Service]
Type=simple
ExecStart=/usr/bin/process-b <arguments>
Restart=always

[Install]
WantedBy=multi-user.target

process-a.sh checks whether the aformentioned conditions are met, and if so it has a systemctl start B.service.

Now I'm trying to replicate this behaviour in OpenWRT with procd, but it seems that there's no option for respawing a program based on its exit code. The wiki (https://openwrt.org/docs/guide-developer/procd-init-scripts?s=procd_set_param%20respawn) shows only time based options. Here are the two units I've wrote for A and B:

#!/bin/sh /etc/rc.common

USE_PROCD=1

START=95
STOP=01

PROG=/usr/bin/newborn.sh
ARGS=production

start_service() {
    procd_open_instance
    procd_set_param command $PROG $ARGS
 
    # Send stdout/stderr output to the system log
    # Use command logread to see the log
    procd_set_param stderr 1
    procd_set_param stdout 1
    
    # Set respawn
    procd_set_param respawn ${respawn_threshold:-0} ${respawn_timeout:-120} ${respawn_retry:-0}

    procd_close_instance
}

#!/bin/sh /etc/rc.common

USE_PROCD=1

START=99
STOP=01

PROG=/usr/bin/modbus-gateway
ARGS="/etc/secrets/gateway.json /etc/secrets/cert.crt /etc/secrets/private_key.pem /dev/ttyS0"

start_service() {
    procd_open_instance
    procd_set_param command $PROG $ARGS
 
    # Send stdout/stderr output to the system log
    # Use command logread to see the log
    procd_set_param stderr 1
    procd_set_param stdout 1
    
    # Set respawn
    procd_set_param respawn ${respawn_threshold:-0} ${respawn_timeout:-10} ${respawn_retry:-0}

    procd_close_instance
}

The problem is that even if the program exits gracefully with exit 0, the procd daemon tries to restart the process. I thought of making the A script stop itself after launching B, but I would like to know if there's a more elegant solution.

There's also the problem that B is configured to be launched automatically when generating the image from source, is it possible to package a startup script to be disabled by default?

Thank you
Matteo

[JackHack96]

I've partially solved this problem by just making A top stop itself after it launches B, then I used UCI defaults to make B disabled by default in the image.