Re: [systemd] Behavior of "Requires" and "After" in service unit

[Mantas Mikulėnas <grawity@xxxxxxxxx>]

On Mon, May 9, 2022 at 7:17 AM Hamza, Muhammad <Muhammad_Hamza@xxxxxxxxxx> wrote:

Hello,

 

I am trying to use swupdate services to perform updates on my BSP but there is a problem that I am facing.

When update usb is plugged in during the boot time, swupdate core and swupdate-usb service run right after

each-other where swupdate-usb service is dependent on swupdate-core service. Therefore, the update fails as

core has not initialized yet when swupdate-usb launches the client.

I have tried adding service type as “exec” in swupdate core service and have tried both Requires and After in swupdate-usb

service but in all case swupdate-usb does not wait for core to be initialized completely before launching.

My understanding is that when service type is set to exec service manager will only assume that swupdate core service is

launched once main process of core is executed and with “Requires” set in swupdate-usb service it should wait till

swupdate core service has executed its main process and is assumed launched. 

Even with Type=exec, systemd won't automatically know when your process has "initialized completely". Type=exec only means that the kernel call to start executing the process has been fired off – systemd has no automatic visibility beyond that, since the initialization stage varies greatly from one program to another.

(In fact, looking at your example, systemd thinks that *the shell script* is the main process, it doesn't find out about the actual daemon until later. Please get rid of the .sh script and change your ExecStart= to directly launch the daemon binary.)

What you need is either Type=notify or Type=forking, and the process needs to explicitly notify systemd that it has reached initialization – for Type=notify it must send a READY=1 message over a socket (see sd_notify), and for Type=forking it needs to "daemonize" at the correct point.

I checked the source code of swupdate on GitHub, and it seems it already has support for sending READY – there's even a whole section about systemd integration in its docs. So in short, you need to compile it with CONFIG_SYSTEMD, then you'll be able to use Type=notify in the .service unit. Example taken from the swupdate docs:

[Service]

Type=notify

ExecStart=/usr/bin/swupdate -u '-t default -u http://localhost -i 25'

(The "socket activation" part isn't required, but may help as well.)

Finally, Type=oneshot is for services where systemd waits for the main process to start *and finish* (e.g. for services that perform a single update and don't have a daemon at all). It sounds a bit like it would be the right choice for swupdate-usb, but it's never correct to use oneshot for daemons like swupdate-core.

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Mantas Mikulėnas