[systemd-devel] How to start a service when a device is created under /dev/pts/

[Mantas Mikulėnas]

On Mon, Feb 13, 2023 at 5:16 PM Giuseppe Sacco <giuseppe at sguazz.it> wrote:

> Hello Mantas,
>
> Il giorno lun, 13/02/2023 alle 14.32 +0100, Giuseppe Sacco ha scritto:
> > Il giorno lun, 13/02/2023 alle 15.25 +0200, Mantas Mikulėnas ha scritto:
> > > On Mon, Feb 13, 2023 at 3:21 PM Giuseppe Sacco <giuseppe at sguazz.it>
> > > wrote:
> > > > [...]
> > > > In this scenario, the ptys are created by software like iaxmodem
> > > > (that uses
> > > > asterisk) when its daemon starts. Some more information is available
> > > > at
> > > > Debian bug 1031200 (http://bugs.debian.org/1031200) and a suggested
> > > > solution is to change the unit and depend on the pty daemon service
> > > > and
> > > > on
> > > > the a file system link in /dev that points to the real device in
> > > > /dev/pty/
> > > > (the link being created by the pty service at startup time).
> > > >
> > > > Something like:
> > > >
> > > > [Unit]
> > > > Description=HylaFAX faxgetty %I
> > > > Wants=iaxmodem.service dev-%i.device
> > >
> > > I can't see how that would work. Udev doesn't issue events for symlinks
> > > –
> > > a symlink to something that's not a "device" is still not a device.
> >
> > Yes, that's my point of view too. That's way I am asking here.
> >
> > > Would it be enough to order faxgetty After=iaxmodem.service, i.e. wait
> > > until it has created *all* of the ptys that it was set up to create?
> > > Are
> > > they preconfigured or do they show up dynamically?
> >
> > I don't know. I am going to check.
>
> The link in /dev is created by the iaxmodem service when it starts, and
> deleted when it stops. I think you may configure more than one line in
> iaxmodem, so you will have many links, like /dev/ttyIAX0 and /dev/ttyIAX1.
>
> BTW, this seems to be working: the dev-%i.device listed in Wants, even if
> it only refers to a link, seems to be working.
>

If you mean the service starts, then it's only working as in "it does
nothing at all" – Wants= does not imply After=, and a plain Wants= without
After= just becomes a no-op if it refers to an nonexistent unit. It might
as well be "Wants=dev-foobar.device".

Wants= *with* After= would wait for the .device to appear even if it
doesn't exist (I think that's special-cased for .device and .mount
dependencies), but if you tried to use a symlink to a non-device you'd
likely get a "Timed out waiting for dev-ttyFOO.device".

-- 
Mantas Mikulėnas
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