systemd-udevd: Race condition when rule starts both a systemd-mount and an unit accessing that mount

["Manuel Wagesreither" <ManWag@xxxxxxxxxxx>]

Hello all,

this is my first post on this mailing list and, first of all, I'd like to thank you and appreciate your work on systemd in general. I admire the logic, the completeness of the manpages and in general how beautifully things are engineered. I'm no unix graybeard and systemd saved me from having to learn all that legacy stuff systemd replaces. Compared to fstab, /etc/network/interfaces and init.d, systemd is a piece of art.

---

I'm working on an embedded device which should access and scan connected usb drives for certain files. I seem to witness a race condition with my current solution. I would ask for advice on how to implement this functionality in a better way.

When a device /dev/sdb1 is connected, the udev rule below starts BOTH
* a systemd-service "start-standalone-mender-deployment@media-sdb1.service"
* `systemd-mount --no-block --automount=no --options=ro --collect /dev/sdb1 /media/sdb1`

The service then starts a shell script accessing the usb drive. Occasionally, it says the directory the usb drive is mounted at is empty. When checking manually, I see it's not. I strongly suspect the script accessed the directory before the usb drive got mounted there.

Here's the udev rule:
```
ACTION=="add", SUBSYSTEMS=="usb", SUBSYSTEM=="block", KERNEL=="*[0-9]*", ENV{ID_FS_USAGE}=="filesystem", TAG+="systemd", ENV{SYSTEMD_WANTS}+="start-standalone-mender-deployment@media-$name.service", RUN{program}+="/usr/bin/systemd-mount --no-block --automount=no --options=ro --collect $devnode /media/$name"
```

And here's the systemd service:
It is templated and gets instantiated with "media-sdb1". It therefore has an "After=media-sdb1.mount". I suspect Systemd-udevd executes the ENV{SYSTEMD_WANTS} part before the RUN{program} part. Hence, "media-sdb1.mount" doesn't yet exist when the service gets started, as it gets created a tad later by systemd-mount.

```
[Unit]
Description=Start standalone Mender deployment (%i)
After=%i.mount

[Service]
Type=oneshot
Restart=no
ExecStart=/bin/sh /usr/bin/start-standalone-mender-deployment.sh /%I
```

Can you confirm my theory?

The only alternative I see is to invoke systemd-mount without --no-block from the shell script itself. Instead of communicating the mount point (media-sdb1) via unit template parameter, I would communicate the device path (/dev/sdb1) to the template unit and pass it on to the shell script, which would determine mount point based on that.

I'm all ears if you have comments or advice on that. I guess I'm not the first one implementing something like this.


Regards,
Manuel

P.S.: I won't be able to respond until Sunday Aug 29th.