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.
