Due to udev changes SANE stopped working - verified by downgrading to udev from stable branch

[Dirk Ritter]

Hello!

It drove me almost insane to try and debug SANE over
the Weekend, but today I downgraded from udev / libudev1
221-1 to 215-17+deb8u1 and suddenly my flatbed scanner
started to work again. No other changes whatsoever,
using current SANE from testing, i.e. 1.0.24-13. The
sole SANE backend used is "snapscan", driving the
"EPSON Perfection 3490 Photo" which has (vendor=0x04b8
[EPSON], product=0x0122 [EPSON Scanner]) at libusb:008:004
according to sane-find-scanner. With current udev, this
tool still finds the device (it merely scans the USB bus
and reports everything advertising to be a scanner!),
but SANE completely fails to recognize it, i.e.
"scanimage -L" yields no results whatsoever for udev from
testing, but works just fine with udev from stable.

The scanner needs a firmware file to be uploaded
which is handled by the SANE snapscan backend, but
merely downgrading udev suggests it may be a
permission issue or whatever else on the device end.
Downside is that it is pretty hard to tell where down
the road exactly it stopped working since I don't use
it often enough. The other downside ist that right now
I have no way to tell how it fails. I would need to
pass debugging options to the saned daemon, but the
older rc.d thing does not seem to do the trick any
longer and there were no clues from the daemon in any
of the log files so far. Question is if saned would
treat the absence of any scanner as an error at all,
so even passing debug options to saned may not get us
any further.

Hope this information is still of use to you. At least
I would be happy to do further debugging and testing
if time permits and provided you can tell me what
would be of help to you and what to look for. I can
at least switch back and forth between stable and testing
without assistance if need be, i.e. in order to check
certain permissions or whatever else might go wrong.

Kind Regards,
Dirk Ritter