CTRL-C does not cancel scanning completely

[Johannes Meixner]

See
https://bugzilla.novell.com/show_bug.cgi?id=631294

Using HPLIP 3.10.2 with my HP LaserJet 1220 all-in-one
I let it pull in the paper, scan a bit and then I press CTRL-C:
---------------------------------------------------------------------
user@host:/tmp $ scanimage \
  -d hpaio:/usb/HP_LaserJet_1220?serial=00XXXXXXXXXX >foo.pnm
^Cscanimage: received signal 2
scanimage: trying to stop scanner
---------------------------------------------------------------------
After several seconds, the HP LaserJet 1220 ejects the paper
but scanimage still hangs.
I need to do as root "kill -9 <PID_of_scanimage_process>"
to get it finally terminated.

In contrast for e.g. my Canon CanoScan N1240U/LiDE30
which uses the "plustek" driver in sane-backends it works:
---------------------------------------------------------------------
user@host:/tmp $ scanimage -d plustek:libusb:002:005 >foo.pnm
^Cscanimage: received signal 2
scanimage: trying to stop scanner
scanimage: sane_read: Operation was cancelled
user@host:/tmp $
---------------------------------------------------------------------

A more detailed analysis for the hpaio backend shows
that while it hangs after a CTRL-C, it is actually hanging
in a "futex(...)" system call which is perhaps the result
of a pthread_mutex_lock call in the hpaio driver
or of such a call in whatever lower-level library, see
https://bugzilla.novell.com/show_bug.cgi?id=631294
for details.

I guess that the hpaio backend should release a lock
when it gets a CTRL-C (i.e. SIGINT) or when whatever
termination signal is sent to it.