CUPS backends hp and hpfax cannot run at the same time

[Johannes Meixner]

Running HPLIP 3.11.3 on openSUSE 11.4 x86_64 hardware
with CUPS 1.4.6 (the same happens with HPLIP 3.11.1).

Summary:

Running hp and hpfax the same time e.g. via
# /usr/lib/cups/backend/hpfax & /usr/lib/cups/backend/hp
or
# /usr/lib/cups/backend/hp & /usr/lib/cups/backend/hpfax
hangs both up - usually both with no output at all.

One may even have to kill one of both to get it unlocked.

Running them one after the other works:
------------------------------------------------------------
# /usr/lib/cups/backend/hp ; /usr/lib/cups/backend/hpfax
direct hp:/usb/HP_LaserJet_1020?serial=JL50HRE "HP LaserJet 1020" ...
direct hp:/usb/HP_LaserJet_1220?serial=00XXXXXXXXXX "HP LaserJet 1220" ...
direct hp:/par/DESKJET_950C?device=/dev/parport0 "HP DESKJET 950C" ...
direct hpfax "Unknown" "HP Fax (HPLIP)" ""

# /usr/lib/cups/backend/hpfax ; /usr/lib/cups/backend/hp
direct hpfax "Unknown" "HP Fax (HPLIP)" ""
direct hp:/usb/HP_LaserJet_1020?serial=JL50HRE "HP LaserJet 1020"...
direct hp:/usb/HP_LaserJet_1220?serial=00XXXXXXXXXX "HP LaserJet 1220" ...
direct hp:/par/DESKJET_950C?device=/dev/parport0 "HP DESKJET 950C" ...
------------------------------------------------------------

Consequences:

# lpinfo -v
does not show the "hp:/..." connections
and therefore printer setup tools like YaST
also do not show the "hp:/..." connections.

I also had strange hang-ups with "hp-setup"
which could be related to this issue.

The reason is that "lpinfo -v" (or matching CUPS library
calls to detect devices) treigger that the cupsd launces the
cups-deviced which runs all backends in parallel,
see /var/log/cups/error_log after "lpinfo -v" was run:
--------------------------------------------------------------------
I [23/Mar/2011:16:04:52 +0100] Started "/usr/lib/cups/daemon/cups-deviced" (pid=11788)
D [23/Mar/2011:16:04:52 +0100] cupsdSendCommand: 12 file=10
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/http (PID 11789)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/beh (PID 11790)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/hp (PID 11791)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/parallel (PID 11792)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/ipp (PID 11793)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/lpd (PID 11794)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/smb (PID 11795)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/scsi (PID 11797)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/serial (PID 11798)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/snmp (PID 11799)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/socket (PID 11800)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/usb (PID 11801)
D [23/Mar/2011:16:04:52 +0100] [cups-deviced] Started backend /usr/lib/cups/backend/hpfax (PID 11803)
--------------------------------------------------------------------
so that in particular hp and hpfax run at the same time
but because both block each other the cups-deviced
gets no output from them and after 15 seconds of
timeout the cups-deviced gives up and in the end
no "hp:/..." connections are detected.

[Johannes Meixner (jsmeix)]

The easiest workaround for me is

# chmod a-x /usr/lib/cups/backend/hpfax

to disable the hpfax backend
(I have no HP device with fax).