Comment 6 for bug 702934

I've just encountered this exact issue, which is impressive four years later.

First, details: Debian stretch (currently testing), hplip 3.16.11; the device is HP OfficeJet 6950, handled for faxing as "HP Fax4 hpcups".

It was harder to debug, because the relevant error messages were no longer in the debug output of hp-sendfax; the only place I've seen them was the cups error log, where it said:

D [05/Jan/2017:23:50:52 +0200] [Job 105] Traceback (most recent call last):
D [05/Jan/2017:23:50:52 +0200] [Job 105] File \"/usr/lib/cups/backend/hpfax\", line 263, in <module>
D [05/Jan/2017:23:50:52 +0200] [Job 105] os.mkfifo(pipe_name)
D [05/Jan/2017:23:50:52 +0200] [Job 105] PermissionError: [Errno 13] Permission denied
D [05/Jan/2017:23:50:52 +0200] [Job 105] During handling of the above exception, another exception occurred:
D [05/Jan/2017:23:50:52 +0200] [Job 105] Traceback (most recent call last):
D [05/Jan/2017:23:50:52 +0200] [Job 105] File \"/usr/lib/cups/backend/hpfax\", line 265, in <module>
D [05/Jan/2017:23:50:52 +0200] [Job 105] os.unlink(pipe_name)
D [05/Jan/2017:23:50:52 +0200] [Job 105] FileNotFoundError: [Errno 2] No such file or directory: \'/home/shai/.hplip/hp_fax-pipe-105\'

So, the pipes are now in ~/.hplip/, and the code just assumes that if the folder exists, it can write to it. But it seems like some of the relevant dbus processes run under user "lp", which, by default, has no such permissions.

So, I was able to get hp-sendfax to work by "sudo chgrp lp ~/.hplip; chmod g+w ~/.hplip". I'm not sure if hpfax was the actual process running into the problem -- I attempted to add debug printings into it, and couldn't reproduce the logging. But changing the permissions to what I suspected would be necessary did solve things.