Python Xlib applications report 'Can't connect to display ":0": No protocol specified' if XAUTH authentication is needed

[Dan Astoorian]

The python-xlib package does not appear to correctly use X11 cookies from $XAUTHORITY under Ubuntu 18.04 (and possibly others).

GNOME3 appears to store the cookies in /run/user/$UID/gdm/Xauthority (and set $XAUTHORITY to this value for the user), but gdm does not appear to include a display number in the authorization entry; e.g., "xauth list" shows "HOSTNAME/unix:" instead of "HOSTNAME/unix:0" for the entry. python-xlib is apparently unable to locate and use the cookie without the display number being present.

This deficiency is typically masked by the presence of "si:localuser:" entries in the xhost table; however, it is sometimes necessary to use XAUTH rather than xhost (e.g., when using certain Tcl/Tk applications which insist on the xhost table being empty for secure communication through the X server).

To reproduce:

  xhost -si:localuser:$USER
  python -c 'from Xlib.display import Display; Display()'

This results in:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/djast/.local/lib/python2.7/site-packages/Xlib/display.py", line 89, in __init__
    self.display = _BaseDisplay(display)
  File "/home/djast/.local/lib/python2.7/site-packages/Xlib/display.py", line 71, in __init__
    protocol_display.Display.__init__(self, *args, **keys)
  File "/home/djast/.local/lib/python2.7/site-packages/Xlib/protocol/display.py", line 166, in __init__
    raise error.DisplayConnectionError(self.display_name, r.reason)
Xlib.error.DisplayConnectionError: Can't connect to display ":0": No protocol specified

If the xauth entry is updated to include ":0", e.g., via

    xauth add $DISPLAY $(xauth list $DISPLAY | cut -d: -f2- | tail -1)

then running "python -c 'from Xlib.display import Display; Display()'" succeeds.

python3-xlib appears to have the same deficiency.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: python-xlib 0.20-3
ProcVersionSignature: Ubuntu 4.15.0-108.109-generic 4.15.18
Uname: Linux 4.15.0-108-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Fri Jun 26 11:28:32 2020
Dependencies: python-six 1.11.0-2
InstallationDate: Installed on 2018-07-31 (695 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
SourcePackage: python-xlib
UpgradeStatus: No upgrade log present (probably fresh install)

[Dan Astoorian (djast-ecf)]

Please note that although the Python traceback I posted cited a user-installed (via pip) version of python-xlib, the same symptom does also occur with the Ubuntu-packaged version in /usr/lib/python2.7/dist-packages/Xlib/.

It is possible that the actual bug is GDM failing to correctly populate the display number in the $XAUTHORITY file /run/user/$UID/gdm/Xauthority; this bug report presumes (without justification) that an empty display number in the Xauthority file has a valid interpretation, but that python-xlib is not applying it. I note that other X applications (e.g., xclock) still work with "si:localuser:$USER" removed from the xhost list; I have only observed the failing behaviour with python-xlib and python3-xlib.

[Dan Astoorian (djast-ecf)]

Changing

            if efam == family and eaddr == address and num == enum:

to

            if efam == family and eaddr == address and (num == enum or enum == ""):

in get_best_auth() in Xlib/xauth.py appears to be an effective fix/workaround; this allows an Xauthority entry with a missing display number to match any display.

Alternatively, and slightly more conservatively, adding:

            if enum == "" and ename not in matches:
                enum = num

before the original line will match the entry only if a display-specific entry is not also present in the Xauthority file.