/etc/X11/Xsession.d/35x11-common_xhost-local is in x11-common 1:7.7+19ubuntu7.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | # This file is sourced by Xsession(5), not executed.
# If xhost (from x11-xserver-utils) is installed, use it to give access
# to the X server to any process from the same user on the local host.
# Unlike other uses of xhost, this is safe since the kernel can check
# the actual owner of the calling process.
if type xhost >/dev/null 2>&1; then
xhost +si:localuser:$(id -un) || :
fi
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