policykit failures due to internal user id mismatch

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Status changed to 'Confirmed' because the bug affects multiple users.

See the same error with two admin accounts.
synaptic-pkexec brings up the admin confirmation dialog but fails to authorize.
"Error executing command as another user: Not authorized"

Removed second user account with admin privileges and synaptic started.

There's a SIGSEGV; gdb bt full attached.

#0 0x00007f1d14271201 in g_type_check_instance_is_fundamentally_a
    (type_instance=type_instance@entry=0x55b74c6fd010, fundamental_type=fundamental_type@entry=0x50 [GObject]) at ../../../gobject/gtype.c:4025
        node = <optimized out>
#1 0x00007f1d14251399 in g_object_unref (_object=0x55b74c6fd010) at ../../../gobject/gobject.c:3243
        _g_boolean_var_ = <optimized out>
        object = 0x55b74c6fd010
        __FUNCTION__ = "g_object_unref"
#2 0x00007f1d15b5ab9c in PolkitQt1::Agent::Session::Private::completed(_PolkitAgentSession*, int, void*) ()
    at /lib/x86_64-linux-gnu/libpolkit-qt5-agent-1.so.1
#6 0x00007f1d142699f6 in <emit signal ??? on instance 0x55b74c84d760 [PolkitAgentSession]> (instance=0x55b74c84d760, detailed_signal=<optimized out>)
    at ../../../gobject/gsignal.c:3487

Looks like there may be two problems but I'm not familiar enough with the code bases to figure it out.

1) should a user already a member of "sudo" group even be prompted to select the user they wish to authenticate as?

2) when they do something goes wrong and there's a SIGSEGV which looks like it may be caused in libpolkit-qt5

I've found that there appears to be a fundamental bug in policykit and have added the upstream bug report covering it, which has been around for several years but the developers don't seem able to come up with a fix for.

Tests done using the Lubuntu 19.10 Live ISO (June 24th 2019 build) where an additional user "test" is created with the same supplementary group memberships as the "lubuntu" user. Both have passwords set so that these tests can be done over SSH as well as at the console, or in the GUI.

The fact that "pkexec" itself is broken proves that lxqt nor the qt-policykit code is at fault here.

test@lubuntu:~$ groups
test adm cdrom sudo dip plugdev lpadmin

$ sudo ls -l /root/
[sudo] password for test:
total 0
-rw-r--r-- 1 root root 0 Jun 26 23:35 file

test@lubuntu:~$ pkexec ls -l /root/

==== AUTHENTICATING FOR org.freedesktop.policykit.exec ===
Authentication is needed to run `/usr/bin/ls' as the super user
Multiple identities can be used for authentication:
 1. Lubuntu2,,, (lubuntu)
 2. Test3,,, (test)
Choose identity to authenticate as (1-2): 2
Password:
polkit-agent-helper-1: error response to PolicyKit daemon: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: No session for cookie

lubuntu@lubuntu:~$ groups
lubuntu adm cdrom sudo dip plugdev lpadmin sambashare

lubuntu@lubuntu:~$ sudo ls -l /root/
total 0
-rw-r--r-- 1 root root 0 Jun 26 23:35 grep

lubuntu@lubuntu:~$ pkexec ls -l /root/
Error executing command as another user: Not authorized

This incident has been reported.

Using gparted (which requires root access to the block storage devices) "test" user fails whilst original "lubuntu" user is successful:

# In "lubuntu" user's GUI terminal emulator:

lubuntu@lubuntu:~$ gparted
localuser:root being added to access control list
======================
libparted : 3.2
======================
localuser:root being removed from access control list

# In "test" user's GUI terminal emulator:

test@lubuntu:~$ gparted
localuser:root being added to access control list
Error executing command as another user: Not authorized

This incident has been reported.
localuser:root being removed from access control list

Possibly related to Bug #1821415 "pkexec fails in a non-graphical environment"

Status changed to 'Confirmed' because the bug affects multiple users.