From the beginning of term.log:
Use of uninitialized value $template in exists at /usr/share/perl5/Debconf/Template.pm line 81, <GEN2> chunk 1.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN2> chunk 1.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN2> chunk 1.
Use of uninitialized value $template in exists at /usr/share/perl5/Debconf/Template.pm line 81, <GEN2> chunk 2.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN2> chunk 2.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN2> chunk 2.
Use of uninitialized value $template in exists at /usr/share/perl5/Debconf/Template.pm line 81, <GEN2> chunk 3.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN2> chunk 3.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN2> chunk 3.
Use of uninitialized value $template in exists at /usr/share/perl5/Debconf/Template.pm line 81, <GEN2> chunk 4.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN2> chunk 4.
Then further in the log
debconf: Unable to initialise frontend: Gnome
debconf: (Unable to load Gtk -- is libgtk2-perl installed?)
debconf: falling back to frontend: Dialog
Use of uninitialized value $template in exists at /usr/share/perl5/Debconf/Template.pm line 81, <GEN0> chunk 1.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN0> chunk 1.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN0> chunk 1.
Use of uninitialized value $template in exists at /usr/share/perl5/Debconf/Template.pm line 81, <GEN0> chunk 2.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN0> chunk 2.
Use of uninitialized value $item in exists at /usr/share/perl5/Debconf/DbDriver/Cache.pm line 39, <GEN0> chunk 2.
As a consequent, the dialog asking to restart services on libc6 upgrade didn't show up and the question was invisible with the Dialog debconf frontend. From a user perspective the upgrade hanged.
From the beginning of term.log: perl5/Debconf/ Template. pm line 81, <GEN2> chunk 1. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN2> chunk 1. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN2> chunk 1. perl5/Debconf/ Template. pm line 81, <GEN2> chunk 2. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN2> chunk 2. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN2> chunk 2. perl5/Debconf/ Template. pm line 81, <GEN2> chunk 3. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN2> chunk 3. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN2> chunk 3. perl5/Debconf/ Template. pm line 81, <GEN2> chunk 4. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN2> chunk 4.
Use of uninitialized value $template in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Use of uninitialized value $template in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Use of uninitialized value $template in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Use of uninitialized value $template in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Then further in the log perl5/Debconf/ Template. pm line 81, <GEN0> chunk 1. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN0> chunk 1. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN0> chunk 1. perl5/Debconf/ Template. pm line 81, <GEN0> chunk 2. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN0> chunk 2. perl5/Debconf/ DbDriver/ Cache.pm line 39, <GEN0> chunk 2.
debconf: Unable to initialise frontend: Gnome
debconf: (Unable to load Gtk -- is libgtk2-perl installed?)
debconf: falling back to frontend: Dialog
Use of uninitialized value $template in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Use of uninitialized value $template in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
Use of uninitialized value $item in exists at /usr/share/
As a consequent, the dialog asking to restart services on libc6 upgrade didn't show up and the question was invisible with the Dialog debconf frontend. From a user perspective the upgrade hanged.