Upgrade from bionic to focal broke for this system which has chrony installed.

After do-release-upgrade exited (warning that the system was in an indeterminate state) I was unable to fix the system initially. It seemed that systemd-timesyncd was somehow interacting badly with chrony:

$ sudo apt --fix-broken install 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Correcting dependencies... Done
The following packages were automatically installed and are no longer required:
  libboost-filesystem1.65.1 libboost-iostreams1.65.1 libboost-system1.65.1 libcwidget3v5 libip4tc2 libnspr4 libnss3 python3.6 python3.6-minimal
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
  libboost-iostreams1.71.0 libcwidget4 libxapian30
Suggested packages:
  libcwidget-dev xapian-tools
The following packages will be REMOVED:
  chrony
The following NEW packages will be installed:
  libboost-iostreams1.71.0 libcwidget4
The following packages will be upgraded:
  libxapian30
1 upgraded, 2 newly installed, 1 to remove and 439 not upgraded.
2 not fully installed or removed.
Need to get 0 B/1204 kB of archives.
After this operation, 3022 kB of additional disk space will be used.
Do you want to continue? [Y/n] 
(Reading database ... 171920 files and directories currently installed.)
Removing chrony (3.2-4ubuntu4.2) ...
Job for systemd-timesyncd.service failed because the control process exited with error code.
See "systemctl status systemd-timesyncd.service" and "journalctl -xe" for details.
dpkg: error processing package chrony (--remove):
 installed chrony package post-removal script subprocess returned error exit status 1
Errors were encountered while processing:
 chrony
E: Sub-process /usr/bin/dpkg returned an error code (1)

I turned off the systemd-timesync ntp and that then allowed chrony to be removed (not quite what I wanted but that is another story):

mark@bigben:/var/log/chrony$ sudo timedatectl status
                      Local time: Sat 2020-04-11 10:55:31 UTC
                  Universal time: Sat 2020-04-11 10:55:31 UTC
                        RTC time: Sat 2020-04-11 10:55:31
                       Time zone: Etc/UTC (UTC, +0000)
       System clock synchronized: yes
systemd-timesyncd.service active: yes
                 RTC in local TZ: no
mark@bigben:/var/log/chrony$ sudo timedatectl set-ntp false
mark@bigben:/var/log/chrony$ sudo timedatectl status
                      Local time: Sat 2020-04-11 10:55:50 UTC
                  Universal time: Sat 2020-04-11 10:55:50 UTC
                        RTC time: Sat 2020-04-11 10:55:50
                       Time zone: Etc/UTC (UTC, +0000)
       System clock synchronized: yes
systemd-timesyncd.service active: no
                 RTC in local TZ: no

$ sudo apt --fix-broken install 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Correcting dependencies... Done
The following packages were automatically installed and are no longer required:
  libboost-filesystem1.65.1 libboost-iostreams1.65.1 libboost-system1.65.1 libcwidget3v5 libip4tc2 libnspr4 libnss3 python3.6 python3.6-minimal
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
  libboost-iostreams1.71.0 libcwidget4 libxapian30
Suggested packages:
  libcwidget-dev xapian-tools
The following packages will be REMOVED:
  chrony
The following NEW packages will be installed:
  libboost-iostreams1.71.0 libcwidget4
The following packages will be upgraded:
  libxapian30
1 upgraded, 2 newly installed, 1 to remove and 439 not upgraded.
2 not fully installed or removed.
Need to get 0 B/1204 kB of archives.
After this operation, 3022 kB of additional disk space will be used.
Do you want to continue? [Y/n] 
(Reading database ... 171920 files and directories currently installed.)
Removing chrony (3.2-4ubuntu4.2) ...

And then installing chrony again worked.

So it looks like we need to harden the chrony packaging for the world of systemd-timesync in general if we want upgrades to focal with chrony not to have problems.