upgrading Cantarell causes visual glitches in GNOME Shell until restarted

[Frédéric Meynadier]

Upgrading fonts-cantarell may cause visual glitches in GNOME Shell. You need to restart your computer after upgrading this font.

Original Bug Report
===================

Hello,

On Ubuntu 16.04 LTS, with unattended upgrades, using gnome3 as desktop environment, I have just witnessed the following problem :

- The upgrade to the latest release occurred a few minutes ago :

$ tail /var/log/apt/history.log
[...]
Start-Date: 2016-07-08 11:10:07
Commandline: /usr/bin/unattended-upgrade
Upgrade: fonts-cantarell:amd64 (0.0.21-1, 0.0.24-1~ubuntu16.04.0), lsb-release:amd64 (9.20160110ubuntu0.1, 9.20160110ubuntu0.2), lsb-base:amd64 (9.20160110ubuntu0.1, 9.20160110ubuntu0.2), tzdata:amd64 (2016d-0ubuntu0.16.04, 2016f-0ubuntu0.16.04)
End-Date: 2016-07-08 11:10:15

- Instantly, all characters in the top bar (open windows, date, dropdown menus) have been replaced by small solid rectangles (sorry, did not think of capturing a screenshot). If important : language is french (locale : fr_FR.UTF-8)

- Logging out from my gnome3 session, and then logging back in fixes the problem.

I would have expected the font package upgrade to go more smoothly (possibly refresh the gnome3 interface at the end of the process ?)

Thanks for your efforts,

[Jeremy Bícha (jbicha)]

While I did not experience this bug, another user did:
comment #6 on bug 1592833

The fonts are not updated very often in stable Ubuntu releases so maybe you won't experience this bug again in Ubuntu 16.04 LTS.

[Frédéric Meynadier (frederic-meynadier-b)]

Ok, thanks. I also think this will probably not occur very often, but I just thought it would be worth documenting to help polishing the upgrade process...

[Jeremy Bícha (jbicha)]

I experienced this when I upgraded an Ubuntu GNOME 16.04 install. :(

[Nikolaus Waxweiler (madleser)]

You must restart all applications actively using a font if you upgrade it in-place. This has to do with the glyph cache (FreeType's?) getting messed up IIRC.