incompatible host fonts cache causing snap app instability

Bug #1877109 reported by Maciej Borzecki on 2020-05-06
This bug affects 11 people
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Bug Description

Originally reported and investigated in the snapcraft forum here:

The problem appears to be caused by making the fonts cache from the host accessible inside the snap mount namespace. The reason we are doing this is to avoid rebuilding the cache on the first run of an application. However, this only works then the binary fonts cache generated on the host is compatible with the fontconfig inside the snap.

As we later discovered, despite advertising a compatible (cache-7) format, the newer version of fontconfig available in Arch and Fedora 32, generate cache files that are not properly handled by fontconfig from gnome-3-28-1804 snap (or basically 20.04 and earlier).

When the incompatible cache is present, we have seen the fonts being rendered as boxes (fontconfig could not identify glyphs?), or plain segfaults with backtrace going to font related code (eg. zoom-client, glimpse-editor).

Deeper investigation led to identifying a commit that introduced breakage: Even if this is fixed upstream (or the older versions are patched to handle the new format), there is no guarantee when the updates appear in affected distro or whether the snaps will be rebuilt.

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