Today I upgraded my office desktop from Fedora 40 to Fedora 41, and as traditional there was a little issue:

Current status: it has been '0' days since a Fedora upgrade caused X font problems, this time because xft apparently no longer accepts 'encoding=...' as a font specification argument/option.

One of the small issues with XFT fonts is that they don't really have canonical names. As covered in the "Font Name" section of fonts.conf, a given XFT font is a composite of a family, a size, and a number of attributes that may be used to narrow down the selection of the XFT font until there's only one option left (or no option left). One way to write that in textual form is, for example, 'Sans:Condensed Bold:size=13'.

For a long time, one of the 'name=value' properties that XFT font matching accepted was 'encoding=<something>'. For example, you might say 'encoding=iso10646-1' to specify 'Unicode' (and back in the long ago days, this apparently could make a difference for font rendering). Although I can't find 'encoding=' documented in historical fonts.conf stuff, I appear to have used it for more than a decade, dating back to when I first converted my fvwm configuration from XLFD fonts to XFT fonts. It's still accepted today on Fedora 40 (although I suspect it does nothing):

: f40 ; fc-match 'Sans:Condensed Bold:size=13:encoding=iso10646-1'
DejaVuSans.ttf: "DejaVu Sans" "Regular"

However, it's no longer accepted on Fedora 41:

: f41 ; fc-match 'Sans:Condensed Bold:size=13:encoding=iso10646-1'
Unable to parse the pattern

Initially I thought this had to be a change in fontconfig, but that doesn't seem to be the case; both Fedora 40 and Fedora 41 use the same version, '2.15.0', just with different build numbers (partly because of a mass rebuild for Fedora 41). Freetype itself went from version 2.13.2 to 2.13.3, but the release notes don't seem to have anything relevant. So I'm at a loss. At least it was easy to fix once I knew what had happened; I just had to take the ':encoding=iso10646-1' bit out from the places I had it.

(The visual manifestation was that all of my fvwm menus and window title bars switched to a tiny font. For historical reasons all of my XFT font specifications in my fvwm configuration file used 'encoding=...', so in Fedora 41 none of them worked and fvwm reported 'can't load font <whatever>' and fell back to its default of an XLFD font, which was tiny on my HiDPI display.)

PS: I suspect that this change will be coming in other Linux distributions sooner or later. Unsurprisingly, Ubuntu 24.04's fc-match still accepts 'encoding=...'.

PPS: Based on ltrace output, FcNameParse() appears to be what fails on Fedora 41.