texttopdf embeds a font without checking it covers the text, printing every glyph as .notdef

Symptom

With powerline-fonts installed, printing any plain text file produces a page where every character is a hollow box. Every application is affected, since they all go through the same filter.

Cause

cfFilterTextToPDF resolves monospace through fontconfig once for the whole document and embeds whatever comes back, without checking whether that font has glyphs for the text being rendered.

Powerline ships a fontconfig snippet that makes its symbol font the preferred monospace:

<alias><family>monospace</family><prefer><family>PowerlineSymbols</family></prefer></alias>

PowerlineSymbols contains only the powerline glyphs — no letters, no digits:

$ fc-query --format="%{charset}" /usr/share/fonts/OTF/PowerlineSymbols.otf
2588 e0a0-e0a2 e0b0-e0b3

So every character in an ordinary text file falls to .notdef.

Terminals are unaffected because they fall through to another font per glyph. texttopdf resolves once, for the document.

Reproduction

Install powerline-fonts (or any package aliasing monospace to a symbol-only font), then:

CUPS_DATADIR=/usr/share/cups CHARSET=utf-8 \
  /usr/lib/cups/filter/texttopdf 1 u t 1 "" < any.txt | pdffonts -

pdffonts shows PowerlineSymbols-Identity-H embedded, and the output prints as boxes.

Suggested fix

Check glyph coverage for the text before committing to a font, and fall back when the resolved font cannot render it — fontconfig can answer this via the font's charset. Anything that avoids embedding a font guaranteed to produce .notdef for the entire document would do.

Environment

cups 2.4.19, libcupsfilters from cups-filters 2.0.1, fontconfig 2.17, Arch/CachyOS.

I have reported the packaging side separately, since shipping that alias enabled by default is arguably also wrong — but a filter that embeds a font which cannot render the document seems worth fixing regardless of what any given package does.

Start at cfFilterTextToPDF, the shared texttopdf filter described in the report, and reproduce the issue with the provided CUPS_DATADIR, CHARSET, and pdffonts command. Trace how monospace is resolved and embedded for the document, then verify that symbol-only fonts do not produce an all-.notdef PDF and that ordinary text still renders correctly.