feat: add disable_system_fonts option to ignore system typefaces#28
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lhoward wants to merge 1 commit into
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feat: add disable_system_fonts option to ignore system typefaces#28lhoward wants to merge 1 commit into
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Adds a new boolean engine property that installs a private, in-memory fontconfig configuration containing only the directories of fonts declared in the bundle's FontManifest.json. With no <dir> entries pointing at /usr/share/fonts (or any other system path), Skia inside the engine sees only bundled fonts and renders missing glyphs as tofu rather than falling back to a system typeface. Useful for embedded targets where the host font set may be unpredictable, sparse, or visually inconsistent with the app's design. The config is materialized via memfd_create() and exposed to the engine through FONTCONFIG_FILE=/proc/self/fd/N, so nothing is written to the on-disk filesystem and the anonymous file is freed by the kernel when the engine fd is closed at shutdown. No new build-time dependencies — fontconfig itself is reached transitively through libflutter_engine.so's existing libfontconfig.so.1 runtime link, and the embedder source never includes a fontconfig header. The option is exposed on FlutterDesktopEngineProperties (defaults to false, so existing callers are source- and behaviorally unchanged) and threaded through DartProject in the C++ wrapper. Fixes: flutter-elinux/flutter-elinux#14 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
Adds a new
disable_system_fontsboolean toFlutterDesktopEngineProperties. When set, the engine installs a private fontconfig configuration containing only the directories of fonts declared in the bundle'sFontManifest.json, so Skia sees no system fonts and renders missing glyphs as tofu rather than falling back to a system typeface.Useful on embedded targets where the host font set may be unpredictable, sparse, or visually inconsistent with the app's intended design.
How it works
flutter_assets/FontManifest.jsonand collect the unique parent directories of each declared font asset.<dir>entries, no system paths, no<cachedir>(the bundle is small; in-memory rebuild on each startup is microseconds, and we avoid assuming any writable home directory exists on the target).memfd_create()anonymous file and expose it to the engine viaFONTCONFIG_FILE=/proc/self/fd/N. Nothing is written to the on-disk filesystem; the kernel frees the memfd when the fd is closed at engine shutdown.The embedder itself never
#includes a fontconfig header or links a fontconfig symbol — the actual parsing happens insidelibflutter_engine.so, which already haslibfontconfig.so.1as a runtime dependency. So no new build-time dependencies (nolibfontconfig-dev, no CMake changes).API surface
FlutterDesktopEngineProperties.disable_system_fonts(new trailing field, defaults tofalsevia zero-init — source-compatible)flutter::DartProject::set_disable_system_fonts(bool)/disable_system_fonts()in the C++ wrapperTest plan
swift buildof the FlutterSwift wrapper that consumes this embedder builds cleanmemfd_create()+/proc/self/fd/Nsemantics: opening the proc path gives a fresh open file description with offset 0, so fontconfig reads the full XML from byte 0 even though the writer's fd is at EOF--enable-fontconfigengine and a bundle containing only a custom font — confirm system Roboto/DejaVu is no longer reachable🤖 Generated with Claude Code