runtime.md

Embedded native runtime

The runtime is embedded below weasyprint/_weasyprint_libs. The patched weasyprint/__init__.py calls _weasyprint_libs_loader.activate() before the normal WeasyPrint imports continue.

Source and generated files

The maintained loader source is:

runtime/loader.py

The central runtime configuration is:

config/runtime-libraries.toml

During wheel patching they become:

weasyprint/_weasyprint_libs_loader.py
weasyprint/_weasyprint_libs_config.py

No standalone Python package or .pth activation file is involved.

Staging

All platforms stage their payload in:

_build/runtime/
  lib/ or bin/
  etc/
  share/

Linux uses $ORIGIN RPATHs. macOS rewrites install names to @loader_path/<name>. Windows registers the embedded bin directory with os.add_dll_directory before preloading the configured entry DLLs.

Fontconfig is pointed at the embedded configuration and an embedded cache location through FONTCONFIG_FILE, FONTCONFIG_PATH and XDG_CACHE_HOME.

Avoiding mixed runtimes

Pango, HarfBuzz, HarfBuzz subset, Fontconfig and FreeType are explicitly part of the configured runtime entry set. The Linux integration test inspects /proc/<pid>/maps and rejects system copies of these libraries after rendering a PDF.

The CFFI alias map is used on macOS because WeasyPrint probes several Linux, Windows and macOS library names. Known aliases are redirected to absolute paths inside the embedded runtime.