Foreign engines — Firefox & Servo

The Chromium family (Chrome / Chromium / Edge) is driven in-process through shirabe's own CDP engine. Firefox and Servo take a different path: their cores are huge, so we let the browser vendors (or anyone building those cores) compile a tiny adapter against a fixed C ABI and ship it as a dynamic library — the same model ort uses for ONNX Runtime. shirabe is the "thin C-binding wrapper": it dlopens the vendor lib and routes calls through a generic Engine trait.

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your app ── shirabe (CDP engine) ── Chrome / Chromium / Edge   (in-process)
        └─ shirabe (FFI wrapper) ── libshirabe_engine_firefox ── Firefox core
                                 └ libshirabe_engine_servo   ── Servo core

Enabling

toml
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shirabe = { version = "0.1", features = ["foreign-engine"] }

Then select a foreign backend:

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SHIRABE_BACKEND=firefox shirabe debug --port 3001
SHIRABE_BACKEND=servo   shirabe debug --port 3001

The C ABI a vendor lib exports

c
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typedef struct shirabe_engine shirabe_engine;

shirabe_engine *shirabe_engine_new(const char *options_json);   /* JSON opts   */
void  shirabe_engine_destroy(shirabe_engine *eng);
int   shirabe_engine_navigate(shirabe_engine *eng, const char *url);
char *shirabe_engine_evaluate(shirabe_engine *eng, const char *js); /* JSON out */
void  shirabe_engine_free_string(shirabe_engine *eng, char *s);
int   shirabe_engine_screenshot(shirabe_engine *eng,
                                unsigned char **out, size_t *out_len);   /* PNG */
void  shirabe_engine_free_pixels(shirabe_engine *eng, unsigned char *buf, size_t len);
const char *shirabe_engine_id(void);                              /* "firefox" … */

A few hundred lines of adapter code against this ABI is enough to drive a whole browser core; everything shirabe exposes over HTTP is built on these five operations.

Where the lib comes from

CdylibEngine::open looks for libshirabe_engine_<id>.{so,dylib,dll} in:

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    SHIRABE_ENGINE_PATH — explicit override.
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    next to the current executable.
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    <cache>/shirabe/engines/<id>/ — where the release-fetch step places downloaded copies (a release workflow publishes prebuilt libs to GitHub Releases under their own tags).

Until a vendor publishes a lib, selecting Firefox/Servo returns a clear error pointing at the FFI contract — shirabe never tries to spawn firefox as if it spoke CDP.