Improving support for testing native modules (JSI / N-API)

[kewde]

Problem

I'm writing a native module using the JSI interface and I'd like to have a headless runner for hermes to run my tests against my native code. The pattern that I typically see people do is build a full blown react native application to run their test suite in. It's not really a pleasant experience to say the least, it's also quite slow..

At the moment we use https://github.com/ExodusMovement/test to run our "plain" JavaScript tests with the hermes binary shipped in hermes-engine-cli package. It takes a bit of mocking to make everything run but at least it's fast enough.

At first I thought the JSI is more a react native thing but there is an experimental runtime wrapper for the JSI in the ABI folder so it seems to be on track to become a first class citizen?

hermes/API/hermes_abi/HermesABIRuntimeWrapper.cpp

Line 122 in 038bfdb

class HermesABIRuntimeWrapper : public Runtime {

JSI missed his older brother N-API, so he also came around the corner, developers from the NodeJS ecosystem may want to test their code across multiple JS engines.

Perhaps we can have a solution that works for both?

Solution

Solution 1: Write your own runtime

Slap some extra code on to this and build your own runtime:
https://github.com/facebook/hermes/blob/main/tools/jsi/jsi.cpp#L101-L111

Solution 2: dynamically hook functions in the binary

Another solution could be to use LD_PRELOAD / DYLD_INSERT_LIBRARIES and intercept some function like:

hermes/lib/VM/Runtime.cpp

Line 1699 in 038bfdb

void Runtime::initNativeBuiltins() {

Then call it using dlsym, and afterwards do our own thing to .

dlsym(RTLD_NEXT, "_ZNK3Runtime8initNativeBuiltins");

Seems very hacky, but also quite versatile at the same time.

Solution 3: ???

Happy to explore any solutions from the comments.

[tmikov]

https://github.com/tmikov/hermes-jsi-demos/tree/master/hf-runner