Makes HermesRuntimeImpl::isArray proxy-compatible

[dorentus]

When using hermes engine, dynamicFromValue in React Native calls isArray here to check if the input object is an array.

Consider this piece of code:

const list = ['a', 'b', 'c', 'd'];
const proxyed_list = new Proxy(list, {});

The proxyed_list behaves exactly the same as list in Javascript. But when been passed to native and converted to folly::dynamic by dynamicFromValue, proxyed_list will be converted to {"0": "a", "1": "b", "2": "c", "3": "d"} but not the expected ["a", "b", "c", "d"].

This patch implements similar routines in commit 26840ed441d614a07809c612521074ca9544086c and makes isArray proxy-compatible.

Summary

Test Plan

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[neildhar]

Hey @dorentus, thanks for submitting this. This is a little tricky because Hermes today assumes that any jsi::Array must be an instance of a vm::JSArray. Supporting this would require changing this assumption throughout hermes.cpp. Given that the API is documented to be equivalent to Array.isArray(), that is probably the right thing to do, but we'll look into it a bit first.

[neildhar]

To support this properly, we should fix all users of jsi::Array in this file to no longer imply that the underlying object is a vm::JSArray. In most cases, this should just be a matter of using handle() instead of arrayHandle(). But for length we will need to have a fast path that follows the current behaviour, and a slow path that does a regular property lookup.

[neildhar]

We have an isArray in Operations.h that you can call into that will provide the right functionality here.

[dorentus]

Code updated.

Question: Should I call runtime_.clearThrownValue() on ExecutionStatus::EXCEPTION before returning false?

[neildhar]

You should use checkStatus to rethrow the exception as a C++ exception

[dorentus]

checkStatus isn't const thus cannot be used here without modification...

[dorentus]

It should be safe to remove the constness and call checkStatus here. Code updated.

[dorentus]

https://github.com/facebook/react-native/blame/321f7dbcadb78dede9048500ab8abe86af863061/packages/react-native/ReactCommon/jsc/JSCRuntime.cpp#L993

JSC used to implement isArray by calling Array.isArray when _JSC_FAST_IS_ARRAY is defined, but now it only calls JSValueIsArray.

And JSValueIsArray behaves the same as HermesRuntimeImpl::isArray and returns false for new Proxy([], {}).

So yes, changes suggested by this PR might cause more problems than expected.

---

As for the dynamicFromValue problem, I'm trying the Array.isArray approach in my local copy of code to see if is has any performance glitches.

[tmikov]

Closing for now, since no progress. Please feel free to re-open when there are updates.