I'm looking for feedback on this experiment. This is helpful to observe promise resolution without considering the promise as handled. This allows the unhandledRejection event to still be produced if no other resolution handling occurs. This includes a very small V8 change. Assuming people find the intent of this change reasonable, I would try to get that upstreamed to V8. I just don't want to put in all the effort to make that happen if the thing itself is not likely to be accepted.

The particular use case I have in mind for this is the tracePromise method in TracingChannel. Currently it wraps thenables in a Promise.resolve(...) and then calls the then(...) on it. This is problematic though as the thenable could have other methods which disappear as the return value becomes a native promise instead. Rather than using chaining there which produces a new and crucially different promise, I would like to return the original promise. The issue is that if we just branch internally for our observation and then return the original that original is considered handled and so would no longer trigger unhandledRejection if no other continuation was attached. This change would provide an interface such that we can observe that result but opt-out of the handled transition.

I imagine there might be other use cases for this in other parts of the codebase. This is intended purely as an internal utility, I don't expect we'd actually expose it in the public API anywhere.

What do folks think? Is this a reasonable solution we should be applying?

cc @nodejs/diagnostics @nodejs/v8