re-implement errname resolution

[jamestalmage]

#27 dropped execFile in favor of using spawn directly, so we've lost some features.

One is whatever these two lines do:

1. https://github.com/nodejs/node/blob/31600735f40e02935b936802ff42d66d49e9769b/lib/child_process.js#L8
2. https://github.com/nodejs/node/blob/31600735f40e02935b936802ff42d66d49e9769b/lib/child_process.js#L204

Can we reliably just do process.binding('uv')? Or is that asking for problems. I know it means importing some native lib, but not much beyond that.

Maybe we look back all the way to Node 0.12 and make sure it was still imported and used the same back then?

[sindresorhus]

Can we reliably just do process.binding('uv')?

Yes, but we probably shouldn't as they're trying to deprecate it: nodejs/node#2768 I think we should rather open an issue requesting a public API for what we need.

[jamestalmage]

they're trying to deprecate it ...

That is labeled as "stalled", and there are strong arguments for only preventing access to bindings which might be harmful (I doubt an errname lookup would count as harmful, but not sure what else the uv binding does).

I think we should rather open an issue requesting a public API for what we need

Yes, absolutely. In the meantime let's just write a defensive wrapper around process.binding('uv') so execa will only blow up if we can't access the binding, and they have a negative exit code. (it's only needed for negative exit codes). That allows us to stay backward compatible and provide the errname until a public API is available.

Assuming they deprecate before providing a public alternative, we might also be able to extract it from util._errnoException. It builds an error object from an exit code that has the errname lookup attached. That's obviously a private method, but we could just write more defensive wrappers around our access to it. That method is used liberally in the Node codebase, so no matter what, I think we will be able to hack a workable solution one way or the other until someone creates the "right way" to do it.