Document that the ObjectWrap example code doesn't work for multi-context usage

[geekuillaume]

After spending a day trying to debug why I had so many seemingly random issues with my code, I finally understood why: the object wrap example code is not compatible with a multi-context usage of the addon.

Explanation: this example set the constructor static member of the class to a reference of the constructor function but if the addon is loaded multiple time (with worker_threads for example), the value is overwritten by a new reference in the new context. That makes this reference unusable to create new instances of this wrapped class from other C++ code and makes any code wanting to use this reference in another context crash.

We should at least make sure there is a warning about this in the documentation. The best would be to provide a workaround with the new addon instance data support maybe.

[mhdawson]

@geekuillaume any chance you'd like to submit a PR to the docs to cover this?

[markirish]

@geekuillaume what kind of errors were you seeing?

I am seeing some errors with HandleScopes in an AsyncWorker's OnOk() when using worker_threads. Looking at similar issues across other repos, it seems the issue is 'context-aware addons', but being a node-addon-api addon (and therefore a N-API addon) I don't think I'm doing anything wrong as far as declaring the module.

I'm not sure if this is the same issue you are running into, but seems possible.