Question about how Error::New() handles last error and exceptions

[JckXia]

I was writing a small Cpp addon trying to see if there exists a scenario where the _ref class variable of Error class might be a nullptr.

Since it looks like the error_code napi_boolean_expected isn't handled by the switch statement inside Error::New().

Though it looks like even though we are able to retrieve the error info using napi_get_last_error_info , once we call napi_is_exception_pending we will clear the last error and thus lose the error info from the previous invocation.

I am just wondering if this is the expected behaviour? Since it looks like the original error info( a const) is intended to be used elsewhere in the function body. Thanks!

[KevinEady]

Whenever I see a napi_* without a napi_status status = or NAPI_THROW_ in front of it, it triggers a red flag for me. You should check the return status of your napi_get_value_bool immediately after invocation. We provide the NAPI_THROW_* macros for that. And when the APIs are used incorrectly, I am not sure the intended behavior. Eg you immediately create an Error object not really knowing if one should be made (which is handled by the NAPI_THROW_* macros).

What happens if you swap the order of the get_last_error_info and napi_is_exception_pending calls?

[JckXia]

Hmm, I think in this instance we know that napi_get_value_bool will register an error upon invocation since "asd" is not a bool value, and NAPI_THROW_* looks like it will construct an Error::New(env) value regardless (with the same behaviour as aforementioned, the initial error code being overwritten with a subseq call to napi_is_exception_pending).

When I swap the get_last_error_info and napi_is_exception_pending, the error code napi_boolean_expected gets overwritten completely. I think this is because inside napi_is_exception_pending, we call napi_clear_last_error(env) which simply updates the error code to napi_ok.

It also looks like even though the error code gets overwritten, the error message persists because napi_clear_last_error does not mutate that field. So we have a situation where the error code is napi_ok but the error message is inconsistent with the error code.

[mhdawson]

This section describes the planned error handling: https://nodejs.org/api/n-api.html#n_api_return_values

The TLDR; is that the doc covers

1. Always check the status returned from a call immediately after it is called
2. If the result is not napi_ok or napi_pending_exception you must also call napi_pending_exception to see if there is a pending exception.
3. napi_get_last_error_info can be called to get additional info on a status code. What it does not state specifically is that you need to call this immediately after making the original call that returned something other than napi_ok. We could add that. @JckXia do you think that would help?

Separately, we might also want to look at napi_get_last_error_info, and see if we should reset the napi_extened_error_info returned. We could quite likely defer that until napi_get_last_error_info is called (ie clear if the last error is now napi_ok) which would mean we would not add overhead unless napi_get_last_error_info is called when the error has been reset.

[JckXia]

Hey @mhdawson , yeah I think adding that to the doc could help. Though the issue with the Error::New(env) implementation could that the napi_is_exception_pending call mutates the napi_extended_error_info struct, while the switch statement in later parts of the code seems to work with the assumption that info->error_code has not been changed . In this case, should we create a copy of the info struct?

[mhdawson]

Yes, I see that the code code does not expect that the data pointed to by info be changed by later calls. We re-use the same structure to avoid allocating/de-allocating but does result in this issue.

As you suggest the method should pull the fields it uses out of info earlier to avoid that.

[mhdawson]

I created this PR to address the doc side: nodejs/node#40458

@JckXia do you want to submit a PR to clear info in napi_get_last_error_info() if the last status was napi_ok?

[RaisinTen]

PR: #1092

[JckXia]

Currently working on a PR to reset the error info if previous status is napi_ok inside node core