Implement the File API's File.close method

[oleiade]

As part of #3141, implementing the agreed-upon design for k6's File API, we intend to implement a File.close method.

The close method marks a File as closed and unfit for further use. We, however, anticipate its implementation to be a noop at the moment.

However, depending on the implementation itself, it might turn out that we want to make some reference counting of some sort to make sure to free the file resources from memory as soon as no VU interacts with it anymore.

prototype

interface File {
    close()
}

example

const file = fs.openSync("fou.txt")
file.close()

[oleiade]

We have discussed this topic heavily with @codebien and concluded that in the current state of k6, introducing this API wouldn't bring outstanding benefits and probably wouldn't lead to a better user experience either.

The current implementation of the fs module does not rely on file descriptors under the hood but rather behaves like a filesystem cache, and the benefit of closing files, for that matter, is not immediately apparent.

We have looked into introducing reference-counted file opening/closing. Still, because we are constrained to opening them in the init context at the moment, the performance benefits we wanted to introduce by doing so are unfortunately canceled.

As a result we decided to put the introduction of this API on hold until we have a good enough reason to do so, both from the technical and/or the user-experience perspective. A good technical reason to introduce it in the future might come as a result of #1079.