Allow listing of files in the init context

[na--]

Currently, users can open() files if they know their filenames, but they can't traverse the filesystem and open files dynamically. A few functions to navigate around the filesystem would help with the UX of writing file-heavy tests, as described in this forum post.

This should probably happen after we have a reasonable way to not read the contents of every file in every VU, as open() currently does. So, as a quick fix, having the __VU variable available in the init context would do, but longer-term, something like #592 and #532 would be ideal.

And an open question is whether we should try to emulate even parts of the fs node API, or if we should do something much simpler and narrower in scope. For now I'm definitely leaning toward the latter option...

Current workarounds:

Describe desired files in a .json file:

If I have a testfile.json with an array of file names (or something more complicated with additional metadata), I can just open it, and then iterate over its contents to open all of the other files:

let fileContents = [];
let filePaths = JSON.parse(open("filenames.json"));

filePaths.forEach(fileName => fileContents.push({
    name: fileName,
    contents: open(fileName, "b"),
}));


export default function () {
    // Do something with fileContents
    // ...
}

Pass a list of file as an environment variable

You could run k6 run --env filelist="file1;file2;file3" script.js and then use __ENV.filelist in the k6 script's init context to open all of the supplied files. Of course, the filelist value could also be dynamically generated (e.g. --env filelist="$(find ./some/folder -type f -print0 | tr '\0' ';')"), as long as we pay attention that the filenames don't contain the chosen separator. And, for the moment, that the separator isn't a comma 🪲 ...

Load the files from an HTTP server

As the forum post suggested, the files could be located on a simple HTTP server. This way they can be retrieved in the setup() function or in the future per-VU init function.

[oleiade]

ref #2977
ref #3141
ref #557