WIP: async read function by default

[drzraf]

• Remove the distinction between sync/async readFileFn in order to use a unified codepath.
• Remove the need for calling the preprocessFinished() function after defining a (now async) preprocess() callback.

[drzraf]

Tests are failing, this is related to the fact that once a queue is initialized, it relies upon recursion (so that xhr.load event handler calls the doneHandler which in turn enqueue the next chunk).

The problem with this is that while event handlers used to be synchronous... they now triggers async' function calls without returning the corresponding Promise to upper levels.
In consequence, tests fail because they run assertions meanwhile the async XHR event handlers are still running in the background.
(see #346 (comment))

[drzraf]

I'm pretty sure I'd have to rewrite uploadNextChunk and drop recursion altogether unless someone can provide an elegant alternative.

[drzraf]

Next problem is that XHR onload callback behavior is clearly not the best choice in a Promise-based world. I know of project Promisifying XHR, but the best actual candidate to remove recursion would be to switch to fetch (which probably offer many other benefits).

But the problem with fetch is that upload-progress is standardized but not implemented (https://developer.mozilla.org/en-US/docs/Web/API/Request) except on Chrome > 85 under an experimental flag (https://web.dev/fetch-upload-streaming/)

• https://bugs.chromium.org/p/chromium/issues/detail?id=688906
• Tests : http://w3c-test.org/fetch/api/basic/request-upload.any.html

We definitely don't want to hold v3 until major browsers supports fetch upload Stream out of the box. But I think v3 uploads are not correct until and revamp of the recursion (regarding doneHandler) is done (@ilessiivi)

[ilessiivi]

I agree that there is room for improvement in uploadNextChunk and I'd welcome a rewrite if you have an idea for it.

I'm not sure though, if the synchronous upload tests will ever work with async readFileFn even if the entire function chain up to the web request (regardless if it's XHR or later on, Fetch) is Promise based. There are plenty of tests with await flow.upload() and the progress tests would be executed too late if the web requests were awaited for.

Perhaps the networking related tests should be rewritten with events (flow.on('file-progress') etc.)? That would reflect the code's real world usage better, too.

[drzraf]

Regarding fetch upload progress: https://developer.mozilla.org/en-US/docs/Web/API/BackgroundFetchRegistration/progress_event#browser_compatibility (still not in Safari nor Firefox)

[AidasK]

@drzraf looks good, if you want, you can merge this. But please update the docs accordingly