Proof of concept fix for #2556

[bjeanes]

This fixes the issue I reported in #2556.

It would however not post as multipart without another present file unless run against (rack/rack-test#303. A workaround could be to do something like:

merge_param!(params, "_multipart_workaround", NilUploadedFile.new))

when not running under a new enough rack-test, if any future Capybara version which might include such a fix needs to maintain compatibility with older rack-tests.

However, I am not sure which is more problematic: submitting as single part when a <form enctype="multipart/form-data"> has no filled file inputs or submitting params which were not present in the form in the first place.

My gut is that the latter is actually worse because a single-part form with no file fields should only be a problem if a user is asserting the lower-level data submitted -- which is patently not in the spirit of testing with Capybara. The user wouldn't care, nor should any Capybara test, I would think.

[bjeanes]

Just added some of my thoughts along the way about my proof-of-concept to seed discussion and weigh caveats.

[bjeanes]

This was the easiest way to communicate the multipart knowledge from the form processing to the browser instance, however this interface change would be breaking if there are any callers which rely on implicit Hash arguments:

def submit(method, path, attributes, multipart: false)
  # ...
end

submit("GET", "/", "foo" => "bar")   # ! ArgumentError: wrong number of arguments (given 2, expected 3)
submit("GET", "/", {"foo" => "bar"}) # OK
submit("GET", "/", params_hash)      # OK

However, the headers sent for a form submission by any browser ultimately do depend on the form value, so I am not sure what would be a superior way to do this.

Perhaps implementing a submit_multipart() instead?

[bjeanes]

I would have to set up a test case with other browsers to be sure, but I think this kind of short circuit would need to be in place for other <input> fields with blank values.

I'm not sure if there is in practice a distinction between an empty string value and a nil value, but the code here already does field['value'].to_s above.

[twalpole]

This would be a breaking change and not one I'm willing to make. If/when rack releases a new version which allows to automatically trigger multipart upload without having to pass the nil upload file then we will support that for those versions, however we will not be dropping current support for existing rack versions. This driver attempts to mimic a real browser but is not 100% the same and isn't guaranteed to be.