When parsing a multi-part POST, retain original pairs

[matthewd]

The input is not guaranteed to be rewindable, and the default parameter expansion loses details of the parameter names.

For form-encoded data, we (already) retain the full string: it contains only simple key/value pairs, and will be of a manageable size.

Multipart data may contain uploaded files, so we don't wish to retain the full input string -- that's why we dropped the rewindability requirement on the input stream. Instead, we retain an array of two-element arrays representing the "raw" key-value pairs as we parsed the stream. This minimizes additional memory use, because the values are the same objects we use in the params hash, while still allowing an interested consumer to apply their own logic to parameter name interpretation.

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@jeremyevans @ioquatix I believe this is the minimal version of what we discussed at Kaigi. Apologies for the delay.

For a consumer to use it in this state, they'd need to call Rack::Request#POST, relying on its side-effect, then manually pull rack.request.form_pairs. Neither of those steps feels like a great API, so I think it would make sense to provide a body_param_list-like method as in #2038, but it might be easier to discuss that in a separate follow-up PR, if this seems independently reasonable as the direct implementation / extra retained values?

[jeremyevans]

Thank you for the discussion regarding this at RubyKaigi, it helped me better understand the issue.

This looks good to me. I have one inline comment, but it can be done after this is merged if you and @ioquatix agree.

[ioquatix]

I discussed this with Matthew.

There is an expected follow up PR, creating a method, like form_pairs with the following possible implementation:

def form_pairs
  if form_data? || parseable_data?
    if pairs = Rack::Multipart.parse_multipart(env, Rack::Multipart::ParamList)
      set_header RACK_REQUEST_FORM_PAIRS, pairs
    end
  end

  get_header RACK_REQUEST_FORM_PAIRS # cached value.
end

And then in theory POST is form_pairs |> expand_param_pairs, + hash caching