Manually give content-length of FormData fields

[jennydaman]

Is your feature request related to a problem?

Oh, the terrors of the internet.

The backend I am working with requires Content-Length to be specified for the whole multipart request, which is discussed in the documentation here:

https://docs.aiohttp.org/en/stable/multipart.html#hacking-multipart

However, it seems like it is only possible to figure out what Content-Length is by buffering the data to be sent.

In my situation, sometimes I have a large file on disk with a known size beforehand which I would like to include in a multi-part post request. The documentation also discusses "streaming uploads:"

https://docs.aiohttp.org/en/stable/client_quickstart.html#streaming-uploads

It would be nice if I could combine these solutions:

1. multi-part post request
2. Content-Length for the whole request
3. streaming upload

Currently, (1) and (2) can be achieved using aiohttp.FormData with file-like objects, but using io.IOBase should be avoided because we are calling blocking IO inside an async context.

Describe the solution you'd like

aiohttp.FormData.add_field should be extended in two ways:

1. support being given AsyncIterable[bytes] as value
2. accept optional parameter content_length which, if given, manually sets the length of the part.

When trying to send the FormData: if the lengths of all parts are known, then set the header Content-Length.

Describe alternatives you've considered

Currently I am using aiohttp.FormData with file-like objects and accepting the performance penalty. If the files get too large, I will write a custom multi-part deserializer, hard-coding its parts into a string template and calculating the Content-Length manually. The hard-coded parts plus file "stream" would be wrapped similarly to how it's done here: #3598 (comment)

Related component

Client

Additional context

No response

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