Long story short

I'm using the MultipartReader to read multi-part data from a 3rd party API. It's response has an extra CRLF pair before the first boundary marker.

It appears from reading the RFC that it's valid to include a preamble before the first boundary, and it should just be ignored

I'm passing a response from this server through aiohttp.MultipartReader.from_response and then calling .next()

Expected behaviour

aiohttp ignores the preamble and returns the first part.

Actual behaviour

I get the exception:

ValueError: Invalid boundary b'', expected b'------=_Part_125656_1263945784.1463662001461'

Steps to reproduce

I've added a failing unit test here: obmarg@8724035

Your environment

Not sure any of it's relevant, but: MacOS, python 3.5.1, aiohttp 0.19.0 (though reproduced with master).