For some background, the reason I want to use multipart.Reader with an open stream is because of Amazon's new AVS API which involves creating a long-lived HTTP/2 request that progressively receives data in the form of multipart parts.

I did some experiments and the multipart.Part's Read method seems to block on bufio.Reader's Peek method. It doesn't seem possible to make bufio work with open streams that are shorter than the buffer length, so this might be tricky to fix. I even tried making a non-blocking reader to see how it would behave, but bufio.Reader just errors out.

For anyone else who runs into this issue, here's a working reimplementation of mime/multipart which works on both open and closed streams: https://github.com/rogertalk/go-avs/tree/master/multipart2

A few caveats:

• I didn't implement quoted-printable support
• It won't be as forgiving for incorrect implementations (e.g., whitespace after boundary)
• It uses a bufio.Reader on top of a private implementation similar to bufio which wastes some memory (the reason is because textproto.NewReader(…) requires *bufio.Reader instead of io.Reader)
• Probably a few other things are missing too since I only needed this for a specific use case

CL https://golang.org/cl/32032 mentions this issue.

This is fixed by CL https://golang.org/cl/32092 instead so I'm closing this issue.

We still need to add a test.