why are you dropping this code?

ping @vieux as original author of such trick. Do we need it for import and push?

from what I understand is job.Stdout.Used() is replaced by output.Flushed() it's used to check if we already wrote on the socket, is we did, we cannot return an error (otherwise we will have duplicate write headers and such things)

Here I don't see job.Stdout.Used() on the old version so I don't think it's needed. I think Import will never write and return an error, either it returns an error or it writes, that why the check isn't needed.

I hope this is understandable.

Here is the problem. Things are quite similar there for pull and import.
Assume you wanna pull an non-existing image with current code, youll get an error http: multiple response.WriteHeader calls`.Seems that any attempt to write to output without check for Flused() would case such problems to my best knowledge.
https://github.com/docker/docker/blob/master/graph/pull.go#L46

Pretty weird stuff :/ Import indeed writing somewhere.

I would prefer to move this to api/types

@cpuguy83 may I ask why? asking just because this is a struct needed only as a configuration paramenter to Push and not what we return through the api. Maybe what we write to OutStream should go to api/types

Ah, ok, I didn't realize that.
If it's only used by graph that's probably fine.

I think maybe it makes sense to structure what's written to the out stream in api/types as you said not needed atm..