drpcmanager: cancel stream with error when terminated

[kylecarbs]

Terminations were canceling streams with context.Canceled instead
of surfacing the error, causing unexpected behavior.

[kylecarbs]

@zeebo could you take a look?

[zeebo]

Hmm.. I think this changes some semantics. Specifically, a MsgRecv returning an io.EOF indicates that the other end has cleanly closed the stream. That is now happening even in the case of a "hard close" of the underlying transport. That's why the manager was ignoring the error and terminating the stream with context.Canceled. The test as written was to enforce that property, actually.

[kylecarbs]

Is there another way we should surface this error?

Because this error wasn't being surfaced, it was very difficult for us to log the true reason a stream was being terminated.

[zeebo]

Perhaps the manager code can check to see if the error was io.EOF and terminate with context.Canceled, and otherwise terminate with the provided error. I think that would be fine to both surface unexpected transport errors as well as maintain the property that clean, intentional closes can be detected.

[zeebo]

Or, maybe we do something like io.ReadFull and change io.EOF into io.ErrUnexpectedEOF instead of the previously suggested context.Canceled. I think that's better.

[storjrobot]

This pull request has been mentioned on Storj Community Forum (official). There might be relevant details there:

https://forum.storj.io/t/introducing-drpc-our-replacement-for-grpc/13486/13