Why doesn't Channels.newChannel(OutputStream) flush?

[Charles Oliver Nutter]

While debugging some subprocess logic in JRuby today I came to the
realization that the WritableByteChannel returned by
Channels.newChannel(OutputStream) is "broken".

https://github.com/jruby/jruby/pull/6649/commits/5a6912caf3adb70fb4c210c73ae02fbf16f404d9

The issue arises when the provided stream is itself buffered, as is
the case for Process.getOutputStream. Because neither Channel nor
WritableByteChannel provide a way to flush (as Channels are expected
to be "raw" unbuffered IO), data written to the wrapper will not
propagate to the the other end of the stream until the intermediate
buffer has been filled. A flush can only be performed if you have
direct access to the underlying stream, which is generally impossible
if using this wrapper to interact with a Channel-related API.

I would propose that the default OutputStream Channel wrapper should
flush() after every non-zero write (or potentially after every write,
if the underlying stream is not altogether honest about how many bytes
were written).

Can someone provide a counter example for why the wrapper should *not*
flush (current behavior)? What would be harmed by making this change?
I can come up with nothing.

 Additional reasons to flush:

* Early propagation of IO errors due to the underlying stream no
longer being flushable.
* More responsive, possibly reducing IO stutter for APIs using the wrapper.

That first one could be seen as a negative but I can only see it as a
positive (raise the error at the time of the bad write, not once the
buffer is filled many writes later).

- Charlie