os: fix handling of ErrShortWrite in (*File).Write
Restore the handling of io.ErrShortWrite in (*File).Write:
if we write less than the requested amount, and there is no error from
the syscall, then return io.ErrShortWrite.
I can't figure out how to write a test for this. It would require a
non-pollable file (not a pipe) on a device that is almost but not
quite entirely full. The original code (https://golang.org/cl/
36800043,
committed as part of https://golang.org/cl/
36930044) does not have a test.
Fixes #20386.
Change-Id: Ied7b411e621e1eaf49f864f8db90069f276256f5
Reviewed-on: https://go-review.googlesource.com/43558
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>