internal/poll: handle the special case of sendfile(2) sending the full chunk
CL 622235 would fix #70000 while resulting in one extra sendfile(2) system
call when sendfile(2) returns (>0, EAGAIN).
That's also why I left sendfile_bsd.go behind, and didn't make it line up
with other two implementations: sendfile_linux.go and sendfile_solaris.go.
Unlike sendfile(2)'s on Linux and Solaris that always return (0, EAGAIN),
sendfile(2)'s on *BSD and macOS may return (>0, EAGAIN) when using a socket
marked for non-blocking I/O. In that case, the current code will try to re-call
sendfile(2) immediately, which will most likely get us a (0, EAGAIN).
After that, it goes to `dstFD.pd.waitWrite(dstFD.isFile)` below,
which should have been done in the first place.
Thus, the real problem that leads to #70000 is that the old code doesn't handle
the special case of sendfile(2) sending the exact number of bytes the caller requested.
Fixes #70000
Change-Id: I6073d6b9feb58b3d7e114ec21e4e80d9727bca66
Reviewed-on: https://go-review.googlesource.com/c/go/+/622255
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>