Windows sendfile optimization is skipped since CL 472475, which started
passing an os.fileWithoutWriteTo instead of an os.File to sendfile,
and that function was only implemented for os.File.
This CL fixes the issue by asserting against an interface rather than
a concrete type.
Some tests have been reenabled, triggering bugs in poll.SendFile which
have been fixed in this CL.
Fixes #67042.
Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest
Change-Id: Id6f7a0e1e0f34a72216fa9d00c5bf36f5a994219
Reviewed-on: https://go-review.googlesource.com/c/go/+/664055 Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alex Brainman <alex.brainman@gmail.com>