internal/poll: advance file position in windows sendfile

Some versions of Windows (Windows 10 1803) do not set file
position after TransmitFile completes. So just use Seek
to set file position before returning from sendfile.

Fixes #25722

Change-Id: I7a49be10304b5db19dda707b13ac93d338aeb190
Reviewed-on: https://go-review.googlesource.com/131976
Reviewed-by: Brad Fitzpatrick <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Yasuhiro MATSUMOTO <[email protected]>
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: alexbrainman

src/internal/poll/sendfile_windows.go

@@ -38,5 +38,11 @@ func SendFile(fd *FD, src syscall.Handle, n int64) (int64, error) {
 	done, err := wsrv.ExecIO(o, func(o *operation) error {
 		return syscall.TransmitFile(o.fd.Sysfd, o.handle, o.qty, 0, &o.o, nil, syscall.TF_WRITE_BEHIND)
 	})
+	if err == nil {
+		// Some versions of Windows (Windows 10 1803) do not set
+		// file position after TransmitFile completes.
+		// So just use Seek to set file position.
+		_, err = syscall.Seek(o.handle, curpos+int64(done), 0)
+	}
 	return int64(done), err
 }