1 files changed, 16 insertions, 0 deletions
diff --git a/src/internal/poll/sendfile_windows.go b/src/internal/poll/sendfile_windows.go
index 4a15b75236..17a3681064 100644
--- a/src/internal/poll/sendfile_windows.go
+++ b/src/internal/poll/sendfile_windows.go
@@ -25,8 +25,24 @@ func SendFile(fd *FD, src syscall.Handle, n int64) (int64, error) {
 	o := &fd.wop
 	o.qty = uint32(n)
 	o.handle = src
+
+	// TODO(brainman): skip calling syscall.Seek if OS allows it
+	curpos, err := syscall.Seek(o.handle, 0, 1)
+	if err != nil {
+		return 0, err
+	}
+
+	o.o.Offset = uint32(curpos)
+	o.o.OffsetHigh = uint32(curpos >> 32)
+
 	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
 }