From 8359b5e134052db0e5f1bc2257d496b0a81aa4fb Mon Sep 17 00:00:00 2001
From: Alex Brainman <alex.brainman@gmail.com>
Date: Sun, 26 Aug 2018 16:45:10 +1000
Subject: [PATCH] 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 <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
---
 src/internal/poll/sendfile_windows.go | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/internal/poll/sendfile_windows.go b/src/internal/poll/sendfile_windows.go
index dc93e851d63..17a3681064b 100644
--- a/src/internal/poll/sendfile_windows.go
+++ b/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
 }