From d045236a8d4b76a8a8d1fee1c3db5e78a2871381 Mon Sep 17 00:00:00 2001
From: qmuntal <quimmuntal@gmail.com>
Date: Wed, 23 Oct 2024 10:24:06 +0200
Subject: [PATCH] windows: implement Ftruncate using a single syscall on
 Windows

Ftruncate can be implemented on Windows using a single syscall. This
makes the implementation more efficient and less prone to races when
used in combination with other Seek calls.

Note that this is the x/sys counterpart for CL 618835.

Change-Id: Ie9be356bd953ccce85c0dd87a5dcc6ccf4fec464
Reviewed-on: https://go-review.googlesource.com/c/sys/+/621935
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
---
 windows/syscall_windows.go | 18 +++++-------------
 1 file changed, 5 insertions(+), 13 deletions(-)

diff --git a/windows/syscall_windows.go b/windows/syscall_windows.go
index 63471b0e4..a07bf34af 100644
--- a/windows/syscall_windows.go
+++ b/windows/syscall_windows.go
@@ -725,20 +725,12 @@ func DurationSinceBoot() time.Duration {
 }
 
 func Ftruncate(fd Handle, length int64) (err error) {
-	curoffset, e := Seek(fd, 0, 1)
-	if e != nil {
-		return e
-	}
-	defer Seek(fd, curoffset, 0)
-	_, e = Seek(fd, length, 0)
-	if e != nil {
-		return e
+	type _FILE_END_OF_FILE_INFO struct {
+		EndOfFile int64
 	}
-	e = SetEndOfFile(fd)
-	if e != nil {
-		return e
-	}
-	return nil
+	var info _FILE_END_OF_FILE_INFO
+	info.EndOfFile = length
+	return SetFileInformationByHandle(fd, FileEndOfFileInfo, (*byte)(unsafe.Pointer(&info)), uint32(unsafe.Sizeof(info)))
 }
 
 func Gettimeofday(tv *Timeval) (err error) {