net: avoid using Windows' TransmitFile on non-server machines

src/internal/syscall/windows/types_windows.go

@@ -256,3 +256,7 @@ type FILE_COMPLETION_INFORMATION struct {
 	Port syscall.Handle
 	Key  uintptr
 }
+
+// https://learn.microsoft.com/en-us/windows/win32/api/winnt/ns-winnt-osversioninfoexa
+// https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/ns-wdm-_osversioninfoexw
+const VER_NT_WORKSTATION = 0x0000001

src/internal/syscall/windows/version_windows.go

@@ -11,28 +11,53 @@ import (
 	"unsafe"
 )
 
-// https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/ns-wdm-_osversioninfow
-type _OSVERSIONINFOW struct {
+// https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/ns-wdm-_osversioninfoexw
+type _OSVERSIONINFOEXW struct {
 	osVersionInfoSize uint32
 	majorVersion      uint32
 	minorVersion      uint32
 	buildNumber       uint32
 	platformId        uint32
 	csdVersion        [128]uint16
+	servicePackMajor  uint16
+	servicePackMinor  uint16
+	suiteMask         uint16
+	productType       byte
+	reserved          byte
 }
 
 // According to documentation, RtlGetVersion function always succeeds.
-//sys	rtlGetVersion(info *_OSVERSIONINFOW) = ntdll.RtlGetVersion
+//sys	rtlGetVersion(info *_OSVERSIONINFOEXW) = ntdll.RtlGetVersion
+
+// Retrieves version information of the current Windows OS
+// from the RtlGetVersion API.
+func getVersionInfo() *_OSVERSIONINFOEXW {
+	info := _OSVERSIONINFOEXW{}
+	info.osVersionInfoSize = uint32(unsafe.Sizeof(info))
+	rtlGetVersion(&info)
+	return &info
+}
 
 // Version retrieves the major, minor, and build version numbers
 // of the current Windows OS from the RtlGetVersion API.
 func Version() (major, minor, build uint32) {
-	info := _OSVERSIONINFOW{}
-	info.osVersionInfoSize = uint32(unsafe.Sizeof(info))
-	rtlGetVersion(&info)
+	info := getVersionInfo()
 	return info.majorVersion, info.minorVersion, info.buildNumber
 }
 
+// SupportUnlimitedTransmitFile indicates whether the current
+// Windows version's TransmitFile function imposes any
+// concurrent operation limits.
+// Workstation and client versions of Windows limit the number
+// of concurrent TransmitFile operations allowed on the system
+// to a maximum of two. Please see:
+// https://learn.microsoft.com/en-us/windows/win32/api/mswsock/nf-mswsock-transmitfile
+// https://golang.org/issue/73746
+var SupportUnlimitedTransmitFile = sync.OnceValue(func() bool {
+	info := getVersionInfo()
+	return info.productType != VER_NT_WORKSTATION
+})
+
 var (
 	supportTCPKeepAliveIdle     bool
 	supportTCPKeepAliveInterval bool

src/net/sendfile.go

@@ -12,8 +12,6 @@ import (
 	"syscall"
 )
 
-const supportsSendfile = true
-
 // sendFile copies the contents of r to c using the sendfile
 // system call to minimize copies.
 //
@@ -22,6 +20,9 @@ const supportsSendfile = true
 //
 // if handled == false, sendFile performed no work.
 func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
+	if !supportsSendfile() {
+		return 0, nil, false
+	}
 	var remain int64 = 0 // 0 writes the entire file
 	lr, ok := r.(*io.LimitedReader)
 	if ok {

src/net/sendfile_nonwindows.go

@@ -0,0 +1,12 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux || (darwin && !ios) || dragonfly || freebsd || solaris
+
+package net
+
+// Always true except for workstation and client versions of Windows
+func supportsSendfile() bool {
+	return true
+}

src/net/sendfile_stub.go

@@ -8,7 +8,9 @@ package net
 
 import "io"
 
-const supportsSendfile = false
+func supportsSendfile() bool {
+	return false
+}
 
 func sendFile(c *netFD, r io.Reader) (n int64, err error, handled bool) {
 	return 0, nil, false

src/net/sendfile_test.go

@@ -31,11 +31,11 @@ const (
 // expectSendfile runs f, and verifies that internal/poll.SendFile successfully handles
 // a write to wantConn during f's execution.
 //
-// On platforms where supportsSendfile is false, expectSendfile runs f but does not
+// On platforms where supportsSendfile() is false, expectSendfile runs f but does not
 // expect a call to SendFile.
 func expectSendfile(t *testing.T, wantConn Conn, f func()) {
 	t.Helper()
-	if !supportsSendfile {
+	if !supportsSendfile() {
 		f()
 		return
 	}

src/net/sendfile_windows.go

@@ -0,0 +1,16 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package net
+
+import "internal/syscall/windows"
+
+// Workstation and client versions of Windows limit the number
+// of concurrent TransmitFile operations allowed on the system
+// to a maximum of two. Please see:
+// https://learn.microsoft.com/en-us/windows/win32/api/mswsock/nf-mswsock-transmitfile
+// https://golang.org/issue/73746
+func supportsSendfile() bool {
+	return windows.SupportUnlimitedTransmitFile()
+}