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

Windows API's TransmitFile function is limited to two concurrent
operations on workstation and client versions of Windows. This change
modifies the net.sendFile function to perform no work in such cases
so that TransmitFile is avoided.

Fixes #73746

Change-Id: Iba70d5d2758bf986e80c78254c8e9e10b39bb368
GitHub-Last-Rev: 315ddc0cd8
GitHub-Pull-Request: golang/go#73758
Reviewed-on: https://go-review.googlesource.com/c/go/+/673855
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

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/internal/syscall/windows/zsyscall_windows.go

@@ -539,7 +539,7 @@ func NtSetInformationFile(handle syscall.Handle, iosb *IO_STATUS_BLOCK, inBuffer
 	return
 }
 
-func rtlGetVersion(info *_OSVERSIONINFOW) {
+func rtlGetVersion(info *_OSVERSIONINFOEXW) {
 	syscall.Syscall(procRtlGetVersion.Addr(), 1, uintptr(unsafe.Pointer(info)), 0, 0)
 	return
 }

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()
+}