net,internal/poll: load ws2_32.dll only when net is imported

On Windows, ws2_32.dll is loaded and WSA initialized even if websockets
are not used.

This CL delays loading of ws2_32.dll and starting WSA until net is
initialized.

Change-Id: I07ea8241d79709cd4e80d29ba0d792c7444bbfe9
Reviewed-on: https://go-review.googlesource.com/c/go/+/557015
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/go/testdata/script/ws2_32.txt

@@ -0,0 +1,48 @@
+[!GOOS:windows] skip
+
+go run .
+stdout 'ws2_32.dll: not found'
+
+go run -tags net .
+stdout 'ws2_32.dll: found'
+
+-- go.mod --
+module m
+
+go 1.21
+
+-- utils.go --
+package main
+
+import (
+	"fmt"
+	"syscall"
+	"unsafe"
+)
+
+func hasModuleHandle() {
+	const ws2_32 = "ws2_32.dll"
+	getModuleHandle := syscall.MustLoadDLL("kernel32.dll").MustFindProc("GetModuleHandleW")
+	mod, _, _ := getModuleHandle.Call(uintptr(unsafe.Pointer(syscall.StringToUTF16Ptr(ws2_32))))
+	if mod != 0 {
+		fmt.Println(ws2_32+":", "found")
+	} else {
+		fmt.Println(ws2_32+":", "not found")
+	}
+}
+-- net.go --
+//go:build net
+package main
+
+import _ "net"
+
+func main() {
+    hasModuleHandle()
+}
+-- nonet.go --
+//go:build !net
+package main
+
+func main() {
+    hasModuleHandle()
+}

src/internal/poll/fd_windows.go

@@ -53,14 +53,17 @@ func checkSetFileCompletionNotificationModes() {
 	useSetFileCompletionNotificationModes = true
 }
 
-func init() {
+// InitWSA initiates the use of the Winsock DLL by the current process.
+// It is called from the net package at init time to avoid
+// loading ws2_32.dll when net is not used.
+var InitWSA = sync.OnceFunc(func() {
 	var d syscall.WSAData
 	e := syscall.WSAStartup(uint32(0x202), &d)
 	if e != nil {
 		initErr = e
 	}
 	checkSetFileCompletionNotificationModes()
-}
+})
 
 // operation contains superset of data necessary to perform all async IO.
 type operation struct {

src/internal/poll/fd_windows_test.go

@@ -41,6 +41,8 @@ func logFD(net string, fd *poll.FD, err error) {
 func init() {
 	loggedFDs = make(map[syscall.Handle]*loggedFD)
 	*poll.LogInitFD = logFD
+
+	poll.InitWSA()
 }
 
 func findLoggedFD(h syscall.Handle) (lfd *loggedFD, found bool) {

src/net/fd_windows.go

@@ -23,6 +23,10 @@ const (
 	writeMsgSyscallName = "wsasendmsg"
 )
 
+func init() {
+	poll.InitWSA()
+}
+
 // canUseConnectEx reports whether we can use the ConnectEx Windows API call
 // for the given network type.
 func canUseConnectEx(net string) bool {