net: use closesocket when closing socket os.File's on Windows

The WSASocket documentation states that the returned socket must be
closed by calling closesocket instead of CloseHandle. The different
File methods on the net package return an os.File that is not aware
that it should use closesocket. Ideally, os.NewFile should detect that
the passed handle is a socket and use the appropriate close function,
but there is no reliable way to detect that a handle is a socket on
Windows (see CL 671455).

To work around this, we add a hidden function to the os package that
can be used to return an os.File that uses closesocket. This approach
is the same as used on Unix, which also uses a hidden function for other
purposes.

While here, fix a potential issue with FileConn, which was using File.Fd
rather than File.SyscallConn to get the handle. This could result in the
File being closed and garbage collected before the syscall was made.

Fixes #73683.

Change-Id: I179405f34c63cbbd555d8119e0f77157c670eb3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/672195
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: qmuntal

src/internal/poll/fd_windows.go

@@ -318,7 +318,7 @@ type FD struct {
 	// message based socket connection.
 	ZeroReadIsEOF bool
 
-	// Whether this is a file rather than a network socket.
+	// Whether the handle is owned by os.File.
 	isFile bool
 
 	// The kind of this file.
@@ -368,6 +368,7 @@ const (
 	kindFile
 	kindConsole
 	kindPipe
+	kindFileNet
 )
 
 // Init initializes the FD. The Sysfd field should already be set.
@@ -388,6 +389,8 @@ func (fd *FD) Init(net string, pollable bool) error {
 		fd.kind = kindConsole
 	case "pipe":
 		fd.kind = kindPipe
+	case "file+net":
+		fd.kind = kindFileNet
 	default:
 		// We don't actually care about the various network types.
 		fd.kind = kindNet
@@ -453,7 +456,7 @@ func (fd *FD) destroy() error {
 	fd.pd.close()
 	var err error
 	switch fd.kind {
-	case kindNet:
+	case kindNet, kindFileNet:
 		// The net package uses the CloseFunc variable for testing.
 		err = CloseFunc(fd.Sysfd)
 	default:
@@ -494,7 +497,7 @@ func (fd *FD) Read(buf []byte) (int, error) {
 		return 0, err
 	}
 	defer fd.readUnlock()
-	if fd.isFile {
+	if fd.kind == kindFile {
 		fd.l.Lock()
 		defer fd.l.Unlock()
 	}
@@ -747,7 +750,7 @@ func (fd *FD) Write(buf []byte) (int, error) {
 		return 0, err
 	}
 	defer fd.writeUnlock()
-	if fd.isFile {
+	if fd.kind == kindFile {
 		fd.l.Lock()
 		defer fd.l.Unlock()
 	}

src/net/fd_windows.go

@@ -233,6 +233,9 @@ func (fd *netFD) accept() (*netFD, error) {
 	return netfd, nil
 }
 
+// Defined in os package.
+func newWindowsFile(h syscall.Handle, name string) *os.File
+
 func (fd *netFD) dup() (*os.File, error) {
 	// Disassociate the IOCP from the socket,
 	// it is not safe to share a duplicated handle
@@ -251,5 +254,8 @@ func (fd *netFD) dup() (*os.File, error) {
 	if err != nil {
 		return nil, err
 	}
-	return os.NewFile(uintptr(h), fd.name()), nil
+	// All WSASocket calls must be match with a syscall.Closesocket call,
+	// but os.NewFile calls syscall.CloseHandle instead. We need to use
+	// a hidden function so that the returned file is aware of this fact.
+	return newWindowsFile(h, fd.name()), nil
 }