wgengine/winnet: invoke some COM methods directly instead of through IDispatch.

Intermittently in the wild we are seeing failures when calling
`INetworkConnection::GetNetwork`. It is unclear what the root cause is, but what
is clear is that the error is happening inside the object's `IDispatch` invoker
(as opposed to the method implementation itself).

This patch replaces our wrapper for `INetworkConnection::GetNetwork` with an
alternate implementation that directly invokes the method, instead of using
`IDispatch`. I also replaced the implementations of `INetwork::SetCategory` and
`INetwork::GetCategory` while I was there.

This patch is speculative and tightly-scoped so that we could possibly add it
to a dot-release if necessary.

Updates https://github.com/tailscale/tailscale/issues/4134
Updates https://github.com/tailscale/tailscale/issues/6037

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

wgengine/winnet/winnet.go

@@ -9,6 +9,7 @@ package winnet
 
 import (
 	"fmt"
+	"syscall"
 	"unsafe"
 
 	"github.com/go-ole/go-ole"
@@ -45,6 +46,23 @@ type INetwork struct {
 	ole.IDispatch
 }
 
+type INetworkVtbl struct {
+	ole.IDispatchVtbl
+	GetName                    uintptr
+	SetName                    uintptr
+	GetDescription             uintptr
+	SetDescription             uintptr
+	GetNetworkId               uintptr
+	GetDomainType              uintptr
+	GetNetworkConnections      uintptr
+	GetTimeCreatedAndConnected uintptr
+	Get_IsConnectedToInternet  uintptr
+	Get_IsConnected            uintptr
+	GetConnectivity            uintptr
+	GetCategory                uintptr
+	SetCategory                uintptr
+}
+
 func NewNetworkListManager(c *ole.Connection) (*NetworkListManager, error) {
 	err := c.Create(CLSID_NetworkListManager)
 	if err != nil {
@@ -124,16 +142,35 @@ func (n *INetwork) GetName() (string, error) {
 }
 
 func (n *INetwork) GetCategory() (int32, error) {
-	v, err := n.CallMethod("GetCategory")
-	if err != nil {
-		return 0, err
+	var result int32
+
+	r, _, _ := syscall.SyscallN(
+		n.VTable().GetCategory,
+		uintptr(unsafe.Pointer(n)),
+		uintptr(unsafe.Pointer(&result)),
+	)
+	if int32(r) < 0 {
+		return 0, ole.NewError(r)
 	}
-	return v.Value().(int32), err
+
+	return result, nil
 }
 
-func (n *INetwork) SetCategory(v uint32) error {
-	_, err := n.CallMethod("SetCategory", v)
-	return err
+func (n *INetwork) SetCategory(v int32) error {
+	r, _, _ := syscall.SyscallN(
+		n.VTable().SetCategory,
+		uintptr(unsafe.Pointer(n)),
+		uintptr(v),
+	)
+	if int32(r) < 0 {
+		return ole.NewError(r)
+	}
+
+	return nil
+}
+
+func (n *INetwork) VTable() *INetworkVtbl {
+	return (*INetworkVtbl)(unsafe.Pointer(n.RawVTable))
 }
 
 func (v *INetworkConnection) VTable() *INetworkConnectionVtbl {
@@ -141,17 +178,16 @@ func (v *INetworkConnection) VTable() *INetworkConnectionVtbl {
 }
 
 func (v *INetworkConnection) GetNetwork() (*INetwork, error) {
-	nraw, err := v.CallMethod("GetNetwork")
-	if err != nil {
-		return nil, err
+	var result *INetwork
+
+	r, _, _ := syscall.SyscallN(
+		v.VTable().GetNetwork,
+		uintptr(unsafe.Pointer(v)),
+		uintptr(unsafe.Pointer(&result)),
+	)
+	if int32(r) < 0 {
+		return nil, ole.NewError(r)
 	}
 
-	n := nraw.ToIDispatch()
-	if n == nil {
-		return nil, fmt.Errorf("GetNetwork: nil IDispatch")
-	}
-	if err != nil {
-		return nil, err
-	}
-	return (*INetwork)(unsafe.Pointer(n)), nil
+	return result, nil
 }