ipn/{ipnext,ipnlocal}: add a SafeBackend interface

Updates #12614

Change-Id: I197e673666e86ea74c19e3935ed71aec269b6c94
Co-authored-by: Nick Khyl <nickk@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -525,7 +525,7 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 		}
 	}
 
-	if b.extHost, err = NewExtensionHost(logf, sys, b); err != nil {
+	if b.extHost, err = NewExtensionHost(logf, b); err != nil {
 		return nil, fmt.Errorf("failed to create extension host: %w", err)
 	}
 	b.pm.SetExtensionHost(b.extHost)
@@ -589,6 +589,7 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 }
 
 func (b *LocalBackend) Clock() tstime.Clock { return b.clock }
+func (b *LocalBackend) Sys() *tsd.System    { return b.sys }
 
 // FindExtensionByName returns an active extension with the given name,
 // or nil if no such extension exists.
@@ -3187,6 +3188,12 @@ func (b *LocalBackend) send(n ipn.Notify) {
 	b.sendTo(n, allClients)
 }
 
+// SendNotify sends a notification to the IPN bus,
+// typically to the GUI client.
+func (b *LocalBackend) SendNotify(n ipn.Notify) {
+	b.send(n)
+}
+
 // notificationTarget describes a notification recipient.
 // A zero value is valid and indicate that the notification
 // should be broadcast to all active [watchSession]s.