ipn/{ipnlocal,ipnserver}: add some comments

Change-Id: Ieb5917edaf572342b755caa458693512c7aece81
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -2012,6 +2012,17 @@ func (b *LocalBackend) State() ipn.State {
 	return b.state
 }
 
+// InServerMode reports whether the Tailscale backend is explicitly running in
+// "server mode" where it continues to run despite whatever the platform's
+// default is. In practice, this is only used on Windows, where the default
+// tailscaled behavior is to shut down whenever the GUI disconnects.
+//
+// On non-Windows platforms, this usually returns false (because people don't
+// set unattended mode on other platforms) and also isn't checked on other
+// platforms.
+//
+// TODO(bradfitz): rename to InWindowsUnattendedMode or something? Or make this
+// return true on Linux etc and always be called? It's kinda messy now.
 func (b *LocalBackend) InServerMode() bool {
 	b.mu.Lock()
 	defer b.mu.Unlock()

ipn/ipnserver/server.go

@@ -100,6 +100,8 @@ type Server struct {
 	bsMu sync.Mutex // lock order: bsMu, then mu
 	bs   *ipn.BackendServer
 
+	// mu guards the fields that follow.
+	// lock order: mu, then LocalBackend.mu
 	mu            sync.Mutex
 	lastUserID    string                             // tracks last userid; on change, Reset state for paranoia
 	allClients    map[net.Conn]*ipnauth.ConnIdentity // HTTP or IPN