state: Cache CurrentApplication

This commit adds these methods into *state.State:

    - (*State).CurrentApplication
    - (*State).CurrentApplicationID

This should make interacting with API functions that require AppIDs much
more pleasant.

state/state.go

@@ -77,9 +77,6 @@ type State struct {
 
 	// *: State doesn't actually keep track of pinned messages.
 
-	readyMu *sync.Mutex
-	ready   gateway.ReadyEvent
-
 	// StateLog logs all errors that come from the state cache. This includes
 	// not found errors. Defaults to a no-op, as state errors aren't that
 	// important.
@@ -95,6 +92,9 @@ type State struct {
 	// with the State.
 	*handler.Handler
 
+	readyMu *sync.Mutex
+	ready   *gateway.ReadyEvent
+
 	// List of channels with few messages, so it doesn't bother hitting the API
 	// again.
 	fewMessages map[discord.ChannelID]struct{}
@@ -109,6 +109,10 @@ type State struct {
 	// they will be removed.
 	unreadyGuilds map[discord.GuildID]struct{}
 	guildMutex    *sync.Mutex
+
+	// TODO: box this into a Once[T] for v4
+	currentApp *discord.Application
+	appMutex   *sync.Mutex
 }
 
 // New creates a new state.
@@ -148,6 +152,7 @@ func NewFromSession(s *session.Session, cabinet *store.Cabinet) *State {
 		unavailableGuilds: make(map[discord.GuildID]struct{}),
 		unreadyGuilds:     make(map[discord.GuildID]struct{}),
 		guildMutex:        new(sync.Mutex),
+		appMutex:          new(sync.Mutex),
 	}
 	state.hookSession()
 	return state
@@ -174,7 +179,10 @@ func (s *State) Ready() gateway.ReadyEvent {
 	r := s.ready
 	s.readyMu.Unlock()
 
-	return r
+	if r != nil {
+		return *r
+	}
+	return gateway.ReadyEvent{}
 }
 
 //// Helper methods
@@ -328,6 +336,28 @@ func (s *State) Permissions(
 
 ////
 
+func (s *State) CurrentApplication() (*discord.Application, error) {
+	s.appMutex.Lock()
+	defer s.appMutex.Unlock()
+
+	var err error
+	if s.currentApp == nil {
+		s.currentApp, err = s.Client.CurrentApplication()
+	}
+
+	return s.currentApp, err
+}
+
+func (s *State) CurrentApplicationID() discord.AppID {
+	app, _ := s.CurrentApplication()
+	if app != nil {
+		return app.ID
+	}
+	return 0
+}
+
+////
+
 func (s *State) Me() (*discord.User, error) {
 	u, err := s.Cabinet.Me()
 	if err == nil {