Fixed a potential race condition in the default state storage

state/store.go

@@ -16,6 +16,11 @@ type Store interface {
 // All methods in StoreGetter will be wrapped by the State. If the State can't
 // find anything in the storage, it will call the API itself and automatically
 // add what's missing into the storage.
+//
+// Methods that return with a slice should pay attention to race conditions that
+// would mutate the underlying slice (and as a result the returned slice as
+// well). The best way to avoid this is to copy the whole slice, like
+// DefaultStore does.
 type StoreGetter interface {
 	Self() (*discord.User, error)
 

state/store_default.go

@@ -114,7 +114,7 @@ func (s *DefaultStore) Channels(
 		return nil, ErrStoreNotFound
 	}
 
-	return chs, nil
+	return append([]discord.Channel{}, chs...), nil
 }
 
 func (s *DefaultStore) PrivateChannels() ([]discord.Channel, error) {
@@ -221,7 +221,7 @@ func (s *DefaultStore) Emojis(
 		return nil, ErrStoreNotFound
 	}
 
-	return gd.Emojis, nil
+	return append([]discord.Emoji{}, gd.Emojis...), nil
 }
 
 func (s *DefaultStore) EmojiSet(
@@ -353,7 +353,7 @@ func (s *DefaultStore) Members(
 		return nil, ErrStoreNotFound
 	}
 
-	return ms, nil
+	return append([]discord.Member{}, ms...), nil
 }
 
 func (s *DefaultStore) MemberSet(
@@ -437,7 +437,7 @@ func (s *DefaultStore) Messages(
 		return nil, ErrStoreNotFound
 	}
 
-	return ms, nil
+	return append([]discord.Message{}, ms...), nil
 }
 
 func (s *DefaultStore) MaxMessages() int {
@@ -561,7 +561,7 @@ func (s *DefaultStore) Presences(
 		return nil, ErrStoreNotFound
 	}
 
-	return ps, nil
+	return append([]discord.Presence{}, ps...), nil
 }
 
 func (s *DefaultStore) PresenceSet(
@@ -640,7 +640,7 @@ func (s *DefaultStore) Roles(
 		return nil, ErrStoreNotFound
 	}
 
-	return gd.Roles, nil
+	return append([]discord.Role{}, gd.Roles...), nil
 }
 
 func (s *DefaultStore) RoleSet(