Add RoomInfo cache, remove RoomServerRoomNIDsCache (#1646)

* Add RoomInfo cache, remove RoomServerRoomNID cache, ensure caches are thread-safe

* Don't panic if the roomInfo isn't known yet

* LRU package is already threadsafe

* Use RoomInfo cache to find room version if possible in Events()

* Adding comments about RoomInfoCache safety

internal/caching/cache_roominfo.go

@@ -0,0 +1,45 @@
+package caching
+
+import (
+	"github.com/matrix-org/dendrite/roomserver/types"
+)
+
+// WARNING: This cache is mutable because it's entirely possible that
+// the IsStub or StateSnaphotNID fields can change, even though the
+// room version and room NID fields will not. This is only safe because
+// the RoomInfoCache is used ONLY within the roomserver and because it
+// will be kept up-to-date by the latest events updater. It MUST NOT be
+// used from other components as we currently have no way to invalidate
+// the cache in downstream components.
+
+const (
+	RoomInfoCacheName       = "roominfo"
+	RoomInfoCacheMaxEntries = 1024
+	RoomInfoCacheMutable    = true
+)
+
+// RoomInfosCache contains the subset of functions needed for
+// a room Info cache. It must only be used from the roomserver only
+// It is not safe for use from other components.
+type RoomInfoCache interface {
+	GetRoomInfo(roomID string) (roomInfo types.RoomInfo, ok bool)
+	StoreRoomInfo(roomID string, roomInfo types.RoomInfo)
+}
+
+// GetRoomInfo must only be called from the roomserver only. It is not
+// safe for use from other components.
+func (c Caches) GetRoomInfo(roomID string) (types.RoomInfo, bool) {
+	val, found := c.RoomInfos.Get(roomID)
+	if found && val != nil {
+		if roomInfo, ok := val.(types.RoomInfo); ok {
+			return roomInfo, true
+		}
+	}
+	return types.RoomInfo{}, false
+}
+
+// StoreRoomInfo must only be called from the roomserver only. It is not
+// safe for use from other components.
+func (c Caches) StoreRoomInfo(roomID string, roomInfo types.RoomInfo) {
+	c.RoomInfos.Set(roomID, roomInfo)
+}