LRU cache for room versions in RS query API (#976)

* Experimental LRU cache for room versions

* Don't accidentally try to type-assert nil

* Also reduce hits on query API

* Use hashicorp implementation which mutexes for us

* Define const for max cache entries

* Rename to be specifically immutable, panic if we try to mutate a cache entry

* Review comments

* Remove nil guards, give roomserver integration test a cache

* go mod tidy

common/caching/immutablecache.go

@@ -0,0 +1,12 @@
+package caching
+
+import "github.com/matrix-org/gomatrixserverlib"
+
+const (
+	RoomVersionMaxCacheEntries = 128
+)
+
+type ImmutableCache interface {
+	GetRoomVersion(roomId string) (gomatrixserverlib.RoomVersion, bool)
+	StoreRoomVersion(roomId string, roomVersion gomatrixserverlib.RoomVersion)
+}

common/caching/immutableinmemorylru.go

@@ -0,0 +1,43 @@
+package caching
+
+import (
+	"fmt"
+
+	lru "github.com/hashicorp/golang-lru"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+type ImmutableInMemoryLRUCache struct {
+	roomVersions *lru.Cache
+}
+
+func NewImmutableInMemoryLRUCache() (*ImmutableInMemoryLRUCache, error) {
+	roomVersionCache, rvErr := lru.New(RoomVersionMaxCacheEntries)
+	if rvErr != nil {
+		return nil, rvErr
+	}
+	return &ImmutableInMemoryLRUCache{
+		roomVersions: roomVersionCache,
+	}, nil
+}
+
+func checkForInvalidMutation(cache *lru.Cache, key string, value interface{}) {
+	if peek, ok := cache.Peek(key); ok && peek != value {
+		panic(fmt.Sprintf("invalid use of immutable cache tries to mutate existing value of %q", key))
+	}
+}
+
+func (c *ImmutableInMemoryLRUCache) GetRoomVersion(roomID string) (gomatrixserverlib.RoomVersion, bool) {
+	val, found := c.roomVersions.Get(roomID)
+	if found && val != nil {
+		if roomVersion, ok := val.(gomatrixserverlib.RoomVersion); ok {
+			return roomVersion, true
+		}
+	}
+	return "", false
+}
+
+func (c *ImmutableInMemoryLRUCache) StoreRoomVersion(roomID string, roomVersion gomatrixserverlib.RoomVersion) {
+	checkForInvalidMutation(c.roomVersions, roomID, roomVersion)
+	c.roomVersions.Add(roomID, roomVersion)
+}