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

3 files changed, 29 insertions, 7 deletions

diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index 5f024d26..b272b1eb 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -21,6 +21,7 @@ import (
 	"errors"
 	"net/http"
 
+	"github.com/matrix-org/dendrite/common/caching"
 	commonHTTP "github.com/matrix-org/dendrite/common/http"
 	"github.com/matrix-org/gomatrixserverlib"
 	opentracing "github.com/opentracing/opentracing-go"
@@ -411,16 +412,17 @@ const RoomserverQueryRoomVersionForRoomPath = "/api/roomserver/queryRoomVersionF
 
 // NewRoomserverQueryAPIHTTP creates a RoomserverQueryAPI implemented by talking to a HTTP POST API.
 // If httpClient is nil an error is returned
-func NewRoomserverQueryAPIHTTP(roomserverURL string, httpClient *http.Client) (RoomserverQueryAPI, error) {
+func NewRoomserverQueryAPIHTTP(roomserverURL string, httpClient *http.Client, cache caching.ImmutableCache) (RoomserverQueryAPI, error) {
 	if httpClient == nil {
 		return nil, errors.New("NewRoomserverQueryAPIHTTP: httpClient is <nil>")
 	}
-	return &httpRoomserverQueryAPI{roomserverURL, httpClient}, nil
+	return &httpRoomserverQueryAPI{roomserverURL, httpClient, cache}, nil
 }
 
 type httpRoomserverQueryAPI struct {
-	roomserverURL string
-	httpClient    *http.Client
+	roomserverURL  string
+	httpClient     *http.Client
+	immutableCache caching.ImmutableCache
 }
 
 // QueryLatestEventsAndState implements RoomserverQueryAPI
@@ -585,9 +587,18 @@ func (h *httpRoomserverQueryAPI) QueryRoomVersionForRoom(
 	request *QueryRoomVersionForRoomRequest,
 	response *QueryRoomVersionForRoomResponse,
 ) error {
+	if roomVersion, ok := h.immutableCache.GetRoomVersion(request.RoomID); ok {
+		response.RoomVersion = roomVersion
+		return nil
+	}
+
 	span, ctx := opentracing.StartSpanFromContext(ctx, "QueryRoomVersionForRoom")
 	defer span.Finish()
 
 	apiURL := h.roomserverURL + RoomserverQueryRoomVersionForRoomPath
-	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	err := commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	if err == nil {
+		h.immutableCache.StoreRoomVersion(request.RoomID, response.RoomVersion)
+	}
+	return err
 }
diff --git a/roomserver/query/query.go b/roomserver/query/query.go
index 12d8436e..224d9fa2 100644
--- a/roomserver/query/query.go
+++ b/roomserver/query/query.go
@@ -22,6 +22,7 @@ import (
 	"net/http"
 
 	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/common/caching"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/auth"
 	"github.com/matrix-org/dendrite/roomserver/state"
@@ -97,7 +98,8 @@ type RoomserverQueryAPIDatabase interface {
 
 // RoomserverQueryAPI is an implementation of api.RoomserverQueryAPI
 type RoomserverQueryAPI struct {
-	DB RoomserverQueryAPIDatabase
+	DB             RoomserverQueryAPIDatabase
+	ImmutableCache caching.ImmutableCache
 }
 
 // QueryLatestEventsAndState implements api.RoomserverQueryAPI
@@ -896,11 +898,17 @@ func (r *RoomserverQueryAPI) QueryRoomVersionForRoom(
 	request *api.QueryRoomVersionForRoomRequest,
 	response *api.QueryRoomVersionForRoomResponse,
 ) error {
+	if roomVersion, ok := r.ImmutableCache.GetRoomVersion(request.RoomID); ok {
+		response.RoomVersion = roomVersion
+		return nil
+	}
+
 	roomVersion, err := r.DB.GetRoomVersionForRoom(ctx, request.RoomID)
 	if err != nil {
 		return err
 	}
 	response.RoomVersion = roomVersion
+	r.ImmutableCache.StoreRoomVersion(request.RoomID, response.RoomVersion)
 	return nil
 }
 
diff --git a/roomserver/roomserver.go b/roomserver/roomserver.go
index 2ffbf67d..fa4f2062 100644
--- a/roomserver/roomserver.go
+++ b/roomserver/roomserver.go
@@ -48,7 +48,10 @@ func SetupRoomServerComponent(
 
 	inputAPI.SetupHTTP(http.DefaultServeMux)
 
-	queryAPI := query.RoomserverQueryAPI{DB: roomserverDB}
+	queryAPI := query.RoomserverQueryAPI{
+		DB:             roomserverDB,
+		ImmutableCache: base.ImmutableCache,
+	}
 
 	queryAPI.SetupHTTP(http.DefaultServeMux)