Add prefer_direct_fetch option

dendrite-config.yaml

@@ -274,6 +274,11 @@ server_key_api:
     - key_id: ed25519:a_RXGa
       public_key: l8Hft5qXKn1vfHrg3p4+W8gELQVo8N13JkluMfmn2sQ
 
+  # This option will control whether Dendrite will prefer to look up keys directly
+  # or whether it should try perspective servers first, using direct fetches as a
+  # last resort.
+  prefer_direct_fetch: false
+
 # Configuration for the Sync API.
 sync_api:
   internal_api:

internal/config/config_serverkey.go

@@ -14,6 +14,9 @@ type ServerKeyAPI struct {
 	// Perspective keyservers, to use as a backup when direct key fetch
 	// requests don't succeed
 	KeyPerspectives KeyPerspectives `yaml:"key_perspectives"`
+
+	// Should we prefer direct key fetches over perspective ones?
+	PreferDirectFetch bool `yaml:"prefer_direct_fetch"`
 }
 
 func (c *ServerKeyAPI) Defaults() {

serverkeyapi/serverkeyapi.go

@@ -51,15 +51,26 @@ func NewInternalAPI(
 		ServerKeyValidity: cfg.Matrix.KeyValidityPeriod,
 		FedClient:         fedClient,
 		OurKeyRing: gomatrixserverlib.KeyRing{
-			KeyFetchers: []gomatrixserverlib.KeyFetcher{
+			KeyFetchers: []gomatrixserverlib.KeyFetcher{},
-				&gomatrixserverlib.DirectKeyFetcher{
-					Client: fedClient,
-				},
-			},
 			KeyDatabase: serverKeyDB,
 		},
 	}
 
+	addDirectFetcher := func() {
+		internalAPI.OurKeyRing.KeyFetchers = append(
+			internalAPI.OurKeyRing.KeyFetchers,
+			&gomatrixserverlib.DirectKeyFetcher{
+				Client: fedClient,
+			},
+		)
+	}
+
+	if cfg.PreferDirectFetch {
+		addDirectFetcher()
+	} else {
+		defer addDirectFetcher()
+	}
+
 	var b64e = base64.StdEncoding.WithPadding(base64.NoPadding)
 	for _, ps := range cfg.KeyPerspectives {
 		perspective := &gomatrixserverlib.PerspectiveKeyFetcher{