fedsender: de-duplicate without sorting server names (#1073)

federationsender/api/perform.go

@@ -46,6 +46,7 @@ func (h *httpFederationSenderInternalAPI) PerformDirectoryLookup(
 type PerformJoinRequest struct {
 	RoomID string `json:"room_id"`
 	UserID string `json:"user_id"`
+	// The sorted list of servers to try. Servers will be tried sequentially, after de-duplication.
 	ServerNames types.ServerNames      `json:"server_names"`
 	Content     map[string]interface{} `json:"content"`
 }

federationsender/internal/perform.go

@@ -46,8 +46,19 @@ func (r *FederationSenderInternalAPI) PerformJoin(
 		supportedVersions = append(supportedVersions, version)
 	}
 
-	// Deduplicate the server names we were provided.
-	util.SortAndUnique(request.ServerNames)
+	// Deduplicate the server names we were provided but keep the ordering
+	// as this encodes useful information about which servers are most likely
+	// to respond.
+	seenSet := make(map[gomatrixserverlib.ServerName]bool)
+	var uniqueList []gomatrixserverlib.ServerName
+	for _, srv := range request.ServerNames {
+		if seenSet[srv] {
+			continue
+		}
+		seenSet[srv] = true
+		uniqueList = append(uniqueList, srv)
+	}
+	request.ServerNames = uniqueList
 
 	// Try each server that we were provided until we land on one that
 	// successfully completes the make-join send-join dance.