From fe5cf6f880cdb906006a6ca215872949c3388272 Mon Sep 17 00:00:00 2001
From: Kegsay <kegan@matrix.org>
Date: Fri, 29 May 2020 13:50:06 +0100
Subject: [PATCH] fedsender: de-duplicate without sorting server names (#1073)

---
 federationsender/api/perform.go      |  5 +++--
 federationsender/internal/perform.go | 15 +++++++++++++--
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/federationsender/api/perform.go b/federationsender/api/perform.go
index a73ba0475..2a1834b61 100644
--- a/federationsender/api/perform.go
+++ b/federationsender/api/perform.go
@@ -44,8 +44,9 @@ func (h *httpFederationSenderInternalAPI) PerformDirectoryLookup(
 }
 
 type PerformJoinRequest struct {
-	RoomID      string                 `json:"room_id"`
-	UserID      string                 `json:"user_id"`
+	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"`
 }
diff --git a/federationsender/internal/perform.go b/federationsender/internal/perform.go
index 9791afefa..383ce4888 100644
--- a/federationsender/internal/perform.go
+++ b/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.