From f7ba3916788907ef4cf716532bba93614344fdf8 Mon Sep 17 00:00:00 2001
From: Jannis Braun <151788261+TheZwiss@users.noreply.github.com>
Date: Thu, 26 Mar 2026 20:26:23 +0100
Subject: [PATCH] feat(federation): handle group DM messages via federatedId
 lookup in relay processor

In processCreateEvent, branch on event.federatedId: group DM messages now
look up the pre-bootstrapped local channel by federatedId instead of
computing a pair hash from two participants. In queueDmRelay, fetch the
channel's federatedId and ownerId and include federatedId in the outgoing
relay payload for group DMs so receiving instances can route correctly.
---
 packages/server/src/routes/federation.ts      | 43 ++++++++++++++-----
 packages/server/src/utils/federationOutbox.ts |  9 ++++
 2 files changed, 42 insertions(+), 10 deletions(-)

diff --git a/packages/server/src/routes/federation.ts b/packages/server/src/routes/federation.ts
index f27ff216..62e7c567 100644
--- a/packages/server/src/routes/federation.ts
+++ b/packages/server/src/routes/federation.ts
@@ -1002,16 +1002,39 @@ function processCreateEvent(
   }
   const authorUser = authorEntry.localUser;
 
-  // Compute federated ID from participants' home user IDs and find/create channel
-  const federatedId = computeFederatedId(
-    resolvedParticipants[0]!.homeUserId,
-    resolvedParticipants[1]!.homeUserId,
-  );
-  const localDmChannelId = findOrCreateDmChannel(
-    federatedId,
-    [resolvedParticipants[0]!.localUser.id, resolvedParticipants[1]!.localUser.id],
-    db,
-  );
+  // Resolve local DM channel: group DMs carry a federatedId and the channel
+  // must already exist (bootstrapped by a prior member_add event); 1-on-1 DMs
+  // are computed from the pair of home user IDs and created on demand.
+  let localDmChannelId: string;
+
+  if (event.federatedId) {
+    // Group DM: look up by federated_id (channel must already exist from member_add bootstrap)
+    const channel = db
+      .select()
+      .from(schema.dmChannels)
+      .where(and(
+        eq(schema.dmChannels.federatedId, event.federatedId),
+        isNull(schema.dmChannels.deletedAt),
+      ))
+      .get();
+
+    if (!channel) {
+      rejected.push({ messageId: event.messageId, reason: 'channel_not_found' });
+      return;
+    }
+    localDmChannelId = channel.id;
+  } else {
+    // 1-on-1 DM: compute federated_id from pair and find/create channel
+    const federatedId = computeFederatedId(
+      resolvedParticipants[0]!.homeUserId,
+      resolvedParticipants[1]!.homeUserId,
+    );
+    localDmChannelId = findOrCreateDmChannel(
+      federatedId,
+      [resolvedParticipants[0]!.localUser.id, resolvedParticipants[1]!.localUser.id],
+      db,
+    );
+  }
 
   // Insert the message
   const localMessageId = generateSnowflake();
diff --git a/packages/server/src/utils/federationOutbox.ts b/packages/server/src/utils/federationOutbox.ts
index de1c7d6e..92e1db10 100644
--- a/packages/server/src/utils/federationOutbox.ts
+++ b/packages/server/src/utils/federationOutbox.ts
@@ -318,8 +318,17 @@ export function queueDmRelay(
 
   const targetOrigins = getGroupDmTargetOrigins(dmChannelId);
 
+  // Fetch channel to check if it's a group DM with a federatedId
+  const db = getDb();
+  const channel = db
+    .select({ federatedId: schema.dmChannels.federatedId, ownerId: schema.dmChannels.ownerId })
+    .from(schema.dmChannels)
+    .where(eq(schema.dmChannels.id, dmChannelId))
+    .get();
+
   appendMutationLog(message.id, dmChannelId, eventType);
   queueOutboxEvent(message.id, dmChannelId, eventType, JSON.stringify({
+    ...(channel?.federatedId && channel.ownerId ? { federatedId: channel.federatedId } : {}),
     message: {
       ...buildRelayPayload(message, message.user),
       attachments: attachments.length > 0 ? attachments : undefined,