fix(federation): add participants array to relay events and fix recipient resolution

The relay was failing because processCreateEvent relied on the friends
table to discover the DM recipient, but friendships aren't federated
across instances. Also, resolveLocalUser matched deleted replicated
users before active ones.

- Add participants[] to FederationRelayEvent with homeUserId/homeInstance
  for all DM channel members
- Add getDmParticipants() helper to look up member identities
- Include participants in outbox payloads (create/update) and sync events
- Rewrite processCreateEvent to resolve participants directly, compute
  canonicalDmPairId, and findOrCreateDmChannel — removing the entire
  friends-list fallback (60+ lines)
- Fix resolveLocalUser to filter out deleted users (is_deleted = 0)
  and prefer the replicated user match when multiple candidates exist

packages/server/src/routes/dm.ts

@@ -21,7 +21,7 @@ import {
 import { sanitizeUser } from '../utils/sanitize.js';
 import { deleteUploadFile, deleteAttachmentFiles } from '../utils/fileCleanup.js';
 import { fetchDmEmbedsForMessages, resolveEmbeds, reResolveEmbeds, embedRowToEmbed } from '../utils/embedResolver.js';
-import { appendMutationLog, queueOutboxEvent, buildRelayPayload } from '../utils/federationOutbox.js';
+import { appendMutationLog, queueOutboxEvent, buildRelayPayload, getDmParticipants } from '../utils/federationOutbox.js';
 
 /**
  * Batch-fetch reactions for a set of DM message IDs.
@@ -973,8 +973,10 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
 
     // Federation: log mutation and queue for relay
     appendMutationLog(messageId, id, 'create');
+    const participants = getDmParticipants(id);
     queueOutboxEvent(messageId, id, 'create', JSON.stringify({
       message: { ...buildRelayPayload(message, message.user), attachments: [] },
+      participants,
     }));
 
     // Resolve embeds asynchronously after responding
@@ -1042,6 +1044,7 @@ export async function dmRoutes(app: FastifyInstance): Promise<void> {
     appendMutationLog(id, msg.dmChannelId, 'update');
     queueOutboxEvent(id, msg.dmChannelId, 'update', JSON.stringify({
       message: buildRelayPayload(updated, updated.user),
+      participants: getDmParticipants(msg.dmChannelId),
     }));
 
     // Resolve new embeds asynchronously (old ones already deleted above)

packages/server/src/routes/federation.ts

@@ -9,7 +9,7 @@ import { config } from '../config.js';
 import { connectionManager } from '../ws/handler.js';
 import { sanitizeUser } from '../utils/sanitize.js';
 import { deleteAttachmentFiles } from '../utils/fileCleanup.js';
-import { canonicalDmPairId } from '../utils/federationOutbox.js';
+import { canonicalDmPairId, getDmParticipants } from '../utils/federationOutbox.js';
 import { broadcastDmMessage } from './dm.js';
 import type { FederationRelayRequest, FederationRelayResponse, FederationRelayEvent, FederationRelayAttachment, FederationSyncRequest, FederationSyncResponse, DmMessageWithUser } from '@backspace/shared';
 
@@ -748,6 +748,7 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
           messageId: message.id,
           encryptionVersion: 0,
           timestamp: mutation.mutated_at,
+          participants: getDmParticipants(mutation.dm_channel_id),
           message: {
             userId: message.userId,
             homeUserId,
@@ -795,16 +796,25 @@ function resolveLocalUser(
   homeUserId: string,
   db: ReturnType<typeof getDb>,
 ): typeof schema.users.$inferSelect | undefined {
-  return db
+  const candidates = db
     .select()
     .from(schema.users)
     .where(
-      or(
-        eq(schema.users.homeUserId, homeUserId),
-        and(eq(schema.users.id, homeUserId), isNull(schema.users.homeInstance)),
+      and(
+        or(
+          eq(schema.users.homeUserId, homeUserId),
+          and(eq(schema.users.id, homeUserId), isNull(schema.users.homeInstance)),
+        ),
+        eq(schema.users.isDeleted, 0),
       ),
     )
-    .get();
+    .all();
+
+  // Prefer non-deleted active users; if multiple, prefer the one with homeUserId set
+  // (replicated user) over a local user match
+  if (candidates.length === 0) return undefined;
+  if (candidates.length === 1) return candidates[0];
+  return candidates.find(u => u.homeUserId === homeUserId) ?? candidates[0];
 }
 
 /**
@@ -933,10 +943,8 @@ function processCreateEvent(
     return;
   }
 
-  // Resolve the message author to a local user
-  const authorUser = resolveLocalUser(event.message.homeUserId, db);
-  if (!authorUser) {
-    rejected.push({ messageId: event.messageId, reason: 'user_not_found' });
+  if (!event.participants || event.participants.length < 2) {
+    rejected.push({ messageId: event.messageId, reason: 'missing_participants' });
     return;
   }
 
@@ -957,126 +965,46 @@ function processCreateEvent(
     return;
   }
 
-  // Resolve the DM recipient. Federated DMs are 1-on-1: one side is the author,
-  // the other is a local user on this instance. We match via canonical_pair_id.
-  const authorHomeUserId = event.message.homeUserId;
+  // Resolve ALL participants to local users
+  const resolvedParticipants: Array<{
+    localUser: typeof schema.users.$inferSelect;
+    homeUserId: string;
+  }> = [];
 
-  // First, search existing DM channels where the author is already a member
-  const authorMemberships = db
-    .select({ dmChannelId: schema.dmMembers.dmChannelId })
-    .from(schema.dmMembers)
-    .where(eq(schema.dmMembers.userId, authorUser.id))
-    .all();
-
-  let localDmChannelId: string | null = null;
-
-  // Check each of the author's DM channels to find the matching one
-  for (const membership of authorMemberships) {
-    const channelMembers = db
-      .select()
-      .from(schema.dmMembers)
-      .where(eq(schema.dmMembers.dmChannelId, membership.dmChannelId))
-      .all();
-
-    // For 1-on-1 DMs, there should be exactly 2 members
-    if (channelMembers.length === 2) {
-      const otherMember = channelMembers.find(m => m.userId !== authorUser.id);
-      if (otherMember) {
-        const otherUser = db
-          .select()
-          .from(schema.users)
-          .where(eq(schema.users.id, otherMember.userId))
-          .get();
-
-        if (otherUser) {
-          const otherHomeUserId = otherUser.homeUserId || otherUser.id;
-          const pairId = canonicalDmPairId(authorHomeUserId, otherHomeUserId);
-          const channel = db
-            .select()
-            .from(schema.dmChannels)
-            .where(eq(schema.dmChannels.id, membership.dmChannelId))
-            .get();
-
-          if (channel?.canonicalPairId === pairId) {
-            localDmChannelId = membership.dmChannelId;
-            break;
-          }
-        }
-      }
+  for (const p of event.participants) {
+    const localUser = resolveLocalUser(p.homeUserId, db);
+    if (localUser) {
+      resolvedParticipants.push({ localUser, homeUserId: p.homeUserId });
     }
   }
 
-  // If no existing channel found, search the author's friends for the recipient.
-  // On cold start (first federated DM), we use the friends list as a hint to
-  // find the local user and create the DM channel.
-  if (!localDmChannelId) {
-    const friendRows = db
-      .select()
-      .from(schema.friends)
-      .where(
-        or(
-          eq(schema.friends.userId, authorUser.id),
-          eq(schema.friends.friendId, authorUser.id),
-        ),
-      )
-      .all();
-
-    const friendIds = friendRows.map(f =>
-      f.userId === authorUser.id ? f.friendId : f.userId,
-    );
-
-    for (const friendId of friendIds) {
-      const friendUser = db
-        .select()
-        .from(schema.users)
-        .where(eq(schema.users.id, friendId))
-        .get();
-
-      if (friendUser) {
-        const friendHomeUserId = friendUser.homeUserId || friendUser.id;
-        const pairId = canonicalDmPairId(authorHomeUserId, friendHomeUserId);
-
-        // Check if a channel already exists with this pair ID
-        const existingChannel = db
-          .select()
-          .from(schema.dmChannels)
-          .where(eq(schema.dmChannels.canonicalPairId, pairId))
-          .get();
-
-        if (existingChannel) {
-          localDmChannelId = existingChannel.id;
-          break;
-        }
-      }
-    }
-
-    // If still no channel with a matching canonical pair ID, create one.
-    // The recipient must be a local (non-federated) user who is friends with the author.
-    if (!localDmChannelId && friendIds.length > 0) {
-      for (const friendId of friendIds) {
-        const friendUser = db
-          .select()
-          .from(schema.users)
-          .where(eq(schema.users.id, friendId))
-          .get();
-
-        if (friendUser && !friendUser.homeInstance) {
-          // This is a local user — they're a candidate recipient
-          const friendHomeUserId = friendUser.homeUserId || friendUser.id;
-          const pairId = canonicalDmPairId(authorHomeUserId, friendHomeUserId);
-
-          localDmChannelId = findOrCreateDmChannel(pairId, authorUser.id, friendId, db);
-          break;
-        }
-      }
-    }
-  }
-
-  if (!localDmChannelId) {
-    rejected.push({ messageId: event.messageId, reason: 'recipient_not_found' });
+  if (resolvedParticipants.length < 2) {
+    rejected.push({ messageId: event.messageId, reason: 'participant_not_found' });
     return;
   }
 
+  // Find the author among the resolved participants
+  const authorEntry = resolvedParticipants.find(
+    p => p.homeUserId === event.message!.homeUserId,
+  );
+  if (!authorEntry) {
+    rejected.push({ messageId: event.messageId, reason: 'author_not_found' });
+    return;
+  }
+  const authorUser = authorEntry.localUser;
+
+  // Compute canonical pair ID from participants' home user IDs and find/create channel
+  const pairId = canonicalDmPairId(
+    resolvedParticipants[0]!.homeUserId,
+    resolvedParticipants[1]!.homeUserId,
+  );
+  const localDmChannelId = findOrCreateDmChannel(
+    pairId,
+    resolvedParticipants[0]!.localUser.id,
+    resolvedParticipants[1]!.localUser.id,
+    db,
+  );
+
   // Insert the message
   const localMessageId = generateSnowflake();
   db.insert(schema.dmMessages)