feat: outbound S2S read state relay

Queue read_state_update events when users ack DM messages on
channels with a federatedId. Translates local message IDs to
federation coordinates using sourceInstance/sourceMessageId.

packages/server/src/utils/federationOutbox.ts

@@ -472,6 +472,76 @@ export async function sendCallRelay(
   }
 }
 
+/**
+ * Queue a read_state_update relay event for cross-instance read state sync.
+ * Translates a local channel ack into federation coordinates using the
+ * message's sourceInstance/sourceMessageId mapping.
+ */
+export function queueReadStateRelay(
+  channelId: string,
+  messageId: string,
+  userId: string,
+): void {
+  if (!isFederationRelayEnabled()) return;
+
+  const db = getDb();
+  const ourOrigin = getOurOrigin();
+
+  // Channel must have a federatedId for cross-instance sync
+  const channel = db.select({ federatedId: schema.dmChannels.federatedId, ownerId: schema.dmChannels.ownerId })
+    .from(schema.dmChannels)
+    .where(eq(schema.dmChannels.id, channelId))
+    .get();
+  if (!channel?.federatedId) return;
+
+  // Resolve the user's federated identity
+  const user = db.select({ homeUserId: schema.users.homeUserId, homeInstance: schema.users.homeInstance })
+    .from(schema.users)
+    .where(eq(schema.users.id, userId))
+    .get();
+  if (!user) return;
+
+  const homeUserId = user.homeUserId || userId;
+  const homeInstance = user.homeInstance || ourOrigin;
+
+  // Determine the acked message's federation coordinates
+  const msg = db.select({ sourceInstance: schema.dmMessages.sourceInstance, sourceMessageId: schema.dmMessages.sourceMessageId })
+    .from(schema.dmMessages)
+    .where(eq(schema.dmMessages.id, messageId))
+    .get();
+
+  let messageRef: { sourceInstance: string; sourceMessageId: string };
+  if (msg?.sourceInstance && msg?.sourceMessageId) {
+    // Message was relayed here — use its original coordinates
+    messageRef = { sourceInstance: msg.sourceInstance, sourceMessageId: msg.sourceMessageId };
+  } else {
+    // Message originated on this instance
+    messageRef = { sourceInstance: ourOrigin, sourceMessageId: messageId };
+  }
+
+  const payload: FederationRelayEvent = {
+    eventType: 'read_state_update',
+    dmChannelId: channelId,
+    messageId: `read_state:${userId}:${Date.now()}`,
+    federatedId: channel.federatedId,
+    encryptionVersion: 0,
+    timestamp: Date.now(),
+    readState: {
+      user: { homeUserId, homeInstance },
+      messageRef,
+    },
+  };
+
+  const targetOrigins = getGroupDmTargetOrigins(channelId);
+  queueOutboxEvent(
+    `read_state:${channel.federatedId}:${userId}`,
+    channelId,
+    'read_state_update',
+    JSON.stringify(payload),
+    targetOrigins,
+  );
+}
+
 /**
  * Send typing indicator events directly to remote peers (bypasses outbox).
  * Fire-and-forget — typing is ephemeral, lost packets are acceptable.

packages/server/src/ws/events.ts

@@ -11,7 +11,7 @@ import { ACTIVITY_LIMITS } from '@backspace/shared/src/activities.js';
 import { sanitizeUser } from '../utils/sanitize.js';
 import { deleteAttachmentFiles } from '../utils/fileCleanup.js';
 import { resolveEmbeds, reResolveEmbeds, embedRowToEmbed } from '../utils/embedResolver.js';
-import { appendMutationLog, queueOutboxEvent, queueDmRelay, getGroupDmTargetOrigins, sendCallRelay, computeFederatedId, sendTypingRelay } from '../utils/federationOutbox.js';
+import { appendMutationLog, queueOutboxEvent, queueDmRelay, getGroupDmTargetOrigins, sendCallRelay, computeFederatedId, sendTypingRelay, queueReadStateRelay } from '../utils/federationOutbox.js';
 import { getOurOrigin } from '../utils/federationAuth.js';
 import { generateFederatedCallToken } from '../routes/livekit.js';
 import { config } from '../config.js';
@@ -1320,6 +1320,9 @@ function handleChannelAck(event: Record<string, unknown>, userId: string, isFede
     channelId,
     messageId,
   });
+
+  // Relay read state to federated peers for cross-instance sync
+  queueReadStateRelay(channelId, messageId, userId);
 }
 
 function handleMarkUnread(event: Record<string, unknown>, userId: string, isFederated: boolean): void {