let devices know we can't claim their OTKs

src/matrix/e2ee/RoomEncryption.js

@@ -26,6 +26,7 @@ const ENCRYPTED_TYPE = "m.room.encrypted";
 // note that encrypt could still create a new session
 const MIN_PRESHARE_INTERVAL = 60 * 1000; // 1min
 
+// TODO: this class is a good candidate for splitting up into encryption and decryption, there doesn't seem to be much overlap
 export class RoomEncryption {
     constructor({room, deviceTracker, olmEncryption, megolmEncryption, megolmDecryption, encryptionParams, storage, sessionBackup, notifyMissingMegolmSession, clock}) {
         this._room = room;
@@ -349,36 +350,63 @@ export class RoomEncryption {
 
     async _processShareRoomKeyOperation(operation, hsApi, log) {
         log.set("id", operation.id);
+
         await this._deviceTracker.trackRoom(this._room, log);
         let devices;
         if (operation.userIds === null) {
             devices = await this._deviceTracker.devicesForTrackedRoom(this._room.id, hsApi, log);
             const userIds = Array.from(devices.reduce((set, device) => set.add(device.userId), new Set()));
             operation.userIds = userIds;
-            const userIdsTxn = await this._storage.readWriteTxn([this._storage.storeNames.operations]);
-            try {
-                userIdsTxn.operations.update(operation);
-            } catch (err) {
-                userIdsTxn.abort();
-                throw err;
-            }
-            await userIdsTxn.complete();
+            await this._updateOperationsStore(operations => operations.update(operation));
         } else {
             devices = await this._deviceTracker.devicesForRoomMembers(this._room.id, operation.userIds, hsApi, log);
         }
         
         const messages = await log.wrap("olm encrypt", log => this._olmEncryption.encrypt(
             "m.room_key", operation.roomKeyMessage, devices, hsApi, log));
+        const missingDevices = devices.filter(d => !messages.some(m => m.device === d));
         await log.wrap("send", log => this._sendMessagesToDevices(ENCRYPTED_TYPE, messages, hsApi, log));
+        if (missingDevices.length) {
+            await log.wrap("missingDevices", async log => {
+                log.set("devices", missingDevices.map(d => d.deviceId));
+                const unsentUserIds = operation.userIds.filter(userId => missingDevices.some(d => d.userId === userId));
+                log.set("unsentUserIds", unsentUserIds);
+                operation.userIds = unsentUserIds;
+                // first remove the users that we've sent the keys already from the operation,
+                // so if anything fails, we don't send them again
+                await this._updateOperationsStore(operations => operations.update(operation));
+                // now, let the devices we could not claim their key
+                const withheldMessage = this._megolmEncryption.createWithheldMessage(operation.roomKeyMessage, "m.no_olm", "OTKs exhausted");
+                await this._sendSharedMessageToDevices("org.matrix.room_key.withheld", withheldMessage, missingDevices, hsApi, log);
+            });
+        }
+        await this._updateOperationsStore(operations => operations.remove(operation.id));
+    }
 
-        const removeTxn = await this._storage.readWriteTxn([this._storage.storeNames.operations]);
+    async _updateOperationsStore(callback) {
+        const writeTxn = await this._storage.readWriteTxn([this._storage.storeNames.operations]);
         try {
-            removeTxn.operations.remove(operation.id);
+            callback(writeTxn.operations);
         } catch (err) {
-            removeTxn.abort();
+            writeTxn.abort();
             throw err;
         }
-        await removeTxn.complete();
+        await writeTxn.complete();
+    }
+
+    async _sendSharedMessageToDevices(type, message, devices, hsApi, log) {
+        const devicesByUser = groupBy(devices, device => device.userId);
+        const payload = {
+            messages: Array.from(devicesByUser.entries()).reduce((userMap, [userId, devices]) => {
+                userMap[userId] = devices.reduce((deviceMap, device) => {
+                    deviceMap[device.deviceId] = message;
+                    return deviceMap;
+                }, {});
+                return userMap;
+            }, {})
+        };
+        const txnId = makeTxnId();
+        await hsApi.sendToDevice(type, payload, txnId, {log}).response();
     }
 
     async _sendMessagesToDevices(type, messages, hsApi, log) {

src/matrix/e2ee/megolm/Encryption.js

@@ -43,6 +43,17 @@ export class Encryption {
         }
     }
 
+    createWithheldMessage(roomMessage, code, reason) {
+        return {
+            algorithm: roomMessage.algorithm,
+            code,
+            reason,
+            room_id: roomMessage.room_id,
+            sender_key: this._account.identityKeys.curve25519,
+            session_id: roomMessage.session_id
+        };
+    }
+
     async ensureOutboundSession(roomId, encryptionParams) {
         let session = new this._olm.OutboundGroupSession();
         try {