only run decryptPending if needed

src/matrix/DeviceMessageHandler.js

@@ -15,7 +15,6 @@ limitations under the License.
 */
 
 import {OLM_ALGORITHM, MEGOLM_ALGORITHM} from "./e2ee/common.js";
-import {groupBy} from "../utils/groupBy.js";
 
 // key to store in session store
 const PENDING_ENCRYPTED_EVENTS = "pendingEncryptedDeviceEvents";
@@ -32,13 +31,20 @@ export class DeviceMessageHandler {
         this._megolmDecryption = megolmDecryption;
     }
 
+    /**
+     * @return {bool} whether messages are waiting to be decrypted and `decryptPending` should be called.
+     */
     async writeSync(toDeviceEvents, txn) {
         const encryptedEvents = toDeviceEvents.filter(e => e.type === "m.room.encrypted");
+        if (!encryptedEvents.length) {
+            return false;
+        }
         // store encryptedEvents
         let pendingEvents = await this._getPendingEvents(txn);
         pendingEvents = pendingEvents.concat(encryptedEvents);
         txn.session.set(PENDING_ENCRYPTED_EVENTS, pendingEvents);
         // we don't handle anything other for now
+        return true;
     }
 
     /**

src/matrix/Session.js

@@ -335,7 +335,11 @@ export class Session {
     }
 
     async writeSync(syncResponse, syncFilterId, txn) {
-        const changes = {};
+        const changes = {
+            syncInfo: null,
+            e2eeAccountChanges: null,
+            deviceMessageDecryptionPending: false
+        };
         const syncToken = syncResponse.next_batch;
         const deviceOneTimeKeysCount = syncResponse.device_one_time_keys_count;
 
@@ -357,7 +361,8 @@ export class Session {
 
         const toDeviceEvents = syncResponse.to_device?.events;
         if (Array.isArray(toDeviceEvents)) {
-            this._deviceMessageHandler.writeSync(toDeviceEvents, txn);
+            changes.deviceMessageDecryptionPending =
+                await this._deviceMessageHandler.writeSync(toDeviceEvents, txn);
         }
 
         // store account data
@@ -382,8 +387,11 @@ export class Session {
         }
     }
 
-    async afterSyncCompleted(isCatchupSync) {
+    async afterSyncCompleted(changes, isCatchupSync) {
-        const promises = [this._deviceMessageHandler.decryptPending(this.rooms)];
+        const promises = [];
+        if (changes.deviceMessageDecryptionPending) {
+            promises.push(this._deviceMessageHandler.decryptPending(this.rooms));
+        }
         // we don't start uploading one-time keys until we've caught up with
         // to-device messages, to help us avoid throwing away one-time-keys that we
         // are about to receive messages for
@@ -394,9 +402,11 @@ export class Session {
                 promises.push(this._e2eeAccount.uploadKeys(this._storage));
             }
         }
+        if (promises.length) {
             // run key upload and decryption in parallel
             await Promise.all(promises);
         }
+    }
 
     get syncToken() {
         return this._syncInfo?.token;

src/matrix/Sync.js

@@ -93,6 +93,7 @@ export class Sync {
         // if syncToken is falsy, it will first do an initial sync ... 
         while(this._status.get() !== SyncStatus.Stopped) {
             let roomStates;
+            let sessionChanges;
             try {
                 console.log(`starting sync request with since ${syncToken} ...`);
                 // unless we are happily syncing already, we want the server to return
@@ -110,6 +111,7 @@ export class Sync {
                 const syncResult = await this._syncRequest(syncToken, timeout);
                 syncToken = syncResult.syncToken;
                 roomStates = syncResult.roomStates;
+                sessionChanges = syncResult.sessionChanges;
                 // initial sync or catchup sync
                 if (this._status.get() !== SyncStatus.Syncing && syncResult.hadToDeviceMessages) {
                     this._status.set(SyncStatus.CatchupSync);
@@ -125,16 +127,21 @@ export class Sync {
                 }
             }
             if (this._status.get() !== SyncStatus.Stopped) {
-                await this._runAfterSyncCompleted(roomStates);
+                // TODO: if we're not going to run this phase in parallel with the next
+                // sync request (because this causes OTKs to be uploaded twice)
+                // should we move this inside _syncRequest?
+                // Alternatively, we can try to fix the OTK upload issue while still
+                // running in parallel. 
+                await this._runAfterSyncCompleted(sessionChanges, roomStates);
             }
         }
     }
 
-    async _runAfterSyncCompleted(roomStates) {
+    async _runAfterSyncCompleted(sessionChanges, roomStates) {
         const isCatchupSync = this._status.get() === SyncStatus.CatchupSync;
         const sessionPromise = (async () => {
             try {
-                await this._session.afterSyncCompleted(isCatchupSync);
+                await this._session.afterSyncCompleted(sessionChanges, isCatchupSync);
             } catch (err) {
                 console.error("error during session afterSyncCompleted, continuing",  err.stack);
             }
@@ -204,6 +211,7 @@ export class Sync {
         return {
             syncToken,
             roomStates,
+            sessionChanges,
             hadToDeviceMessages: Array.isArray(toDeviceEvents) && toDeviceEvents.length > 0,
         };
     }