WIP: online incremental megolm backups

src/client.js

@@ -3625,6 +3625,12 @@ module.exports.CRYPTO_ENABLED = CRYPTO_ENABLED;
  * });
  */
 
+/**
+ * Fires when we want to suggest to the user that they restore their megolm keys
+ * from backup or by cross-signing the device.
+ *
+ * @event module:client~MatrixClient#"crypto.suggestKeyRestore"
+ */
 
 // EventEmitter JSDocs
 

src/crypto/OlmDevice.js

@@ -91,6 +91,22 @@ function OlmDevice(sessionStore, cryptoStore) {
     this.deviceEd25519Key = null;
     this._maxOneTimeKeys = null;
 
+    // track whether this device's megolm keys are being backed up incrementally
+    // to the server or not.
+    // XXX: this should probably have a single source of truth from OlmAccount
+    this.backupKey = null;
+
+    // track which of our other devices (if any) have cross-signed this device
+    // XXX: this should probably have a single source of truth in the /devices
+    // API store or whatever we use to track our self-signed devices.
+    this.crossSelfSigs = [];
+
+    // track whether we have already suggested to the user that they should
+    // restore their keys from backup or by cross-signing the device.
+    // We use this to avoid repeatedly emitting the suggestion event.
+    // XXX: persist this somewhere!
+    this.suggestedKeyRestore = false;
+
     // we don't bother stashing outboundgroupsessions in the sessionstore -
     // instead we keep them here.
     this._outboundGroupSessionStore = {};
@@ -921,6 +937,11 @@ OlmDevice.prototype.addInboundGroupSession = async function(
                         this._cryptoStore.addEndToEndInboundGroupSession(
                             senderKey, sessionId, sessionData, txn,
                         );
+
+                        if (this.backupKey) {
+                            // get olm::Account::generate_backup_encryption_secret
+                            // save sessionData (pickled with this secret) to the server
+                        }
                     } finally {
                         session.free();
                     }

src/sync.js

@@ -1059,6 +1059,15 @@ SyncApi.prototype._processSyncResponse = async function(
         async function processRoomEvent(e) {
             client.emit("event", e);
             if (e.isState() && e.getType() == "m.room.encryption" && self.opts.crypto) {
+
+                // XXX: get device
+                if (!device.getSuggestedKeyRestore() && 
+                    !device.backupKey && !device.selfCrossSigs.length)
+                {
+                    client.emit("crypto.suggestKeyRestore");
+                    device.setSuggestedKeyRestore(true);
+                }
+
                 await self.opts.crypto.onCryptoEvent(e);
             }
         }