Pass along key backup for bootstrap

If we ask for the key backup key early in creating secret storage to ensure we
trust the backup, then we stash it to ensure it's available to bootstrap as well
without prompting again.

Fixes element-hq/element-web#12958

Author: jryans

src/CrossSigningManager.js

@@ -185,7 +185,7 @@ export async function promptForBackupPassphrase() {
 
     const RestoreKeyBackupDialog = sdk.getComponent('dialogs.keybackup.RestoreKeyBackupDialog');
     const { finished } = Modal.createTrackedDialog('Restore Backup', '', RestoreKeyBackupDialog, {
-            showSummary: false, keyCallback: k => key = k,
+        showSummary: false, keyCallback: k => key = k,
     }, null, /* priority = */ false, /* static = */ true);
 
     const success = await finished;

src/async-components/views/dialogs/secretstorage/CreateSecretStorageDialog.js

@@ -70,6 +70,7 @@ export default class CreateSecretStorageDialog extends React.PureComponent {
         this._recoveryKey = null;
         this._recoveryKeyNode = null;
         this._setZxcvbnResultTimeout = null;
+        this._backupKey = null;
 
         this.state = {
             phase: PHASE_LOADING,
@@ -243,7 +244,15 @@ export default class CreateSecretStorageDialog extends React.PureComponent {
                     createSecretStorageKey: async () => this._recoveryKey,
                     keyBackupInfo: this.state.backupInfo,
                     setupNewKeyBackup: !this.state.backupInfo && this.state.useKeyBackup,
-                    getKeyBackupPassphrase: promptForBackupPassphrase,
+                    getKeyBackupPassphrase: () => {
+                        // We may already have the backup key if we earlier went
+                        // through the restore backup path, so pass it along
+                        // rather than prompting again.
+                        if (this._backupKey) {
+                            return this._backupKey;
+                        }
+                        return promptForBackupPassphrase();
+                    },
                 });
             }
             this.setState({
@@ -272,10 +281,18 @@ export default class CreateSecretStorageDialog extends React.PureComponent {
     }
 
     _restoreBackup = async () => {
+        // It's possible we'll need the backup key later on for bootstrapping,
+        // so let's stash it here, rather than prompting for it twice.
+        const keyCallback = k => this._backupKey = k;
+
         const RestoreKeyBackupDialog = sdk.getComponent('dialogs.keybackup.RestoreKeyBackupDialog');
         const { finished } = Modal.createTrackedDialog(
-            'Restore Backup', '', RestoreKeyBackupDialog, {showSummary: false}, null,
-            /* priority = */ false, /* static = */ false,
+            'Restore Backup', '', RestoreKeyBackupDialog,
+            {
+                showSummary: false,
+                keyCallback,
+            },
+            null, /* priority = */ false, /* static = */ false,
         );
 
         await finished;