Prevent restoring newer backups into older versions of Signal.

Relates to #8184

res/values/strings.xml

@@ -1335,6 +1335,7 @@
     <string name="preferences_chats__create_backup">Create backup</string>
     <string name="RegistrationActivity_enter_backup_passphrase">Enter backup passphrase</string>
     <string name="RegistrationActivity_restore">Restore</string>
+    <string name="RegistrationActivity_backup_failure_downgrade">Cannot import backups from newer versions of Signal</string>
     <string name="RegistrationActivity_incorrect_backup_passphrase">Incorrect backup passphrase</string>
     <string name="RegistrationActivity_checking">Checking...</string>
     <string name="RegistrationActivity_d_messages_so_far">%d messages so far...</string>

src/org/thoughtcrime/securesms/RegistrationActivity.java

@@ -366,9 +366,9 @@ public class RegistrationActivity extends BaseActionBarActivity implements Verif
           restoreButton.setIndeterminateProgressMode(true);
           restoreButton.setProgress(50);
 
-          new AsyncTask<Void, Void, Boolean>() {
+          new AsyncTask<Void, Void, BackupImportResult>() {
             @Override
-            protected Boolean doInBackground(Void... voids) {
+            protected BackupImportResult doInBackground(Void... voids) {
               try {
                 Context        context    = RegistrationActivity.this;
                 String         passphrase = prompt.getText().toString();
@@ -383,23 +383,32 @@ public class RegistrationActivity extends BaseActionBarActivity implements Verif
 
                 TextSecurePreferences.setBackupEnabled(context, true);
                 TextSecurePreferences.setBackupPassphrase(context, passphrase);
-                return true;
+                return BackupImportResult.SUCCESS;
+              } catch (FullBackupImporter.DatabaseDowngradeException e) {
+                Log.w(TAG, "Failed due to the backup being from a newer version of Signal.", e);
+                return BackupImportResult.FAILURE_VERSION_DOWNGRADE;
               } catch (IOException e) {
                 Log.w(TAG, e);
-                return false;
+                return BackupImportResult.FAILURE_UNKNOWN;
               }
             }
 
             @Override
-            protected void onPostExecute(@NonNull Boolean result) {
+            protected void onPostExecute(@NonNull BackupImportResult result) {
               restoreButton.setIndeterminateProgressMode(false);
               restoreButton.setProgress(0);
               restoreBackupProgress.setText("");
 
-              if (result) {
+              switch (result) {
+                case SUCCESS:
                   displayInitialView(true);
-              } else {
+                  break;
+                case FAILURE_VERSION_DOWNGRADE:
+                  Toast.makeText(RegistrationActivity.this, R.string.RegistrationActivity_backup_failure_downgrade, Toast.LENGTH_LONG).show();
+                  break;
+                case FAILURE_UNKNOWN:
                   Toast.makeText(RegistrationActivity.this, R.string.RegistrationActivity_incorrect_backup_passphrase, Toast.LENGTH_LONG).show();
+                  break;
               }
             }
           }.execute();
@@ -1115,4 +1124,8 @@ public class RegistrationActivity extends BaseActionBarActivity implements Verif
       this.gcmToken   = previous.gcmToken;
     }
   }
+
+  private enum BackupImportResult {
+    SUCCESS, FAILURE_VERSION_DOWNGRADE, FAILURE_UNKNOWN
+  }
 }

src/org/thoughtcrime/securesms/backup/FullBackupImporter.java

@@ -98,7 +98,11 @@ public class FullBackupImporter extends FullBackupBase {
     EventBus.getDefault().post(new BackupEvent(BackupEvent.Type.FINISHED, count));
   }
 
-  private static void processVersion(@NonNull SQLiteDatabase db, DatabaseVersion version) {
+  private static void processVersion(@NonNull SQLiteDatabase db, DatabaseVersion version) throws IOException {
+    if (version.getVersion() > db.getVersion()) {
+      throw new DatabaseDowngradeException(db.getVersion(), version.getVersion());
+    }
+
     db.setVersion(version.getVersion());
   }
 
@@ -328,4 +332,9 @@ public class FullBackupImporter extends FullBackupBase {
     }
   }
 
+  public static class DatabaseDowngradeException extends IOException {
+    DatabaseDowngradeException(int currentVersion, int backupVersion) {
+      super("Tried to import a backup with version " + backupVersion + " into a database with version " + currentVersion);
+    }
+  }
 }