Better error reporting on backup errors

qvm-backup-restore will now show output from the rpc command
if backup restore fails at the very start (e.g. if file was not
found)

fixes QubesOS/qubes-issues#3446

qubesadmin/backup/restore.py

@@ -1195,15 +1195,16 @@ def _retrieve_backup_header_files(self, files, allow_none=False):
         retrieve_proc_returncode = retrieve_proc.wait()
         if retrieve_proc in self.processes_to_kill_on_cancel:
             self.processes_to_kill_on_cancel.remove(retrieve_proc)
-        extract_stderr = error_pipe.read(MAX_STDERR_BYTES)
+        extract_stderr = error_pipe.read(MAX_STDERR_BYTES).decode(
+            'ascii', 'ignore')
         error_pipe.close()
 
         # wait for other processes (if any)
         for proc in self.processes_to_kill_on_cancel:
             if proc.wait() != 0:
                 raise QubesException(
-                    "Backup header retrieval failed (exit code {})".format(
-                        proc.wait())
+                    "Backup header retrieval failed (exit code {}): {}".format(
+                        proc.wait(), extract_stderr)
                 )
 
         if retrieve_proc_returncode != 0:

qubesadmin/tests/tools/qvm_backup_restore.py

@@ -38,7 +38,9 @@ def tearDown(self):
     @mock.patch('qubesadmin.tools.qvm_backup_restore.input', create=True)
     @mock.patch('getpass.getpass')
     @mock.patch('qubesadmin.tools.qvm_backup_restore.BackupRestore')
-    def test_000_simple(self, mock_backup, mock_getpass, mock_input):
+    @mock.patch('os.path.exists', return_value=True)
+    def test_000_simple(self, _mock_exists,
+                        mock_backup, mock_getpass, mock_input):
         mock_getpass.return_value = 'testpass'
         mock_input.return_value = 'Y'
         vm1 = BackupVM()
@@ -68,7 +70,9 @@ def test_000_simple(self, mock_backup, mock_getpass, mock_input):
     @mock.patch('qubesadmin.tools.qvm_backup_restore.input', create=True)
     @mock.patch('getpass.getpass')
     @mock.patch('qubesadmin.tools.qvm_backup_restore.BackupRestore')
-    def test_001_selected_vms(self, mock_backup, mock_getpass, mock_input):
+    @mock.patch('os.path.exists', return_value=True)
+    def test_001_selected_vms(self, _mock_exist,
+                              mock_backup, mock_getpass, mock_input):
         mock_getpass.return_value = 'testpass'
         mock_input.return_value = 'Y'
         vm1 = BackupVM()
@@ -235,6 +239,14 @@ def test_012_handle_broken_missing_netvm(self):
                     self.app, mock_args, mock_restore_info)
             self.assertAppropriateLogging('NetVM', 'error')
 
+    @mock.patch('argparse.ArgumentParser.error')
+    def test_013_missing_file(self, mock_argparse):
+        mock_argparse.side_effect = qubesadmin.exc.QubesException('Error')
+        with self.assertRaises(qubesadmin.exc.QubesException):
+            qubesadmin.tools.qvm_backup_restore.main(['/some/path'],
+                                                     app=self.app)
+        mock_argparse.assert_called_once_with("File not found: '/some/path'")
+
     def test_100_restore_in_dispvm_parser(self):
         """Verify if qvm-backup-restore tool options matches un-parser
         for paranoid restore mode"""