tests: check importing empty data into ThinVolume

Verify if it really discards old content.

QubesOS/qubes-issues#5192

qubes/tests/storage_lvm.py

@@ -843,6 +843,37 @@ class TC_00_ThinPool(ThinPoolBase):
 
         self.loop.run_until_complete(volume.remove())
 
+    def test_034_import_data_empty(self):
+        config = {
+            'name': 'root',
+            'pool': self.pool.name,
+            'save_on_stop': True,
+            'rw': True,
+            'size': 1024 * 1024,
+        }
+        vm = qubes.tests.storage.TestVM(self)
+        volume = self.app.get_pool(self.pool.name).init_volume(vm, config)
+        with unittest.mock.patch('time.time') as mock_time:
+            mock_time.side_effect = [1521065905]
+            self.loop.run_until_complete(volume.create())
+        p = self.loop.run_until_complete(asyncio.create_subprocess_exec(
+            'sudo', 'dd', 'if=/dev/urandom', 'of=' + volume.path, 'count=1', 'bs=1M'
+        ))
+        self.loop.run_until_complete(p.wait())
+        import_path = self.loop.run_until_complete(volume.import_data())
+        self.assertNotEqual(volume.path, import_path)
+        p = self.loop.run_until_complete(asyncio.create_subprocess_exec(
+            'sudo', 'touch', import_path))
+        self.loop.run_until_complete(p.wait())
+        self.loop.run_until_complete(volume.import_data_end(True))
+        self.assertFalse(os.path.exists(import_path), import_path)
+        p = self.loop.run_until_complete(asyncio.create_subprocess_exec(
+            'sudo', 'cat', volume.path,
+            stdout=subprocess.PIPE
+        ))
+        volume_data, _ = self.loop.run_until_complete(p.communicate())
+        self.assertEqual(volume_data.strip(b'\0'), b'')
+
     def test_040_volatile(self):
         '''Volatile volume test'''
         config = {