iotests.py: add FilePath context manager

The scratch/ (TEST_DIR) directory is not automatically cleaned up after
test execution.  It is the responsibility of tests to remove any files
they create.

A nice way of doing this is to declare files at the beginning of the
test and automatically remove them with a context manager:

  with iotests.FilePath('test.img') as img_path:
      qemu_img(...)
      qemu_io(...)
  # img_path is guaranteed to be deleted here

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170824072202.26818-3-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/qemu-iotests/iotests.py

@@ -160,6 +160,32 @@ class Timeout:
     def timeout(self, signum, frame):
         raise Exception(self.errmsg)
 
+
+class FilePath(object):
+    '''An auto-generated filename that cleans itself up.
+
+    Use this context manager to generate filenames and ensure that the file
+    gets deleted::
+
+        with TestFilePath('test.img') as img_path:
+            qemu_img('create', img_path, '1G')
+        # migration_sock_path is automatically deleted
+    '''
+    def __init__(self, name):
+        filename = '{0}-{1}'.format(os.getpid(), name)
+        self.path = os.path.join(test_dir, filename)
+
+    def __enter__(self):
+        return self.path
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        try:
+            os.remove(self.path)
+        except OSError:
+            pass
+        return False
+
+
 class VM(qtest.QEMUQtestMachine):
     '''A QEMU VM'''