[Trivial] Allow flushing named pipe log handles

The named pipe handler provided by os-win is used by Nova when
consuming instance serial ports. For convenience, it allows logging
the named pipe output to a file (useful for storing the serial
console output).

The issue is that the log file never gets flushed, thus Nova returns
incomplete results. This change adds a method that may be used for
flushing the log file.

Change-Id: I4b5eb7579ac0c92e9c3630539b0c4d76f5b98c6d
Closes-Bug: #1741494
(cherry picked from commit 1b99d30fa2)

os_win/tests/unit/utils/io/test_namedpipe.py

@@ -324,6 +324,16 @@ class NamedPipeTestCase(base.BaseTestCase):
     def test_write_to_log_size_exceeded(self):
         self._test_write_to_log(size_exceeded=True)
 
+    def test_flush_log_file(self):
+        self._handler._log_file_handle = None
+        self._handler.flush_log_file()
+
+        self._handler._log_file_handle = mock.Mock()
+        self._handler.flush_log_file()
+
+        self._handler._log_file_handle.flush.side_effect = ValueError
+        self._handler.flush_log_file()
+
     @mock.patch.object(namedpipe.NamedPipeHandler, '_retry_if_file_in_use')
     @mock.patch.object(builtins, 'open')
     @mock.patch.object(namedpipe, 'os')

os_win/utils/io/namedpipe.py

@@ -221,6 +221,13 @@ class NamedPipeHandler(object):
         except Exception:
             self._stopped.set()
 
+    def flush_log_file(self):
+        try:
+            self._log_file_handle.flush()
+        except (AttributeError, ValueError):
+            # We'll ignore errors caused by closed handles.
+            pass
+
     def _rotate_logs(self):
         self._log_file_handle.flush()
         self._log_file_handle.close()