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Re: [PATCH 4/5] python/qemu: qmp: Make QEMUMonitorProtocol a context man

From: Wainer dos Santos Moschetta
Subject: Re: [PATCH 4/5] python/qemu: qmp: Make QEMUMonitorProtocol a context manager
Date: Wed, 29 Jan 2020 18:07:14 -0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 1/8/20 10:23 PM, John Snow wrote:

On 12/27/19 8:41 AM, Wainer dos Santos Moschetta wrote:
This implement the __enter__ and __exit__ functions on
QEMUMonitorProtocol class so that it can be used on 'with'
statement and the resources will be free up on block end:

with QEMUMonitorProtocol(socket_path) as qmp:

Signed-off-by: Wainer dos Santos Moschetta <address@hidden>
  python/qemu/qmp.py | 15 +++++++++++++--
  1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/python/qemu/qmp.py b/python/qemu/qmp.py
index 914b8c6774..6d55f53595 100644
--- a/python/qemu/qmp.py
+++ b/python/qemu/qmp.py
@@ -139,6 +139,15 @@ class QEMUMonitorProtocol:
                  raise QMPConnectError("Error while reading from socket")
+ def __enter__(self):
+        # Implement context manager enter function.
+        return self
+    def __exit__(self, exc_type, exc_value, exc_traceback):
+        # Implement context manager exit function.
+        self.close()
+        return False
      def connect(self, negotiate=True):
          Connect to the QMP Monitor and perform capabilities negotiation.
@@ -259,8 +268,10 @@ class QEMUMonitorProtocol:
          Close the socket and socket file.
-        self.__sock.close()
-        self.__sockfile.close()
+        if self.__sock:
+            self.__sock.close()
+        if self.__sockfile:
+            self.__sockfile.close()
Not evident on cold read: does self.close() change self.__sock and
self.__sockfile such that they are false-ish?

Because self.__exit__() calls self.close() even when a runtime exception raises, there isn't any guarantee that self.__sockfile and self.__sock were initialized. That's the reason why I added those guards.

close() I suspect might need to actually unset the __sock and __sockfile

The QEMUMonitorProtocol object is designed to be disposed after close() being called. So I don't see any reason to unset those fields. Unless I am missing something here...

Thanks for the comments!

- Wainer

def settimeout(self, timeout):

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